Top Ten Physics Concepts (as told by my trip to South Africa)

As the year comes to a close, let's take a look at some of my favorite Physics concepts we've covered (in no particular order), as told in pictures from my five-week road trip across South Africa last summer.

WEEK 1-CAPE TOWN

1.) Why tides occur

Cape of Good Hope

Tides are caused by the net force of the moon's gravitational force acting on each side of the earth. The side of the earth that is closer to the moon is more strongly affected by the moon's gravitational pull than the farther side. We know this because of the Universal Gravitational Law, which states that F=(G)(m1)(m2)/d^2. Essentially, this means that  since force is inversely proportional to distance, a short distance has a strong force and a long distance has a weak force. This means that the force from the moon on one side of the moon is positive and is negative on the other side. It is this difference in force that causes the tides. 

Check out this diagram for a visual representation of the difference:

2. Why motors work

Okay, okay, I know that motors aren't exactly a concept, but the physics behind how they operate is one of the concepts we've covered that is most easily applied to daily life. 

The two essential parts of a motor are a current-carrying wire and a magnet. When the wire is placed over the magnet, the force from that magnetic field will act on the wire. The force causes a torque (remember: torque is what causes rotation) and the wire will spin. This induces a voltage in the wire and causes a current, which then powers the bus. 

WEEK 3- KAROO AND DRAKENSBERG MOUNTAINS

3. Why wind turbines work

Wind turbines are operated by electromagnetic induction. This is when the magnetic field of a loop of wire is changed by moving a magnet through or over coils of wire without an additional voltage source. The relative motion between the magnet and the loops induces voltage, which causes current. This is a method of transforming mechanical energy to electrical energy. In wind turbines, the wind moves blade that are connected to a magnet, the magnet rotates around wire inside the turbine. The force from the magnet's electric field causes a change in the wire's electric field, which induces voltage. That voltage causes current to flow which can be stored or sent to power homes and buildings. 

4. Newton's 3rd Law of Motion

Now, on to cleaner and less dead things. Newton's 3rd Law of Motion says that for every action, there is an equal and opposite reaction. 

While I was in the Drakensberg Mountains, I went abseiling. 

Me abseiling down a cliff in the Drakensberg.

I was connected to a rope that had a friction device on it. As I walked backward down the cliff, I was kept safe because of Newton's 3rd Law and action-reaction pairs. Since every action has an equal and opposite reaction, while the force of my weight pulled the rope attached to my harness down, the rope pulled me up with an equal and opposite force. Additionally, with each step, I pushed on the cliff with a force equal and opposite to that with which the cliff pushed on me. 

5. Why the auroras happen

The northern and southern lights (aurora borealis and aurora australis, respectively) are both phenomena we can thank physics for. The aurora australis can be seen at the southernmost tips of South Africa. 

The auroras are caused by the entrance of cosmic rays into the earth's atmosphere. Essentially, cosmic rays are charged particles from beyond the earth's atmosphere that enter the earth's magnetic field. Charged particles can only enter a magnetic field when they are perpendicular to the field. That is why the auroras occur in locations at the northern and southern poles, and not in countries on the equator, where the rays would be parallel to the earth's magnetic field. 

WEEK 4- KRUGER NATIONAL PARK

6. Newton's 1st Law of Motion

A physics classic. Now, this may be gruesome, but bear with me. Newton's 1st Law states that an object in motion will remain in motion unless acted upon by an outside force and an object at rest will remain at rest unless acted upon by an outside force. 

Now, how does this apply to my trip to South Africa? Consider the head of this water buffalo carcass (sorry, it's a little gruesome). 

Seen in Kruger National Park.

Now, this water buffalo was killed. It was likely ambushed by a group of lions or hyenas. As it was chased, it remained in motion until acted upon by the outside force of its attackers. Now, it will remain at rest unless acted upon by an outside force, like a vulture. 

7. Newton's 2nd Law of Motion

Newton's 2nd Law states that acceleration is equal to force divided mass. Therefore, acceleration is inversely proportional to mass. 

