Just because you see something done in a movie doesn’t mean you should try it yourself. Take, 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 backgrounds to make fake trains look like they were moving. Now there is CGI. Or they could speed up the movie to make a real train look faster than it really is.
So here’s a question: Is it possible Running on the roof of a train and jumping from one car to another? Or will the train pass you while you’re in the air, so you’ll land behind where you took off? Or worse yet, would you end up falling between the cars because the gap increases, lengthening the distance you have to travel? This, my friend, is why stunt actors study physics.
Frame the action
What is physics anyway? It’s basically a set of real-world models, which we can use to calculate forces and predict how the position and speed of things will change. However, we cannot find the position or velocity of anything without a frame of reference.
Suppose I am standing in a room, holding a ball, and I want to describe its location. I can use Cartesian coordinates for a 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 along the base of two adjacent walls and the z axis vertically upward. Using this system (with units in meters), I find that the ball is at the point (1, 1, 1).
What if my friend Bob is there and measures the location of the ball in a different way? Maybe he puts the origin where the ball starts, in my hand, giving it an initial position of (0, 0, 0). That also seems logical. We could argue about who is right, but that would be foolish. We simply have different frames of reference and they are both arbitrary. (Don’t worry, we’ll get back on the trains.)
Now I throw that ball up in the air. After a short time interval of 0.1 seconds, my coordinate system has the ball at location (1, 1, 2), which means it is 1 meter higher. Bob also has a new location (0, 0, 1). But notice that in both systems the ball rose 1 meter in the z direction. Then we would agree that the ball has an upward velocity of 10 meters per second.
A moving frame of reference
Now suppose I carry that ball on a train traveling at 10 meters per second (22.4 miles per hour). I throw the ball up again. What will happen? I’m inside the car, so I use a coordinate system that moves along with the train. In this moving frame of reference, I am stationary. Bob is standing next to 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