Lesson 7.02 Conservation of Energy

Law of Conservation of Energy

The Law of Conservation of Energy states that energy can be transferred or transformed but it cannot be created or destroyed. Energy can only be converted from one form to another or transferred from one place to another.

For example,

• Falling objects have gravitational potential energy before the fall, and kinetic energy during the fall.
• Roller coasters continue to transfer gravitational potential energy to kinetic energy and back to gravitational potential energy during the course of the ride.

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Energy Transformations

In the image below, peaches hang on a tree. This is an example of a system that has gravitational potential energy.

• While the peach is hanging on the tree, the peach-Earth system does not have kinetic energy.


• When the peach begins to fall, it will inevitably get closer to the earth, decreasing the amount of gravitational potential energy.


• The gravitational potential energy is transformed into kinetic energy as the peach continues to fall.


• During this time, the speed of the peach increases, which increases its kinetic energy

Since the gravitational potential energy is converted into kinetic energy, the mechanical energy of the system remains nearly constant. As the amount of gravitational potential energy changes, that same amount is increased as kinetic energy. This shows us that the total amount of mechanical energy remains constant, but the type of mechanical energy changes.

Watch the video Energy Transfer in a Trebuchet (4:18) to learn more. Login information.

Then, watch the video Energy Assessment and Roller Coasters (5:03) to learn more. Login information.

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Mechanical Energy

Mechanical energy is the sum of the kinetic energy and the potential energy of objects in a system. This includes elastic potential energy and gravitational potential energy. Some examples of mechanical energy are:

• swings,
• bicycles,
• automobiles, and
• roller coasters.

Mechanical energy is not necessarily conserved. The mechanical energy of a system generally remains constant. When the energy remains constant, it is only transformed between different types of mechanical energy.

Read the page Mechanical Energy from the Physics Classroom to learn more.

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