Check out this awesome video. Two skydivers are traveling downward and passing a small ball back and forth. Pretty cool, right? But what's going on here and how can you get something like this to work? Does it have to be a special ball? Let's get to the physics of this cool trick.

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You may have heard something about all objects falling with the same constant acceleration. There was even that story of Galileo dropping two spheres of different masses off the leaning tower of Pisa. Not sure if it's true, but his point was to show that both the gravitational force and the acceleration of an object depend on mass. When you put these two ideas together, the mass cancels. Any object dropped on the surface of Earth will fall and speed up with an acceleration of about 9.8 meters per second squared—except when it doesn't. Yes, this idea is only approximately true.

If you dropped a tennis ball and a basketball from a standing position and you let them go at the exact same time, they would hit the ground at the same time. If you take a rock and a feather, however, they don't hit the ground at the same time. That is because of a force other than the gravitational force: air resistance.

You probably already have some experience with the air resistance force. If you stick your hand out the window of a moving car, you can feel the air pushing against you. As a basic model, this air resistance force has the following properties (this is just a model):

- The air resistance force increases with the speed of the object (
*v*). - The force is in the opposite direction of the velocity of the object.
- It depends on the cross-sectional area (
*A*) and shape of the object---a parameter we call the drag coefficient (*C*). - It depends on the density of air (ρ).