# High School Physics : Energy

## Example Questions

### Example Question #4 : Calculating Kinetic Energy

A book falls off of a  high table. If the book weighs , what will its kinetic energy be right before it hits the ground?

Explanation:

Initially, the book has only potential energy. Right before it hits the ground, all the potential energy will have converted to kinetic energy. The two values will be equal based on the conservation of energy.

If we solve for potential, we can find kinetic energy.

### Example Question #1 : Calculating Kinetic Energy

A rock is dropped from a helicopter hovering at  above the ground. If the rock weighs , what is its kinetic energy right before it hits the ground?

Explanation:

Right before it hits the ground, the initial potential energy and the final kinetic energy will equal each other due to conservation of energy.

If we solve for initial potential, we can find final kinetic energy.

Plug in the values given. Remember that height is the change in height. Since the rock is headed downward, the height will be negative.

Multiply and solve.

### Example Question #6 : Calculating Kinetic Energy

A pendulum with string length  is dropped from rest. If the mass at the end of the pendulum is , what is its maximum velocity?

Explanation:

The maximum velocity of the pendulum will be when the object has only kinetic energy. Using conservation of energy, we can set our initial potential energy to equal our "final" kinetic energy.

Plug in the given values and solve for the velocity.

### Example Question #7 : Calculating Kinetic Energy

book falls off the top of a  bookshelf. What is its final velocity right before it hits the ground?

Explanation:

The relationship between velocity and energy is:

We know the mass, but we need to find the total kinetic energy.

Remember the law of conservation of energy: the total energy at the beginning equals the total energy at the end. In this case, we have only potential energy at the beginning and only kinetic energy at the end. (The initial velocity is zero, and the final height is zero).

If we can find the potential energy, we can find the kinetic energy. The formula for potential energy is .

Using our given values for the mass, height, and gravity, we can solve using multiplication. Note that the height becomes negative because the book is traveling in the downward direction.

The kinetic energy will also equal , due to conservation of energy.

Using this value and our given mass, we can calculate the velocity from our original kinetic energy equation.

Since we are taking the square root, our answer can be either negative or positive. The final velocity of the book will be in the downward direction; thus, our final velocity should be negative.

### Example Question #2 : Calculating Kinetic Energy

book falls off the top of a  bookshelf. What is its velocity right before it hits the ground?

Explanation:

The relationship between velocity and energy is:

We know the mass, but we need to find the total kinetic energy.

Remember the law of conservation of energy: the total energy at the beginning equals the total energy at the end. In this case, we have only potential energy at the beginning and only kinetic energy at the end. (The initial velocity is zero, and the final height is zero).

If we can find the potential energy, we can find the kinetic energy. The formula for potential energy is .

Using our given values for the mass, height, and gravity, we can solve using multiplication. Note that the height becomes negative because the book is traveling in the downward direction.

The kinetic energy will also equal , due to conservation of energy.

Using this value and our given mass, we can calculate the velocity from our original kinetic energy equation.

Since we are taking the square root, our answer can be either negative or positive. The final velocity of the book will be in the downward direction; thus, our final velocity should be negative.

### Example Question #1 : Calculating Kinetic Energy

An orange falls off of a tree  tall. What is the final velocity of the orange before it hits the ground?

Explanation:

For this problem, we need to use the law of conservation of energy. Since we are only looking at gravitational energy here and the orange starts at rest, we can say that the initial potential energy is equal to the final kinetic energy.

.

From here, we can expand the equation, using the formulas for gravitational potential energy and kinetic energy.

Notice that the mass cancels out form both sides.

For the height, make sure to keep that value negative as we are measuring the DISPLACEMENT rather than the distance travelled. Since displacement is a vector (magnitude and direction) we need to be clear that it travels down .

At this point, we need to remember that the square root of a positive number can be either positive or negative. Our velocity is a vector, so we will need to make sure we pick the answer choice with the appropriate direction. Since the orange is traveling downward, we know our final velocity must be negative.

### Example Question #1 : Calculating Kinetic Energy

A baseball has a mass of . If a high school pitcher can throw a baseball at , what is the approximate kinetic energy associated with this pitch?

Explanation:

To solve for the kinetic energy, we will need to use the equation:

Before we can plug in our given values, we must convert the mass from grams to kilograms. Remember, the SI unit for mass is kilograms, so most calculations will require this conversion.

Now we can use this mass and the given velocity to solve for the kinetic energy.

### Example Question #31 : Energy

book falls off the top of a  bookshelf. What is its velocity right before it hits the ground?

Explanation:

The relationship between velocity and energy is:

We know the mass, but we need to find the total kinetic energy.

Remember the law of conservation of energy: the total energy at the beginning equals the total energy at the end. In this case, we have only potential energy at the beginning and only kinetic energy at the end. (The initial velocity is zero, and the final height is zero).

If we can find the potential energy, we can find the kinetic energy. The formula for potential energy is .

Using our given values for the mass, height, and gravity, we can solve using multiplication. Note that the height becomes negative because the book is traveling in the downward direction.

The kinetic energy will also equal , due to conservation of energy.

Using this value and our given mass, we can calculate the velocity from our original kinetic energy equation.

Since we are taking the square root, our answer can be either negative or positive. The final velocity of the book will be in the downward direction; thus, our final velocity should be negative.

### Example Question #12 : Calculating Kinetic Energy

Starting from rest a ball with a mass of 3kg experiences a constant force of 9N for 4s. Find the final kinetic energy (in joules) of the ball after 4s.

Explanation:

Use Newton's laws to find the acceleration of the ball. Find the final speed of the ball, and then solve for kinetic energy.

Acceleration:

Final speed:

Kinetic energy:

### Example Question #11 : Calculating Kinetic 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 final vertical kinetic energy right before the rock hits the ground?