Consider and contrast a rhinoceros and a hyena. 

Seen in Kruger National Park.

Seen in Kruger National Park.

The average running speed of a rhino is 31 miles per hour, while the average running speed of a hyena is 40 miles per hour. The mass of a hyena is significantly lower than that of a rhino, so they are able to accelerate much faster. 

8. Work

Work is defined as the force exerted on an object over a distance (work=force/distance). Consider these two pictures. The same amount of work is being done on the male impala with two birds on his back as the female kudu with no birds on her back.

Seen in Kruger National Park.

Seen in Kruger National Park.

Work can only be done when the force being exerted on an object (or impala) is perpendicular to the distance covered. Therefore, as the impala walks around with two birds on his back, no work is being done on him because the weight of the bird is parallel to the ground.

9. Kinetic Energy and Potential Energy

Kinetic energy is the energy of motion. It is calculated by using the formula KE=1/2(m)(v^2). In words, kinetic energy is equal to half the object's mass times its velocity squared. Kinetic energy is measured in Joules. An object at rest will never have kinetic energy.

Look at this bird at a watering hole. While it is standing there, its kinetic energy is 0J.

Seen in Kruger National Park.

Let's practice calculating kinetic energy. Imagine that this bird saw a crocodile approaching in the water and flew away. Say that its mass is 10kg and its velocity is 4 m/s. 

KE=1/2(m)(v^2)

KE=(0.5)(10)(16)

KE=(0.5)(160)

KE=80J

The bird in flight's kinetic energy would be 80 Joules.

Potential energy, the energy of position, determines the maximum amount of kinetic energy an object can have. It is calculated using the formula PE=(m)(g)(h). In words, potential energy is equal to mass times energy times height. 

Look at this African fish eagle.

Seen in Kruger National Park.

Let's imagine that the tree is 100m tall and the eagle's mass is 40 kg. Acceleration due to gravity will always be 9.8 m/s^2, but let's round to 10 m/s^2. 

Here is how to calculate the eagle's potential energy:

PE=(m)(g)(h)

PE=(40)(100)(10)

PE=40,000J

Here's another cool thing about the relationship between potential and kinetic energy. Imagine that the eagle swoops down to catch a field mouse and then flies back up to the top of the tree to eat its prey. While the eagle is sitting on the top of its perch, its potential energy is 40,000J and its kinetic energy is 0J because it is a rest. While the eagle swoops down, its potential energy converts to kinetic energy. At the bottom of its path, when it grabs the mouse, the energy has converted completely so that the eagle's potential energy is 0J and its kinetic energy is 40,000J. Then, as it flies back to the top of the tree, the kinetic energy converts back to potential energy. While it sits to eat its meal, the eagle's potential energy is back to 40,000J and its kinetic energy is back to 0J.

10. Center of gravity 

An object's center of gravity is its specific point upon which gravity acts.  An object's base of support is just that, its base. Whether or not an object will rotate or fall is based upon the location of its center of gravity over its base of support. The wider an object's base of support is, the harder it is for it to rotate and fall over. 

Seen in Kruger National Park.

This warthog does not fall over as it leans to the ground to eat because its center of gravity is over its base of support. If it were to lean farther forward or if its legs were closer together, it would easily rotate and fall over.

Thanks for coming along for the trip!

DIY Wind Turbine!

This past week in Physics we built wind turbines, building on our knowledge of motors.

Here is a quick recap from my posts about my mini motor and my unit seven summary about electromagnetic induction, which is the big physics concept at work here.

Electromagnetic induction is when a voltage is induced in a current-carrying wire by the relative motion between the coil of wire and a magnet. If a magnet moves in or over a coil of wire, or vice versa, the wire's magnetic field will align with that of the magnet. This change in magnetic field will induce a voltage, which causes current to flow. The force of the magnetic field causes a torque on the wire, which makes it spin. This is a great example of a conversion of energy from mechanical to electrical.

A pure example of this can be seen here, in the video of the motor I made. Notice how, as the wire is positioned over the magnet, the wire spins.



