Lectures on Physics has been derived from Benjamin Crowell's Light and Matter series of free introductory textbooks on physics. See the editorial for more information....

Discussion - Angular Momentum

Discussion Questions

A This series of discussion questions deals with past students' incorrect reasoning about the following problem. Suppose a comet is at the point in its orbit shown in the figure. The only force on the comet is the sun's gravitational force.

Throughout the question, define all torques and angular momenta using the sun as the axis.

(1) Is the sun producing a nonzero torque on the comet? Explain.

(2) Is the comet's angular momentum increasing, decreasing, or staying the same? Explain.

Explain what is wrong with the following answers. In some cases, the answer is correct, but the reasoning leading up to it is wrong.

(a) Incorrect answer to part (1): Yes, because the sun is exerting a force on the comet, and the comet is a certain distance from the sun.

(b) Incorrect answer to part (1): No, because the torques cancel out.

(c) Incorrect answer to part (2): Increasing, because the comet is speeding up.

B Which claw hammer would make it easier to get the nail out of the wood if the same force was applied in the same direction?

C You whirl a rock over your head on the end of a string, and gradually pull in the string, eventually cutting the radius in half. What happens to the rock's angular momentum? What changes occur in its speed, the time required for one revolution, and its acceleration? Why might the string break?
D A helicopter has, in addition to the huge fan blades on top, a smaller propeller mounted on the tail that rotates in a vertical plane. Why?
E The photo shows an amusement park ride whose two cars rotate in opposite directions. Why is this a good design?

Last Update: 2009-06-21