Tides Explanation

The Universal Gravitational Law states that F=(G)(m1)(m2)/d^2, because of this we know that force is proportional to 1/distance squared. In other words, a short distance has a strong force and a long distance has a weak force.






Let's think about this in relationship to the Earth. The side of the Earth closer to the Moon experiences the Moon's gravitational force more than the side that is farther away from the Moon. The net force acting on each side of the Earth is what causes the tides. Say that the on the side closer to the Moon, the Moon's gravitational force exerts 150N and on the other side, only 50N. (note: these numbers are grossly exaggerated just to detail the concept) Additionally, the gravitational force from the center of the Earth is 100N. To find the net force on each side, the Earth's gravitational force is subtracted from the moon's force on each side. Therefore, the net force on the close side is 50N and the net force on the far side is -50N. The positive force indicates that the close side is being pulled toward the Moon and the negative force indicates that the far side is being pulled away from the moon. It is this imbalance that causes the tides. If the forces weren't opposite, tides wouldn't exist. Also, if tides were determined by gravitational pull instead of the relationship between force and distance, tides would be caused by the Sun, not the Moon.




This video explains tides really well:










In each day, there are two high tides and two low tides due to planetary revolution. Six hours pass between each high tide and each low tide, also meaning that from high tide to high tide is 12 hours and between low tide to low tide is 12 hours.

Unit Three Resource

So far, we've covered in class that Newton's third law states that for every action there is an equal and opposite reaction. Meaning, that as I type and rest my hands on my laptop, my laptop is pushing up on my hands with the same amount of force.

That being said, does the Earth pull on the Moon with the same force that the Moon pulls on the Earth?

This video by Veritasium gives a little explanation and debunks a common misconception.



By now, hopefully it is clearer why Newton's third law is possible.

Now: why is it that no one wins the tug of war in this video?
(note: embedding was disabled for this video, but I really thought it was helpful)

No Win Tug of War

Newton's third law also rings true in this case. The two scientists pulled on the rope with the same force, however since there was no interaction between their feet and the ground, the scientists pulled each other together instead of one dominating the other.

See an example of successful tug of war:



The team on the left won not because they were stronger, but because the action-reaction force pair between their feet and the ground was greater than that on the right.