 There are other physics concepts you should be aware of before building your own wind turbine. 

Newton's 1st Law of Motion

An object in motion will remain in motion unless acted upon by an outside force and an object at rest will remain at rest unless acted upon by an outside force. 

That's where friction comes into play: every piece of your turbine needs to be measured, made, and assembled with care so that haphazardly cut pieces do not rub together and cause unwanted friction. The more friction there is, the less the turbine will spin and the more poorly it will produce energy.

Newton's 2nd Law of Motion

Newton's second law of motion states that acceleration is directly proportional to force and inversely proportional to mass. 

This is important when it came to the blades of the turbine. Since we tested them inside with fans, we had to make sure that our blades were lightweight in order for the wind's force to cause the turbine to accelerate.

Newton's 3rd Law of Motion

For every action, there is an equal and opposite reaction.

This is another reason for why the blades have to be made out of a lightweight material. According to Newton's 3rd Law, the blades will push the wind with the same amount of force with which the wind is pushing the blades. Therefore, heavier blades are even less likely to move.

Materials and Method

Like I did for the mini motor, my partners and I kept the materials for the wind turbine pretty basic. We used, for the most part, household items. All in all, we spent less than $15 on materials.

• cardboard-- for the base and the frame
• copper wire-- to carry the current and cause the blades to move
• cut pieces of a recycled plastic water bottle (typical vending machine fare)-- for the blades
• metal dowel-- as an axle 
• wooden dowel-- to stabilize the blades
• masking and electrical tape-- to hold together the magnets and the blades
• hot glue-- to secure the frame and the base
• wooden block-- to provide a level bottom for the base

 We built the frame for the generator using the instructions from this website and its handy-dandy video below.




We didn't follow the instructions to a tee (read on for an explanation about our 200ft wire disaster!) but they were very helpful and I am sure you will be even more successful than we were if you do follow these instructions more carefully.

Like in the video, we placed our magnets inside the cardboard frame, attached to the metal axle. We bought the ceramic block magnets, but they caused us trouble, so we ended up using stacks of small, round magnets instead. In order for electromagnetic induction to occur, we wrapped the wire around the outside of the box. The change in magnet field caused the magnets (taped to the axle) to spin. Since the magnets were spinning, so did the axle, which is how we got our turbine blades to move.

The actual blades were built by cutting strips of plastic water bottle and taping them to a wooden dowel which would then be glued to a cardboard platform on the end of the metal axle.

Here are some images of our final turbine (it's not a work of art, I know).

In the end, we produced 0.008A and 0.005V. It worked! Unfortunately, we (unlike you because I warned you!) were not able to light an LED light bulb because there was too much friction caused by the amount of tape holding the magnets together.

Here is a video of my turbine in action. I know it's horribly pixelated, but maybe you can make out the fact that it really did spin.






In the end, I got to see the concept of more coils = more voltage in action. I'm really glad that we followed a specific design for the generator, even though we needed to tweak it a little. The ceramic block magnets ended up not working for us because once we wrapped them to the axle with tape, there was too much friction inside the frame for it to work well. Overall, I wish we had made the frame bigger, to allow us to have more flexibility with our materials. My biggest piece of advice, similar to my advice from the mousetrap car, be METICULOUS in everything. One of our biggest setbacks with this project was that we did not coil the copper wire around the frame immediately as we unspooled it. As a result, we effectively tangled 200ft of perfectly good copper wire, wasted class time and personal time trying to fix it and ended up wasted the wire we could not salvage. I think that if we had taken a little more time to think through each of our steps, we wouldn't have tangled the wire or made the frame too small.

Unit Seven is Going to Heaven!

It's the end of the school year and we are finishing up our last unit in Conceptual Physics! Unit Seven covered magnets/magnetism, electromagnetism, forces on particles in a charged field, electromagnetic induction, energy production, and energy transfer.

Magnets and Magnetism

Moving charges are the source of all magnetism. On a closer level, consider the fact that all object are made of atoms and that these atoms contain electron clusters. We know that moving electrons cause current, so this should begin to sound familiar. In an unmagnetized object, these electron clusters are all spinning in random directions. A group of electron clusters spinning in the same direction is called a domain. When an object is magnetic, all of its domains will be spinning in the same direction.

