Problems 1-5 Solutions Outline

Problem 1

In what ways is the word “work” as used in everyday language the same as that defined in physics?

In what ways is it different?

Give examples of both.

 

Same as in physics

Some activities involving physical labor, particularly if it involves lifting objects, such as shoveling dirt or carrying shingles up to a roof, are “work” in the physics sense of the word. 

Pushing a lawn mower would be work corresponding to the physics definition. 

Different

When we use the word “work” for employment, such as “go to work” or “school work”, there is often no sense of physical labor or

of moving something through a distance by a force.

 

Problem 2

Can a centripetal force ever do work on an object? Explain.

 

Centripetal means “pointing to the center of curvature”. A centripetal force will not do work on an object, because if an object is moving in a curved path, by definition the direction towards the center of curvature is always perpendicular to the direction of motion. 

For a force to do work, the force must have a component in the direction of displacement.  So the centripetal force does no work.

 

Problem 3

Can the normal force on an object ever do work? Explain.

 

The normal force can do work on an object if the normal force has a component in the direction of displacement of an object. 

If someone were to jump up in the air, then the floor pushing upward on the person (the normal force) would do positive work and increase the person’s kinetic energy.  Likewise when they hit the floor coming back down, the force of the floor pushing upwards (the normal force) would do negative work and decrease the person’s kinetic energy.

 

Problem 4

A woman swimming upstream is not moving with respect to the shore. Is she doing any work?

If she stops swimming and merely floats, is work done on her?

 

Note: You must be careful to clearly who or what is doing the work. Is the work being done by or on the object?

The woman does work by moving the water with her hands and feet, because she must exert a force to move the water some distance.

This distance is parallel to the force.

As she stops swimming and begins to float in the current, the current does work on her because she gains kinetic energy.  Once she is floating the same speed as the water, her kinetic energy does not change, and so no net work is being done on her.

 

 

Problem 5

Is the work done by kinetic friction forces always negative? (Hint: Consider what happens to the dishes when you pull a tablecloth

out from under them.)

 

The kinetic force of friction opposes the relative motion between two objects.  In the example, the tablecloth is pulled from under the dishes, the relative motion is for the dishes to be left behind as the tablecloth is pulled, and so the kinetic friction opposes that and moves the dishes in the same direction as the tablecloth.  This is a force that is in the direction of displacement, and so positive work is done.  Also note that the cloth is moving faster than the dishes in this case, so that the friction is kinetic, not static.