Question

4. Why does the normal force usually not do any work on a moving object?

5. How could you use a graph of force versus position to determine the work done on an object by a non-constant force?

6. Why can't the kinetic energy of an object ever be less than zero? What about the potential energy?

Answer 1

4) Normal force is applied when the object is in contact with a surface. If there is a motion in that direction, it will lose contact and normal force will become zero. So either the displacement in direction of normal is zero, or the the normal itself becomes zero

So work done by normal is zero.

5)We can find the area under force - position curve to find the work done.

6) Kinetic energy = 0.5mv^2. Since mass is never negative, KE can never be negative.

Potential energy can be negative.