Just because you see something in a movie doesn’t mean you should try it yourself. Consider, for example, a human running on top of a moving train. For starters, you can’t be sure it’s real. In early westerns, they used moving sets to make it look like fake trains were moving. Now there’s CGI. Or they could speed up the movie to make a real train look faster than it actually is.
So here’s a question for you: is it possible run on the roof of a train and jump from one carriage to another? Or will the train zoom past you while you’re in the air, so that you land behind where you took off? Or worse, would you end up falling between cars because the gap widens, thus extending the distance you have to cover? This, my friend, is why stuntmen study physics.
Framing the action
By the way, what is physics? Basically, it’s a set of real-world models that we can use to calculate forces and predict how the position and speed of objects will change. However, we cannot find the position or speed of anything without a frame of reference.
Suppose I’m in a room, holding a balloon, and want to describe its location. I can use Cartesian coordinates for 3D space to give the ball a value (x, y, z). But these numbers depend on the origin and orientation of my axes. It seems natural to use a corner of the room as the origin, with the x and y axes running along the base of two adjacent walls and the z axis extending vertically upwards. Using this system (with units in meters), I see that the ball is at point (1, 1, 1).
What if my friend Bob is there and he measures the location of the ball in a different way? Maybe it puts the origin of the ball in my hand, giving it an initial position of (0, 0, 0). That seems logical too. We could argue about who is right, but that would be stupid. We simply have different frames of reference, and they are both arbitrary. (Don’t worry, we’ll get back to trains.)
Now I throw this ball straight into the air. After a short time interval of 0.1 seconds, my coordinate system places the ball at location (1, 1, 2), which means it is 1 meter higher. Bob also has a new location (0, 0, 1). But note that in both systems the ball rises 1 meter in the z direction. So we would agree that the ball has an upward velocity of 10 meters per second.
A mobile frame of reference
Now suppose I take this ball on a train traveling at 10 meters per second (22.4 miles per hour). I throw the ball upwards again: what will happen? I’m inside the train car, so I’m using a coordinate system that moves with the train. In this moving frame of reference, I am stationary. Bob stands on the edge of the tracks (he can see the ball through the windows), so he uses a stationary coordinate system in which I move.
Courtesy of Rhett Allain
