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Relativity in Motion

Teacher/Instructor Matt Jones
Matt Jones

M.Ed., George Washington University
Dept. chair at a high school

Matt is currently the department chair at a high school in San Francisco. In his spare time, Matt enjoys spending time outdoors with his wife and two kids.

The motion of any object is only meaningful when given relative to something else. Relativity in motion assures us that the laws of Physics don't vary depending on how much inertia an object has. This is important because everything has some amount of inertia. Standing on the surface of the earth, we feel that we are not in motion, but because the earth is moving, so are we. If we measure the speed of a ball thrown atop a train, we can either measure the speed of the ball with respect to the train s motion or with respect to the motion of the Earth.

Today we're going to talk about relativity in motion. First for you what is my present speed right now where I stand? Go ahead and think about that, I'll give you some choices is it zero meters per second? Is it 30,000 meters per second? Or is it over 50,000 meters per second? Well if you said zero meters per second, you're correct that is my present speed given the relativity to the floor, where I stand relative to the floor I'm moving at zero meters per second. If you said 30,000 meters per second that is also correct, that is my speed relative to the sun as we stand on earth, we are moving at 30,000 meters per second around the sun. If you said over 50,000 meters per second that is also correct that is our estimated speed and people don't even really know the actual value but that is the distance that we're moving away from the center of the universe. So my speed is relative I could, dependent on what we're talking about and what we're relative to.

Okay let's look at some examples, I've got a race car, that race car is traveling at 300 kilometers per hour okay that speed is relative to the track. If that race car is passing another race car, going 295 kilometers per hour the speed relative to the slower race car is 5 kilometers per hour so again relativity. Let's look at another example, let's say I'm on a train, and that train is traveling at 10 meters per second, I am on top of that train doing a very unsafe thing, I'm throwing a football in the direction that the train is moving. Don't try this at home okay I'm throwing that at 8 meters per second. Okay, well the ball relative to the train is moving at 8 meters per second but the ball relative to the ground or people standing on the ground watching is 10+8, 18 meters per second okay. So that makes sense and this works really well at relatively low speeds and trains and cars and me we're all relatively slow.

Okay it doesn't work so well at really high speeds, so now let's look at another example. I've got a train moving at 10 meters per second and I have a head light on that train and that head light is traveling at the speed of light. Okay the speed of light is 300 million meters per second okay so I say well that's easy I've got 10 meters per second plus 300 million meters per second which is going to be 300 million and 10 meters per second okay. Unfortunately it doesn't work that way, Einstein discovered in what we call "Einstein's special relativity in motion" that nothing can go faster than the speed of light. So that light coming out of the front of the train no matter how fast that train is going, if it's going half the speed of light or if it's going 200 kilometers an hour, that light is traveling at the same speed. So these are the ways we work with relativity in motion.