Potential energy: gravitational potential energy

Kinetic energy

Recall that gravitational potential energy is the energy an object has because of its position above a surface.

In diagram "A" below, the 30kg barrel is rolled to the top of the ramp. During this process 882 joules of GPE are stored. If the barrel falls from the top of the ramp to the ground, where does the 882J of energy go?

Diagram A

The answer is really quite simple. As the barrel falls it accelerates, converting gravitational potential energy, GPE into energy of motion or “Kinetic energy”. Halfway down 50% of the GPE has been converted to KE. By the time the barrel contacts the ground, 100% of the GPE has been converted to KE. Once the barrel strikes the ground the KE is converted into sound and heat. The energy is conserved. It is never destroyed, it just changes form. Diagram "B" illustrates this conversion of GPE into KE and then into sound and heat energy.

Diagram B

Diagram C illustrates another situation in which Gravitational Potential Energy is converted into Kinetic energy. This situation involves a pendulum swinging back and forth. Initially, the pendulum mass is raised to a height of .5m above it's original resting position at point "C". At this point the pendulum has it's greatest GPE. The GPE can be calculated using GPE=mgh. When lifted to point "A" the pendulum mass gains 4.9J of GPE ( Note: This also means that 4.9J of work has to be done to lift the pendulum mass to this starting point. See PS 6.3 ) When the pendulum is released and falls towards point "C" the mass gains velocity, and therefore Kinetic Energy on the way down. By the time the pendulum reaches point "C" it has its maximum velocity and 100% of the GPE has been converted to Kinetic energy. This process reverses as the pendulum continues its swing from point "C up to point "E". By the time the pendulum mass reaches point "E" the Kinetic Energy it had at point "C" has been entirely converted to GPE.

Diagram C