# Classical Mechanics Problems on Energy and Work

1, Three identical balls are thrown from the top of a building, all with the same initial speed. Ball 1 is thrown horizontally, ball 2 is thrown at some angle above the horizontal, and ball 3 is thrown at some below the horizontal. Rank the speed of the balls as they reach the ground from fastest to slowest.

A,1-2-3

B,2-1-3

C,3-2-1

D, All three strike the ground with the same speed.

2, Mark the following statements true (T) or False(F)

A, _______Friction is a conservative force and does negative work.

B, ______The work done by a conservative force between two points depends on the path taken between those points.

C,_______A conservative force cannot change a body's total mechanical energy.

D,________ The work done by a conservative force while a body moves at constant velocity must be zero.

3, A woman runs up a flight of stairs. The gain in her gravitational potential energy is U. If she runs up the same stairs with twice the speed, what is her gain in potential energy.

A, U

B, 2U

C, ½ U

D, 4U

E, ¼ U

4. The reference point for gravitational potential energy

A, must be at the initial position of the object

B, must be at the final position of the object

C, must be at ground level

D, must be at the lowest position ever reached by the object

E, can be chosen arbitrarily

5. A particle that is subjected to a variable force travel a distance of 20m in a straight line in the same direction as the force. The force varies as follows:

First, 5.0m and F=0

Next, 5.0m and F=3.0

Next, 5.0m and F=6.0

Last, 5.0 and F decreases uniformly from 6.0 N to zero.

What was the total work done by this force during the full 20m trip?

A, 15 J

B,30 J

C, 45 J

D.60 J or E. 75 J

#### Solution Preview

1, Three identical balls are thrown from the top of a building, all with the same initial speed. Ball 1 is thrown horizontally, ball 2 is thrown at some angle above the horizontal, and ball 3 is thrown at some below the horizontal. Rank the speed of the balls as they reach the ground from fastest to slowest.

A,1-2-3

B,2-1-3

C,3-2-1

D, All three strike the ground with the same speed.

The correct answer is D. This is because the horizontal motion is completely independent of the vertical motions.

2, ...

#### Solution Summary

Five different classical mechanics questions pertaining to energy, work, and velocity are discussed. The solutions are explained to understand the concepts of conservative force, and potential energy.