In a magnet, field lines explain why like sides repel and opposing sides attract. This is magnetic flux.

The domains in a magnet are all pointing in one direction, when you hold a compass (which is simply a magnet that is free to move) over a magnet, it will move based on the alignment of the magnet's domains. They go from south to north inside the magnet and from north to south outside the magnet.



Two magnets are attracted to each other because their domains are all spinning in the same direction.

So: how does a paper clip stick to  magnet?

The domains in a paper clip are random, so it has no poles. A domain is a cluster of electrons that are spinning in the same direction. The magnet has a magnetic field, which means that it has a north and a south pole. When the magnet is close to the paper clip, the domains of the paper clip align to match the magnetic field of the magnet. The paper clip now has a north and south pole and the north pole of the paper clip is attracted to the south pole of the magnet. Thus, the paper clip sticks to the magnet.

Here is a great video explaining magnetism with helpful visuals:

Electromagnetism

An electromagnet is a magnet that needs a current-carrying wire that has a magnetic field. The domains of a magnetized object can align with that field, and then have a magnetic field of its own.

In order to create an electromagnet, we need a current. That's where electromagnetic induction comes in. Electromagnetic induction is when the magnetic field of a loop of wire is changed by moving a magnet in or over loops of wire without an additional voltage source. The relative motion between the magnet and the loops induces voltage, which causes current. This means that it is a method of transforming mechanical energy to electrical energy.

The relationship between loops of wire and voltage is directly proportional. The greater the number of loops, the greater the voltage. Additionally, slow motion means low voltage and quick motion means high voltage.

Examples of how electromagnetic induction is useful in daily life are: traffic light triggers, hybrid cars converting braking energy to electric energy, credit card machines, and security sensors.

So: how does a credit card machine work?

Credit cards work by electromagnetic induction. This is when an electric current is produced in a wire by moving a magnet near a loop of wire or vice versa. This induces voltage by changing the magnetic field through relative motion between the magnet and the wire, which causes current. Each credit card has a specific pattern of magnets in a strip on the back. Each credit card machine has a loop of wire where you swipe the card. When you swipe your credit card, the voltage induced is specific to your card so the individual current flow that is caused identifies your card to the machine.

Forces on Charged Particles

As charges move around a magnetic field, the charges that are moving perpendicularly to the magnetic field feel the force from the field.

While this seems abstract, there is a natural phenomenon that is caused specifically by this physics concept: the aurora borealis (northern lights). Simply put, cosmic rays are charged particles that enter the earth's magnetic field from outside of the atmosphere. When these cosmic rays are moving parallel to the earth's magnetic field, they cannot enter the atmosphere. This means that the charged particles are deflected along the equator, which runs parallel to the earth. However at the north and south poles, the charged particles can enter the earth's atmosphere which causes the light show often referred to as the northern lights. Aurora borealis is the name for this phenomenon when it occurs in the northern hemisphere, it can often be seen in places like Canada, Norway, Russia, Finland, Scotland, Iceland, Greenland, Denmark, and some parts of Alaska in the United States of America. In the southern hemisphere, it is called aurora australis and can be seen in Australia, Tasmania, New Zealand and Antarctica.

A while back, I posted about the motor I  built using paper clips, a magnet, a 9V-battery, rubber bands, and a copper wire. That exercise is a prime example of how the two essential parts of a wire and a magnet. The key principle that a motor relies on is that the current-carrying wire (remember: moving charges!) will feel a force from the magnetic field which will cause a torque. Remember from unit four that torque is what causes rotation? Well, that is why the current-carrying wire spins. This is a great example of a conversion of electrical energy to mechanical energy.

To figure out which way the magnetic field will make the wire spin, we can use something called the right hand rule. As we can see from the diagram below, we can use our right hand to visualize this concept. The thumb represents the force, the index finger represents the current and the middle finger represents the magnetic field.

