The answer is because no work is done. For work to be done a force must be exerted across a distance parallel to the direction of the force. In this case, a force is exerted by the man but the wall is stationary and since it does not move there is no distance for work to take place on. The formula for work can be written as:

Here, is zero, so is also zero, making the entire term, and thus the work zero.

Work is defined as:

Where is work, is force experienced by the object/person, and is the distance the object/person moves as a result of the force. Recall that force is calculated as follows.

Where is mass and is acceleration; therefore, work depends on the mass and acceleration (force), and distance.

There is no net work done on the ball. The wall stopped the ball, doing negative work, then accelerated the ball, doing positive work. These end up canceling each other out.

From the Work-energy theorem,

.

From conservation of energy, we see that all of the potential energy contained in the spring will be transferred into kinetic energy of the ball, shown as

Noting the initial kinetic energy of the ball is , we can see that the work done by the spring will be equal to the initial potential energy of the spring.

Work is found by finding the change in kinetic energy. Since the car started from rest it had no initial kinetic energy.

Divide by 1000 to convert to kilojoules and get 5400 kilojoules.

Initially the car is at rest at the bottom of a hill, this velocity and height are zero.

Converting to

Plugging in values:

There will be no work done.

Energy is a scalar quantity. Since it has the same amount of energy in the end as the beginning, no work was done.

Alternatively, it can be modeled as the floor did negative work during the first half of the bounce (when the ball was compressing), and did positive work of the same magnitude and opposite sign in the second half of the bounce. This is due to the direction of the ball's motion flipping and the Force vector of the floor staying the same.

In general,

In order to calculate the work done by a force, we need to find the angle between the force and the displacement through which the object moves. Since the displacement is directed down the incline, then this means the friction force acts up the incline. This means . We must remember the frictional force is represented as

Where is the normal force and is the coefficient of kinetic friction. On an inclined plane, we can show that . Therefore, we can finally write

Plugging in everything, and noting the magnitude is the absolute value of a quantity, we can plug in for our answer:

We can use the expression for conservation of energy:

Since the rod is both initially and finally at rest, we can removed both kinetic energies. Also, if we assume point p is at a height of 0, we can removed initial potential energy, leaving us with:

Plugging in the expression for potential energy and expanding for both masses:

Since the rod is vertical, we know that mass A is half a rod's length above our reference height, and mass B is half a rod's length below it. Thus we get:

Factoring to clean up our expression:

We know all of our variables, so time to plug and chug:

From the Work-energy theorem,

.

From conservation of energy, we see that all of the potential energy contained in the spring will be transferred into kinetic energy of the ball, shown as

Noting the initial kinetic energy of the ball is , we can see that the work done by the spring will be equal to the initial potential energy of the spring.