6.1

Work and Kinetic Energy

Student Notes

In this simulation a constant force is applied to a block, which moves on a frictionless surface. The work-kinetic energy theorem can be written KEf=KEi+W, where KEf and KEi are the final and initial kinetic energies, and W is the work done on an object. Since the force applied to the block is constant we can easily calculate the work W using W = (F cos q)* s, where F is the applied force, s is the displacement, and q is the angle between the force and the displacement. Remember that if the block starts and ends at the same position, the displacement is zero.
Run the simulation using the default values.

1. | Since the force is constant, what can you say about the acceleration of the block?
2. | How much work did the force do on the block? There are several different ways to answer this question; answer it in as many ways as you can. For example, you can use the magnitude of the force, the kinetic energy readout, and the velocity readout for three different ways of calculating the work. Are all your answers the same?
3. | Set the initial position of the block to zero, and set the initial velocity to -3 m/s. Run the simulation. When is the work negative? When is it positive? Why?
4. | Given the limits of variables used in the simulation, what is the maximum amount of work than can be done on the block? Does the mass of the block affect your answer? Try it and see.
5. | How can you maximize the final velocity of the block? Can you do the same amount of work and have the block moving more slowly?

Self-test

Describe a situation where the block moves as the force is applied to it, yet the force does no work over a non-zero time interval. How can this be? Set the simulation parameters and check your prediction.

Copyright © 2006 John Wiley & Sons, Inc. All rights reserved.