science fair help?

Question

For my science fair, i'm doing how a pendulum clock works. I found some different info on google, but i don't understand it. Can someone give me a detailed but simple explanation of how a pendulum clock works?

Answer 1

Sure!

The key to understanding a pendulum is to understand that energy must be conserved.

The clock is enclosed to prevent the pendulum from being disturbed by wind. So, you start with the pendulum at the top of it's arc.

At the top of the arc, the weight does not move, but it does have potential energy, defined as it's weight * height. The formula is PE = mgh (mass * acceleration due to gravity * height).

When the weight is released, it swings towards the bottom, since it's tied to the string. At the bottom of its arc, some of the potential energy has converted to kinetic energy (defined as KE = 1/2mv^2, where v = velocity).

Since energy must be conserved, the change in Potential Energy must equal the change in Kinetic Energy.

As the weight passes the bottom of the arc, it slows. Once it reaches its original height, it then stops, because the kinetic energy converts back to potential energy - and it has the exact same potential energy that it had before.

Now that it's stopped, gravity pulls it back down, to start again.

To illustrate it with formulae:
PE at top = mgh
KE at bottom = mgΔh = 1/2mv^2 (Δh = difference in height between top and bottom)
PE at other end = mgh

Note: the pendulum does suffer from air resistance - it loses a small fraction of its energy on each swing due to having to push air molecules out of its way. If the pendulum swings in a vacuum, that would not occur.