As an example, if you know that the magnetic field is going forward, the force will go to the left. Try it!

 

Energy Production

The opposite of a motor, a generator converts the mechanical energy of a spin to electrical energy. A generator uses electromagnetic induction by moving magnets around a wire or a wire around magnets to produce energy. All generators are the same. You are most likely already familiar with the three most common ways that companies generate energy: steam, water, and wind.

Energy Transfer

A transformer is a pair of wire coils that converts alternating current from an outlet to direct current for a device. A transformer has either a high number of coils to a low number, or vice versa. Transformers require alternating current to function because the constant change in direction in the current causes a change in the magnetic field. This changing magnetic field in the primary (the first coil) will induce a voltage in the secondary (the second coil).

Ding! Ding! Ding! What does that sound like? Electromagnetic induction! It's everywhere this unit. 

There are two types of transformers we need to know about: a step-up transformer and a step-down transformer. A step-up transformer increases the amount of voltage going to the device. This could be seen with a large device like a washing machine, which needs more voltage than the typical American outlet provides (120V). A step-down transformer is used for a smaller device like a cellphone, which would likely overheat if it used 120V.

We can use something called Faraday's Law for calculations involving transformers. It looks like this:

# of loops in the primary/ voltage of primary = # of loops in the secondary/ voltage of secondary

We also need to know that the power of the primary and the power of the secondary will always be equal. This means that if the primary has a small current and a high voltage, the secondary must have a high current and a low voltage. This will be the only time that current and voltage are inversely proportional. 

Remember: We can also use I = v/r to find current. 

Something interesting about transformers is that they are all around us, but we barely notice them. Ever wondered what that huge block on your laptop charger is? It's a transformer. Ever wondered what those gray boxes on power lines are? They're transformers, too. Transformers are put on power lines in order to decrease the amount of current flow in each line. This is in order to conserve energy. Remember that energy is released as light, heat, and sound? If transformers were not on power lines, more current than necessary would run through them and would be wasted and released as heat. I think the use of transformers is a pretty neat solution. 

Mini Motor

Today I finished work on a mini-motor made using a copper wire armature, a 9V-battery, two rubber bands, and two paper clips. The motor looked like this: a battery with two paper clips bent into stands on either side (attached with rubber bands) and a magnet on top. The current running through the armature made the motor work.

Here's how: we know that current is equal to I=V/R. Therefore, we must have a source of voltage for the current to run through the system. The battery provides the current's voltage. The paper clips conduct the current and as a result of the current's movement, the armature spins.

In order for the current to flow through the armature, I scraped the tops of the wire on either side of the loop while the loop was perpendicular to the table. I did it this way so that the force of the magnet's magnetic field, when acting on the armature, would cause a torque. The force would have been felt, but not have caused a torque had I scraped the armature while the loop was parallel to the table.

The armature spins because of the torque when it is connected to both sides of the battery, completing the circuit. If I added fan blades to the motor, it could keep me cool as the quickly warming days indicate summer's approach. Additionally, adding sharp blades would make me a blender and adding wheels would make me a car.

This is a video of my motor in action!

That's it for Unit Six!

 

 

Here is a video that I made with two of my classmates explaining Electric fields with more detail.

 

Mousetrap(ped) Car!

This past week in Physics, we, in pairs, built cars that were powered by mousetraps! The purpose of the project was see how all of the concepts we learned over the course of the year could be applied to achieve our goal of building a car that would go at least 5 meters. The group with the fastest car would win a milkshake and a question omitted from the final exam.

Final mousetrap car design made by me and Annie.

So... How does physics apply to this project?

Newton's 1st Law
The car in motion will continue to be in motion unless a force pushes it backwards. In the case of the mousetrap car, this force could come from friction between the axles and the frame, friction on the mousetrap spring or friction between the wheels and the ground.

The only friction on the car that is actually helpful is the friction between the wheels and the ground. As you'll see explained below, the larger the force between the wheels and the ground are, the more the car will be propelled. In order to create more friction there, we added electrical tape to the wheels. The tape had the same gripping effect on the ground that rubber tires have on roads. Contrarily, too much friction between the axles and the frame would cause the wheels to stop turning entirely.

