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Kinetic energy and momentum

Potential energy and kinetic energy

Gravitational potential energy

Any object that is raised against the force of gravity stores gravitational potential energy. For example, if you lift a book up onto a shelf, you have to do work against the force of gravity. The book has gained gravitational potential energy.

Elastic potential energy

Elastic objects such as elastic bands and squash balls can change their shape. They can be stretched or squashed, but energy is needed to change their shape. This energy is stored in the stretched or squashed object as elastic potential energy.

Kinetic energy

Every moving object has kinetic energy (sometimes called movement energy). The more mass an object has, and the faster it is moving, the more kinetic energy it has. You should be able to discuss the transformation of kinetic energy to other forms of energy.

Example 1 - The bouncing ball

Several energy transfers happen when a squash ball is dropped onto a table and bounces up again.

When the ball is stationary above the table, its gravitational potential energy (GPE) is at a maximum. It has no kinetic energy (KE), or elastic potential energy (EPE).

As the ball falls, its GPE is transferred to KE and the ball accelerates towards the table.

When the ball hits the table, the KE is transferred to EPE as the ball squashes. As the ball regains its shape, the EPE is transferred to KE and it bounces upwards.

When the ball reaches the top of its travel, all the KE has been transferred to GPE again. Note that the ball will be lower than it was when it was first dropped, because some energy is also transferred as heat and sound to the surroundings.

  • High up
  • GPE - maximum
  • KE - none
  • EPE - none
  • Falling
  • GPE - decreasing
  • KE - increasing
  • EPE - none
  • On table
  • GPE - minimum
  • KE - none
  • EPE - maximum

Example 2 - The pendulum

The pendulum is a simple machine for transferring gravitational potential energy to kinetic energy, and back again.

When the bob is at the highest point of its swing, it has no kinetic energy, but its gravitational potential energy is at a maximum. As the bob swings downwards, gravitational potential energy is transferred to kinetic energy, and the bob accelerates.

At the bottom of its swing, the bob’s kinetic energy is at a maximum and its gravitational potential energy is at a minimum.

As the bob swings upwards, its kinetic energy is transferred to gravitational potential energy again. At the top of its swing, it once again has no kinetic energy, but its gravitational potential energy is at a maximum.

Note that the bob’s swing will become lower with each swing, because some energy is also transferred as heat to the surroundings.

maximum kinteic energy at lowest point of swing - minimum gravitational potential energy. at the highest point of swing there is no kinetic energy, and maximum gravitational potential energy.

Here is a version of the 'Work and Energy' podcast with drawings to help explain things.

Back to Forces and motion index

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