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Energy from Gravity for Objects Projected Upward

by Ron Kurtus (24 May 2013)

When an object is projected upward from a given height, it travels until it reaches a maximum displacement, at which time its velocity is zero. The object then falls to the ground from that displacement.

You can calculate the PE, KE and total energy (TE) for an object that is 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:

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



Potential energy when projected upward

The initial potential energy of an object projected upward is:

PEi = mgh

As the object is moving upwards, the PE increases according to its displacement. The maximum displacement is:

ym = −vi2/2g

where ym is the maximum displacement from the starting point.

(See Displacement Equations for Objects Projected Upward for more information.)

Note: The value of ym is a negative number, because the motion is in the opposite direction of gravity.

However, the object is projected upward from a given height. Thus, the maximum height above the ground (hm) that it reaches is:

hm = h − ym

The equation for the PE at the maximum height is then:

PEm = mghm

or

PEm = mgh − mgym

Substituting for ym:

PEm = mgh + mvi2/2

Considerations concerning PE can be made from the initial height.

Kinetic energy when projected upward

The initial KE is:

KEi = mvi2/2

At the maximum displacement, v = 0 and thus:

KEm = 0

When the object falls and finally reaches the ground:

KEf = mvf2/2

Total energy and final velocity

To determine the final velocity, consider the total energy at the maximum displacement and compare it with the total energy at the ground:

PEm + KEm = PEf + KEf

(mgh + mvi2/2) + 0 = 0 + mvf2/2

Divide by m, multiply by 2 and rearrange terms to get the final velocity:

vf2 = 2gh + vi2

vf = √(2gh + vi2)

This compares with the equation in Velocity Equations for Objects Projected Upward:

v = √(2gy + vi2)

where y is the displacement below the starting point.

Summary

Potential energy with respect to gravity is PE = mgh. When the object is dropped, thrown downward or projected upward, its kinetic energy becomes KE = mv2/2, along with a factor of the initial velocity.

The sum of the PE and KE is the total energy, which is a constant. Equating the initial total energy with the final total energy, you can determine the final velocity of the object.


Gain confidence through small successes


Resources and references

Ron Kurtus' Credentials

Websites

Gravity and Potential Energy - University of Alaska

Potential Energy Calculator

Gravity Resources

Books

Top-rated books on Simple Gravity Science

Top-rated books on Advanced Gravity Physics


Questions and comments

Do you have any questions, comments, or opinions on this subject? If so, send an email with your feedback. I will try to get back to you as soon as possible.


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