# High School Physics : Energy and Work

## Example Questions

### Example Question #11 : Calculating Potential Energy

book falls off the top of a  bookshelf. What is its potential energy right before it falls?

Explanation:

The formula for potential energy is .

Given the values for the mass, height, and gravity, we can solve using multiplication. Note that the height is negative because the book falls in the downward direction.

### Example Question #12 : Calculating Potential Energy

book falls off the top of a  bookshelf. What is its potential energy right before it falls?

Explanation:

The formula for potential energy is .

Given the values for the mass, height, and gravity, we can solve using multiplication. Note that the height is negative because the book falls in the downward direction.

### Example Question #41 : Energy

What is the potential energy of a  ball held  above the ground?

We need to know the final velocity to solve

Explanation:

The formula for gravitational potential energy is:

We are given the mass of the ball, height, and acceleration from gravity. Remember, since the displacement is downward it must be negative.

### Example Question #14 : Calculating Potential Energy

A box is placed on top of a  tall spring resting on the ground. The box has a weight of , and compresses the spring . What is the spring constant?

Explanation:

We need two formulas for this problem: the gravitational potential energy and the spring potential energy.

We know the weight of the box and the change in height when it is placed on the spring. These values allow us to calculate the change in potential energy. Due to conservation of energy, the total energy must remain constant. Initially, the box only has gravitational potential energy. In its final position, it has both gravitational and spring potential energy.

We know the initial height and final height of the spring. This also gives us the value of , the displacement of the spring.

We also know the weight of the box:

Now we can solve for the spring constant, .

### Example Question #15 : Calculating Potential Energy

Calculate the potential energy of a  branch attached to a tree at a point above the ground.

Explanation:

Potential energy due to gravity is given by the equation:

We are given the mass of the branch and its height. Gravity is constant. Using these values, we can solve for the potential energy.

First, convert the mass of the branch to kilograms.

Then, use the equation to find the energy.

### Example Question #16 : Calculating Potential Energy

A man stands at the top of a  tall building. He holds a  rock over the edge. What is the potential gravitational energy of the rock?

Explanation:

Potential gravitational energy is given by the equation:

We are told the height of the rock and its mass. Using the constant acceleration due to gravity, we can solve for the gravitational potential energy.

### Example Question #17 : Calculating Potential Energy

Sam throws a  rock off the edge of a  tall building at an angle of  from the horizontal. The rock has an initial speed of .

What is the gravitational potential energy of the rock as soon as it leaves Sam's hand?

Explanation:

The formula for gravitational potential energy is . The only relevant variables are mass, gravity, and height. All of the information about velocity and angle are not needed to solve for the initial potential energy.

Plug in the given values for mass and height, and solve using the equation. Keep in mind that the height will be negative because the rock travels downward.

### Example Question #18 : Calculating Potential Energy

Laurence throws a  rock off the edge of a  tall building at an angle of  from the horizontal with an initial speed of .

.

What is the potential energy of the rock at the moment it is released?

Explanation:

The formula for gravitational potential energy is:

We can solve for this value using the given mass of the rock, acceleration of gravity, and initial height.

This value is independent of the kinetic energy of the rock, and is not dependent on initial velocity.

### Example Question #391 : High School Physics

A bodybuilder lifts a  weight upward , and then returns it to its original position, all with a constant speed. What is the net work done on the weight?

Explanation:

Work is a vector quantity equal to the change in energy in a given direction. Our net work will be equal to product of force and displacement.

We can write this equation in terms of Newton's second law to incorporate our given values.

During its motion the weight travels a distance of upward, and then downward. Its total distance will be , however, we are looking for its displacement. The initial height and final height are equal, making the displacement equal to zero.

The total work will be zero because there was no net displacement.

### Example Question #1 : Work

crate slides along the floor for . What other piece of information do we need to find the work done?

Time

Potential energy

Velocity

Power

Acceleration