As an object falls, it gains speed, so you might conclude that it gains energy as well. This isn't true. Rather than gaining energy, the object converts the energy within from gravitational potential energy to kinetic energy. At the beginning of its descent, all the object's energy is potential and at the end, all its energy is kinetic. But the total energy remains the same throughout. This value is the product of the object's mass, its height above the surface and the rate of gravitational acceleration.

Convert the object's mass, in pounds, to kilograms by dividing by 2.2. If the object weighs 15 lb:
15 / 2.2 = 6.818 kg.

Convert the object's initial height, in feet, to meters by dividing by 3.3. If it drops 20 feet:
20 / 3.3 = 6.06 meters.

Multiply together the answers to the previous two steps: 6.818 x 6.06 = 41.32.

Multiply the answer by 9.81, which is the rate of acceleration by gravity:
41.32 x 9.81 = approximately 405 joules. This is the falling object's constant energy.
Tips & Warnings
 To calculate just the object's kinetic energy, multiply 9.81 by the time it has spent falling, square the result and multiply your answer by half the object's mass.
References
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