Newton's 2nd Law
Acceleration is equal to force over mass. The force causes the car's acceleration but if there is too much mass, the car will not be able to accelerate as much as it would be able to with a smaller mass. Therefore, we wanted our frame to be as light as possible.

Newton's 3rd Law
In this situation the action-reaction pairs are: car pushes ground back and ground pushes car forward. This causes a reaction from the axle torque which causes wheel torque. Therefore, the bigger the wheels are, the bigger the lever arm must be and the bigger the torque will be. It is important to remember that torque is what causes rotation. Since we wanted our car to follow a straight course, we wanted our car to have the smallest torque possible.

My partner and I built our car using:

• Small wooden discs for front wheels-- The size of the front wheels is not as important as the size of the back wheels. They mostly serve the purpose of keeping the car moving in a straight line.
• Wooden dowels for axles-- We used a narrow Poplar dowel for the front axle and a thicker Oak axle for the back axle.
• Balsa wood for the frame-- We drilled holes with circumferences a little larger than those of our dowels to allow for easy rotation. Balsa wood worked well for our frame because it is light, so the wheels don't have to support very much weight.
• CDs for back wheels-- CDs are pretty commonly used for wheels on mousetrap cars because they are thin, light and easy to find. However, we needed to wrap our wheels with electrical tape in order to create friction between the ground and our wheels.
• Poplar dowel for the lever arm-- We attached a 2-foot long Poplar wood dowel to the arm of the mousetrap to act as our lever arm. Our car was powered by our back wheels, so we attached a string to the top of our lever with a glue gun and wound it around the back axle. Therefore, when the trap was released, the string would cause the back wheels to turn. This works because the Elastic Potential Energy stored in the spring is converted to Kinetic Energy when it is attached to the spinning axle. In reality, the lever of the mousetrap doesn't actually do work on the car, it just applies force to the axle. We used the torque of the spring to cause the force. We also increased the length of our lever arm from 1.5 feet to 2 feet because we knew that it would increase the distance and time that the force would act on the axle, not actually increasing the force at all.
• Rubber bands and hot glue-- We made sure that our wheels were stable by putting rubber bands on either side of the wheels and gluing the rubber bands down.

Remember:

• We can't calculate the amount of work the spring does on the car because the force of the spring isn't parallel to the axle and we know that work = force multiplied by parallel distance.
• We also can't calculate the amount of PE stored in the spring and the amount of KE the car used because we can only find the average velocity, not the instantaneous speed of the car, because it accelerates at different intervals over the course. Additionally, PE depends on vertical position, and the car only covered a horizontal distance.
• We can't calculate the force of the spring exerted on the car to accelerate it because we don't have a constant acceleration of the car.
• Rotational inertia is the property of an object to resist changes in spin. Rotational velocity is the amount of rotations made over a certain amount of time. Tangential velocity is the linear speed of something moving along a circular path. This means that we needed to find wheels that had a low rotational inertia but a high tangential velocity in order to have wheels that would spin quickly and easily. Our first wheels had a rotational inertia that was too high, so we downsized to CDs to maintain a higher tangential velocity than our small front wheels and with a low rotational inertia.
• The Law of Conservation of Energy states that the input amount of energy is equal to the output. Therefore, the amount of potential energy stored in the spring to be converted to kinetic energy when it started to move was the maximum amount of energy that the car could have. In order to maximize the amount of energy stored in the spring, we pulled the lever to its tightest position and wound the string attached to the lever arm completely taut. Therefore, we were able to house the most potential energy in the spring's release as possible.

Here is a video of the car in action! We came in 3rd place, covering 5m in 3.77s.






