Physics Quiz: Do You Really Understand Angular Momentum?

Image: Rhett Allain. Ball moving in a circle using Web VPython

Here is a question to test your understanding of angular momentum. Don’t worry, I’m going to go over the answer shortly. Actually, there are two questions. This is the first part:

Suppose you have a ball moving in a vertical circle with a constant speed. As the ball moves from the top of the circle to the bottom, what happens to the angular momentum with respect to the center of the circle?

Some optional answers: a) the angular momentum increases, b) the angular momentum decreases, c) the angular momentum changes directions, d) the angular momentum is constant.

Torque and Angular Momentum

Perhaps you already know the answer, but I’m going to model this motion in VPython and calculate the angular momentum. That way we can just see what’s going on. Remember that the angular momentum vector (L) is:

So, how do you make an object move in a circle? Of course there are many ways to do this, but I want to use physics. If you have an object connected to a spring, it’s possible to push it in a way that the force from the spring is always perpendicular to the motion of the object. The problem is that as the spring stretches, the force changes.