# Overview of Energy from Gravity

by Ron Kurtus

Gravity is a force that attracts objects toward the Earth and gives them kinetic energy (KE).

An object at some height above the ground has the potential of accelerating downward, due to the pull of gravity. In other words, in that position, the object has potential energy (PE) that can be turned into the kinetic energy (KE) of motion. The sum of the potential energy and kinetic energy due to gravity for an object is constant unless outside forces come into play.

You can calculate the PE, KE and total energy (TE) for an object that is dropped, thrown downward or projected upward with some simple equations. You can then verify that the final velocity is the same as obtained from the gravity derivations.

Questions you may have include:

• What is the energy of a dropped object?
• What is the energy of an object thrown downward?
• What is the energy of an object projected upward?

This lesson will answer those questions. Useful tool: Units Conversion

## Energy of dropped objects

When an object is held at some height above the Earth's surface, it has an initial potential energy (PE) that is a function of that height. Its kinetic energy (KE) is zero, since the object is not moving.

When an object is dropped, it increases its velocity—and thus its kinetic energy (KE)—as it falls to the ground. The total energy of the object (TE) is the sum of its PE and KE at any given point.

When it hits the ground, the final PE is zero and the KE equals the initial PE.

## Energy of objects projected downward

When an object is projected downward from some height, it has a KE that is a function of the initial downward velocity. However, its initial PE is only a function of the height.

As the object moves downward, the PE decreases and the velocity and KE increases.

When the object reaches the ground, the PE is zero and the KE is the sum of the velocity from the height and the initial velocity.

## Energy of objects projected upward

When an object is projected upward from a given height, it has an initial KE that is a function of the initial upward velocity. However, its initial PE is only a function of the initial height.

As the object is moving upwards, the PE increases according to its displacement and the KE decreases until the object reaches a maximum height, at which time its velocity is zero.

The object then falls from that height similar to falling from any height. The total energy is also a function of the maximum height.

## Summary

An object at some height has potential energy (PE) that can be turned into the kinetic energy (KE) of motion. The sum of the potential energy and kinetic energy due to gravity for an object is constant unless outside forces come into play.

You can calculate the PE, KE and total energy (TE) for an object that is dropped, thrown downward or projected upward with some simple equations.

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## Resources and references

Ron Kurtus' Credentials

### Websites

Gravity and Potential Energy - University of Alaska

Potential Energy Calculator

Gravity and Energy - Exploring the interplay of gravity and mass-energy

Gravity Resources

### Books

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