Originally, we intended to use wooden discs (larger than CDs) for our back wheels because we knew that they would have a high rotational velocity. However, the wheels were so big that their rotational inertia was high enough to make them ineffective. We also increased the length of our lever arm to compensate for the rotational inertia of our CD wheels. Since the wheels were large, our lever arm needed to apply more force to the axle in order make sure that the car would travel 5m. Additionally, our original design had axles underneath the body of the car, attached with hooks. This was not as efficient because the hooks were much larger than the axles, which allowed them to wobble around within the hooks. We fixed this by drilling holes into our new frame that were marginally wider than the axles. The use of the frame also made the car's body more narrow and less subject to air resistance.

My partner and I ended up restarting our project about three times. Initially, I thought that this would be easy-peasy because I made a mousetrap car for another science class about 4 years ago. However, that time I had my science/math geek dad to help me. This time, my partner and I were on our own. Almost all of the issues we encountered resulted from our lack of true preparation. Unlike some of our classmates, we didn't have a specific set of instructions that we were following. Essentially, we were just putting parts together and hoping for the best. The first major goal we had was for our car to go to the finish line after we pushed it. This took until the day before it was due because we didn't think through our entire plan before beginning to build. Since we didn't have a concrete idea of how our car would work and how we would execute its construction, we ran into a lot of issues like gluing things on before considering what that would keep us from being able to do. Another issue we had was that once we got our car running off of the mousetrap, it would stop just before the 5m mark. We didn't stop to think about the physics as much as we should have. I think that had we considered that a little more during each step of the process, we would have realized that our lever arm needed to be about six inches longer than it originally was in order to have a greater force on our axle. The most frustrating mistakes, however, were just results of us rushing through the process. We were each frustrated, stressed and tired of burning ourselves with the glue gun and it was easy to stop ourselves from taking the time to do things correctly. One vivid memory of an example of this is when, after starting our frame over for the second time, the wood cracked. It was the afternoon before the car was due and I felt like the world had come to an end. I had been confident that finishing the frame would mean success for us. Unfortunately, we were back to square one. Again. After that, my partner and I measured everything meticulously and devised a methodical plan for drilling holes for the axles. As a team, we moved up drill-bit size by drill-bit size until we reached our goal. That experience helped me realize that doing things carefully would keep us from having to do things more than once.

In the future, I would follow clear instructions and take my time. I truly believe that my partner, Annie, and I would have finished much sooner had we been methodical from the start. I think that taking the time to make sure that everything was perfectly measured and even may have increased our speed. Additionally, I would have used CDs for the front wheels as well and made the frame a little bit smaller. I hope that the next time we have a building project, I will make sure that I have laid out all of my steps and measurements before I actually begin to build.

Chapter Five is Going Live!

This past unit we covered work, power, kinetic and potential energy, machines and the conservation of energy.

Work


Work can be easily defined as the force exerted on an object over a certain distance. Work is calculated by multiplying force by distance and is measure in Joules (J).

In order to do work on an object, the force and distance must be parallel to each other. For that reason, a server carrying a tray through a dining room is not doing work, but that same server climbing a flight of stairs is doing work.

Work cannot be done if no distance is covered. Anything multiplied by zero is zero, so if you were to push against a wall with no result, no work would be done because the distance covered would be 0m.

Additionally, in questions where a person is climbing the stairs, riding an escalator or an elevator, only the vertical height is relevant.


Here are two examples of work practice problems:

1. An actress carries her Oscar across the stage after winning the Academy Award for Best Actress in a Leading Role. The statue weighs 4N and the distance from the stairs to the podium is 4m, how much work does she do?
2. A woman walks up the stairs to the stage to accept the Academy Award for Best Director. She weighs 550N and the stairs are 2m high. How much work does she do?
3. A server lifts a tray and then carries it to a table. Does he do work when he lifts the tray, when he walks to the table, or both times? Explain.

Answers:

1. The actress does not do any work because the force of the statue and the distance she travels are not parallel, therefore no work can be done.
2. work = force*distance; work = 550N*2m; work = 1100J The Academy Award winner for Best Director does 1100 Joules of work.
3. The server only does work when he lifts the tray. In this situation, the weight of the tray and the distance it is being lifted are parallel. Therefore, work is being done. Contrarily, when the server carries his tray to the table, the weight of the tray is perpendicular to the distance the server is walking. Therefore, work cannot be done.

