Kinetic and Potential Energy 

Lesson Objectives

• I can tell the difference between kinetic and potential energy.
• I can analyze situations where kinetic energy is converted to potential energy.
• I can analyze situations where potential energy is converted to kinetic energy.

Introduction

What could these four photos possibly have in common? Can you guess what it is?

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All of them show things that have kinetic energy.

Guided Learning

Defining Kinetic Energy

Kinetic energy is the energy of moving matter. Anything that is moving has kinetic energy—from atoms in matter to stars in outer space. Things with kinetic energy can do work. For example, the spinning saw blade in the photo above is doing the work of cutting through a piece of metal.

Thinking about Kinetic Energy

The amount of kinetic energy in a moving object depends directly on its mass and velocity. An object with greater mass or greater velocity has more kinetic energy. ^[2]

Let’s consider an example. The figure shows Juan running on the beach with his dad. Juan has a mass of 40 kg and is running at a velocity of 1 m/s.

What about Juan’s dad? His mass 80 kg, and he’s running at the same velocity as Juan (1 m/s). Because his mass is twice as great as Juan’s, his kinetic energy is twice as great.

Review how kinetic energy is related to mass and velocity by watching this cartoon (not iPad friendly).

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This diver has just jumped up from the end of the diving board. After she dives down and is falling toward the water, she’ll have kinetic energy, or the energy of moving matter. But even as she is momentarily stopped high above the water, she has energy. Do you know why?

Stored Energy

The diver has energy because of her position high above the pool. The type of energy she has is called potential energy. Potential energy is energy that is stored in a person or object. Often, the person or object has potential energy because of its position or shape.

Q: What is it about the diver’s position that gives her potential energy?

A: Because the diver is high above the water, she has the potential to fall toward Earth because of gravity. This gives her potential energy.

Gravitational Potential Energy

Potential energy due to the position of an object above Earth’s surface is called gravitational potential energy. Like the diver on the diving board, anything that is raised up above Earth’s surface has the potential to fall because of gravity. You can see another example of people with gravitational potential energy in the figure below, and also in this video. 

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Elastic Potential Energy

Potential energy due to an object’s shape is called elastic potential energy. This energy results when an elastic object is stretched or compressed. The farther the object is stretched or compressed, the greater its potential energy is. A point will be reached when the object can’t be stretched or compressed any more. Then it will forcefully return to its original shape.

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Look at the pogo stick in the figure. Its spring has elastic potential energy when it is pressed down by the boy's weight. When it can’t be compressed any more, it will spring back to its original shape. The energy it releases will push the pogo stick—and the boy—off the ground. You can see how a pogo stick spring compresses and then returns to its original shape. Check out the animation for yourself.^[6]

Q: The boy in the figure below is giving the elastic band of his slingshot potential energy by stretching it. He’s holding a small stone against the stretched band. What will happen when he releases the band?

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A: The elastic band will spring back to its original shape. When that happens, watch out! Some of the band’s elastic potential energy will be transferred to the stone, which will go flying through the air.

Other Forms of Potential Energy

All of the examples of potential energy described above involve movement or the potential to move. The form of energy that involves movement is called mechanical energy. Other forms of energy also involve potential energy, including chemical energy and nuclear energy.

• Chemical energy is stored in the bonds between the atoms of compounds. For example, food and batteries both contain chemical energy.

• Nuclear energy is stored in the nuclei of atoms because of the strong forces that hold the nucleus together. Nuclei of radioactive elements such as uranium are unstable, so they break apart and release the stored energy.

Review

• Kinetic energy (KE) is the energy of moving matter. Anything that is moving has kinetic energy.
• You can increase kinetic energy by….

• increasing mass
• increasing velocity (speed)

• Potential energy (PE) is energy that is stored in a person or object.

• Gravitational potential energy is due to the position of an object above Earth’s surface. The object has the potential to fall due to gravity. Gravitational potential energy depends on an object’s weight and its height above the ground.

• Elastic potential energy is due to an object’s shape. It results when an elastic object is stretched or compressed. The more it is stretched or compressed, the greater its elastic potential energy is.

• Chemical energy and nuclear energy are other forms of potential energy.

• Review concepts by watching this video.

Kinetic energy 

Kinetic energy is energy of moving matter.

Potential energy 

Potential energy is stored energy an object has because of its position or shape.