READ: Kinetic and Potential Energy
READ: Kinetic and Potential Energy
Kinetic energy
What could these four photos possibly have in common? Can you guess what it is? All of them show things that have kinetic energy.
Energy exists in many different forms, but the one we probably use the most is kinetic energy. Kinetic energy is often thought of as the energy of motion because it is used to describe matter that is moving. Kinetic energy is the energy of moving matter. Anything that is moving has kinetic energy—from atoms in matter to stars in outer space. The spinning saw blade, flying bee, racing motorcycle, and the flowing water in the photos are moving; therefore, the common factor in all the pictures is kinetic energy.
Factors Affecting Kinetic Energy
An object’s kinetic energy depends on its mass and speed. The greater the mass, the more kinetic energy the object has. Mass is a measure of the amount of matter in a substance or an object. The basic SI unit for mass is the kilogram (kg), but smaller masses may be measured in grams (g).
Mass is commonly confused with weight. The two are closely related, but they measure different things. Whereas mass measures the amount of matter in an object, weight measures the force of gravity acting on an object. The force of gravity on an object depends on its mass but also on the strength of gravity.
Speed, which is how fast an object is moving (meters/second), also influences kinetic energy. The greater the speed, the greater the kinetic energy.Potential Energy
Potential energy is energy that is stored in an object. Objects have potential energy because of their position or shape. Like a diver on a diving board or the gymnast on the balance beam, anything that is above Earth’s surface has the potential to fall because of gravity therefore it has potential energy. The amount of gravitational potential energy an object has depends on the object’s mass and its height above the ground.
How could the child on the sled increase their potential energy? If the gymnast increased their mass how would that affect their potential energy?
An object’s shape can also give it potential energy. The girl in the photo is giving the elastic band of her slingshot potential energy by stretching it. This is known as elastic potential energy. Stretched rubber bands, inflated balloons, and springs that are uncoiled are examples of objects that have elastic potential energy due to their shape.
How does the amount of potential energy vary as distance changes?
Think about climbers on a cliff. If one climber climbed higher than the other, how would the added distance influence the amount of potential energy involved? The higher climber invested more energy climbing to a greater height, therefore there is more potential energy stored in the higher position. If you stretch out a rubber band on a slingshot to a farther distance it will have more potential energy because of the bigger change in shape.Can you think of other examples of how varying distances change the amount of potential energy involved? What could you do to investigate changes in potential energy?
What about carts on a roller coaster? Where on the roller coaster would the cart have the most potential energy? Where would it have the least?
Kinetic-Potential Energy Changes
Energy is neither created nor destroyed only transferred from one form into another. One of the most common energy changes occurs between kinetic and potential energy. Kinetic energy is the energy of moving objects. Potential energy is energy that is stored in objects, typically because of their position or shape. Kinetic energy can be used to change the position such as climbing to the top of a slide or shape of an object such as pulling back on a rubber-band, giving it potential energy. Potential energy gives the object the potential to move. If it does move, the potential energy changes back to kinetic energy.
The girl in the photo just finished coming down the water slide. When she was at the top of the slide, she had potential energy. Why? She had the potential to slide into the water because of the pull of gravity. As she moved down the slide, her potential energy changed to kinetic energy. By the time she reached the water, the potential energy had changed to kinetic energy.
How could the girl regain her potential energy? She could climb up the steps to the top of the slide. It takes kinetic energy to climb the steps and this energy would be stored in her position on the stairs as she climbed. By the time she got to the top of the slide, she would have the same amount of potential energy as before.
Friction, which is the resistance of an object to movement, also causes changes in kinetic energy. Rub your hands rapidly together. What do you feel? Friction causes the kinetic energy of the rubbing hands to be converted to heat energy which is why your hands feel warm.
On a dry slide friction would slow the girl down more than on a wet slide because water reduces friction. Ice also reduces friction which is why a hockey puck slides much further and faster in an ice rink than it would on a soccer field. Water reduces friction however on this slide she can get going much faster. As a rule of thumb when there is motion there is some amount of friction occurring as well which causes some of the energy to end up as heat.
Summary
- Energy, or the ability to cause changes in matter, can exist in many different forms. Energy can also change from one form to another.
- Kinetic energy (KE) is the energy of moving matter. Anything that is moving has kinetic energy.
- The amount of kinetic energy in a moving object depends directly on its mass and speed.
- Potential energy is energy that is stored in a person or object. Objects have potential energy because of their position or shape.
- The amount of gravitational potential energy an object has depends on the object’s mass and its height above the ground.
- Energy is neither created nor destroyed only transferred from one form into another.
Vocabulary
- Energy: Ability to cause changes in matter, or ability to do work.
- Kinetic energy: Energy of moving matter.
- Mass: The amount of matter in a substance or an object.
- Speed: How fast an object is moving
- Potential energy: Stored energy an object has because of its position or shape.
- Gravitational potential energy: Potential energy due to the position of an object above Earth’s surface.
- 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.
- Friction: the resistance of an object to movement.
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