Power

Power is defined as how quickly work is done.

It is calculated using: power= work/time. It can be measured using two different units. If you literally translate the units used to measure work/time the unit is J/s (Joules per second). However, power is most commonly measured using Watts (W). Joules per second and Watts are equivalent.

It is likely that you have heard them term Watt before, likely when looking at a light bulb. The amount of power light bulbs generate is measured in Watts. Does 60W sound familiar?

Horsepower is another term that should sound familiar. One horsepower = 746 W.

Now that we know how to calculate both work and power, let's revisit one of our previous problems and add a time component to it.

 

A woman walks up the stairs to the stage to accept the Academy Award for Best Director. She weighs 550N and the stairs are 2m high. It takes her 10 seconds. How much work does she do? How much power does she generate? Is it enough to power a standard 60W light bulb?

Well, from our calculations above, we already know that she did 1100J of work. We also know that power is equal to work over time. Therefore we can set up our calculations to look like this:

Work = 1100J

Power = work/time
Power = 1100J/10s
Power = 110W

Yes, she did generate enough power the light a 60W light bulb.


 Kinetic and Potential Energy

Kinetic energy is the energy of motion. In other terms, it is mass and speed's ability to do work.

It is measured using the formula KE = (1/2)mv^2. It is also measured in Joules.

An object must be in motion to have kinetic energy.

Change in kinetic energy is equal to work. Therefore in the problem we did earlier with the Academy Award winner for Best Director, she did 1100J of work and has 1100J of kinetic energy.

We can use our knowledge of work and energy to tell us why airbags keep us safe.

• You go from moving to not moving regardless of how you stop. Therefore the change in kinetic energy is the same with or without the airbag. Change in kinetic energy is equal to work and work can also be written as distance times force. Therefore, if the distance between you and the force that causes you to stop increases, that force must decrease in order for work to remain constant. The smaller the force of impact is, the less injury will be caused.
• work = F*d or work = F*d
• This is why work remains constant.
• Change in kinetic energy = KE final - KE initial
• work = change in kinetic energy

Potential Energy

Potential energy (PE) is the energy of position, and determines the maximum possible kinetic energy an object can have. Consider a pendulum that is about to be released from the highest point of its path. Let's say that it has 200J of potential energy and 0J of kinetic energy here. As soon as it is dropped, the pendulum's potential energy will begin to convert to kinetic energy. When it is halfway between its highest possible and lowest possible points, the pendulum's PE will be equal to 100J and its KE will be equal to 100J. At the lowest possible point on its path, the pendulum will have a PE of 0J and a KE of 200J. Then again, at the highest possible point on the other side, it will have 0J KE and 200J PE.

As shown by this example, an object can be moving and still have potential energy. However, this is only true for PE. An object at rest will never have KE.

PE and KE are why a rollercoaster can complete the track after having been released only once. This is also why no hill on a roller coaster is taller than the first one. The physics reasoning behind this is that the PE that the cars have at the top of the first, tallest hill is the maximum amount of potential energy that the cars can have. Therefore, if no hills are any taller than that first hill, there will always be enough energy to get up and over each one.


Machines

The purpose of a simple machine is to decrease the amount of force you have to use while doing work by increasing the distance you cover. Thus, work in = work out. You can never get more work out of a machine than the amount of work you put in.

For example, if you are moving to a new home and are loading a moving truck-- you can use the amount of work you would do without a ramp to find out how much work you would do with a ramp as long as you know the length of the ramp or the amount of force it would take you with the ramp.

Machines' effectiveness can also be measured. The Law of Conservation of Energy states that energy will always be conserved and that the energy you get out a machine will never be more or less than what you put in. However, a machine can never actually be 100% effective because some of the energy produced must be turned into heat, light or sound. However, the effectiveness of a machine can easily be calculated by setting up a proportion with the amount of Joules of work produced (which will always be the smaller number) over the amount of work you put in.

Here are two helpful videos that talk about machines and include some practice problems.