Energy from Gravity of Falling Objects

by Ron Kurtus (updated 29 May 2023)

An object held at a given height above the ground has an initial potential energy (PE), according to its mass and the initial height.

When the object is released, its velocity increases as it falls. This increase in velocity results in an increase of the object's kinetic energy (KE). But it also results in a decrease in PE.

Because of the Conservation of Energy Law, the total energy (TE) of the object remains constant. This allows you to calculate the object's velocity at any point in its fall.

Questions you may have include:

What is the potential energy for a dropped object?
What is its kinetic energy?
What is the total energy and final velocity for a falling object?

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

Potential energy of an object

The potential energy of an object at some height with respect to gravity is:

PE = mgh

where

PE is the initial potential energy in joules (J) or foot-pounds (ft-lbs)
m is the mass of the object in kg-mass or pound-mass
g is the acceleration due to gravity (9.8 m/s² or 32 ft/s²)
h is the height above the ground in m or ft

Note: Potential energy is also sometimes abbreviated as U.

When the object reaches the ground, h = 0 and thus the final potential energy is:

PE_f = 0

Note: In reality, there is still a gravitational force on the object at the surface of the Earth, so the object has a gravitational potential energy at that point. But since the object cannot go anywhere, we say its PE from gravity is zero.

Kinetic energy of falling object

Kinetic energy (KE) is the energy of motion. Since the object is not moving at the initial position, the initial KE is:

KE_i = 0

Once the object is released, it accelerates downward. When the object reaches the ground, its kinetic energy is:

KE_f = mv_f²/2

where

KE_f is the kinetic energy at the ground in joules (J) or foot-pounds (ft-lbs)
v_f is the downward velocity of the object at the ground in m/s or ft/s

Total energy for falling object

The total energy of the object is:

TE = PE + KE

The total energy is a constant value, provided no external forces besides gravity act on the object. Thus, the initial total energy equals the final total energy:

TE_i = TE_f

PE_i + KE_i = PE_f + KE_f

When the object is simply dropped,

mgh + 0 = 0 + mv_f²/2

mgh = mv_f²/2

Final velocity for falling object

From that equivalence, you can determine the final velocity of the dropped object. Divide by m and multiply by 2:

v_f² = 2gh

v_f = √(2gh)

This is equivalent to v = √(2gy) that is given in Velocity Equations for Falling Objects.

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 (TE), which is a constant. Equating the initial total energy with the final total energy, you can determine the final velocity of the object.

You can succeed