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!