PhysicsLAB: Energy Conservation in Simple Pendulums

Remember the formula used to calculate the gravitational potential energy of a mass given its mass and height above an arbitrary zero level is

PE_gravity = mgh

When a pendulum is pulled back from equilibrium through an angle θ, its height is calculated with the formula

h = L - L cos θ
where θ is the angular displacement

The formula used to calculate the kinetic energy of a massive particle is

KE = ½ mv²

In the absence of non-conservative forces, such as friction or applied, external forces, the mechanical energy in a system is conserved. That is

	During the swing of a simple pendulum, when does the bob possess maximum PE?

	During the swing of a simple pendulum, when does the bob possess maximum KE?

	During the swing of a simple pendulum, what is the magnitude of the bob's maximum velocity?

Another way of looking at conservation of energy is with the following energy diagram. As you can see,

the "purple" curve represents the pendulum bob's KE which during each cycle begins with an initial value of zero, increases to a maximum value, and then returns to zero
the "green" curve represents the PE of the bob which begins each cycle at a maximum value, then becomes zero as the bob passes through its equilibrium position, and returns to its maximum value
the "brown" line represents the total energy of the pendulum bob that always remains constant

	If a pendulum is initially released at an angle of 37º, at what angle will its PE and the KE be equal?

Refer to the following information for the next question.

At any intermediate position during the oscillation, the pendulum bob would have both PE and KE.

PE_max = PE_intermediate + KE_intermediate = KE_max

	If the pendulum was released at point A, derive an expression for the pendulum's instantaneous velocity at point B, an intermediate position in its swing.

See the related lesson on vertical circles if you are asked to calculate the tension of the string during the pendulum's oscillation. Remember that a pendulum is merely the bottom half of a vertical circle! These conservation of energy methods are the easiest way to determine an object's speed so that tensions can be calculated.