The swinging bob

The idea

A pendulum bob has potential energy but no kinetic energy at its extreme points and kinetic energy but no potential energy at the bottom; friction at the support and air resistance eventually stop it.

A swing

A swing goes up on one side, rushes through the lowest point, and climbs on the other side. It stops for a moment at the top of each swing.

At the lowest point, what do you think is the swing's energy mostly of?

• Motion
• Height
• Neither

P, Q and R

In this picture the bob has 2 J of mechanical energy (an example). Visit each place.

Swapping again

At the far points the bob has height but no motion. At the bottom it has motion and no height above it. The energy swaps between the two kinds, and the bob reaches almost the same height on the other side. In real life friction and air resistance take energy away, and the pendulum slows and stops.

In a swinging pendulum the bob has potential energy but no kinetic energy at its extreme points, and kinetic energy but no potential energy at the bottom. Friction at the support and air resistance eventually stop it.

Notes

A pendulum swaps potential energy at the extreme points with kinetic energy at the bottom; friction and air resistance eventually stop it.

Check yourself

At the extreme point of its swing, the pendulum bob has…

At the bottom-most point, the pendulum bob has…

Why does a real pendulum slow down and eventually stop?

The bob starts from rest at P and swings to R on the other side. How high does it reach at R?

• kinetic energy but no potential energy. That describes the bottom-most point.
• potential energy but no kinetic energy — correct. Yes!
• both its greatest potential energy and its greatest kinetic energy. At the extreme point the bob stops for an instant, so it has no kinetic energy.
• potential energy but no kinetic energy. That describes the extreme points.
• neither kind of energy. The bob is moving at the bottom, so it has kinetic energy.
• kinetic energy but no potential energy — correct. Yes!
• Energy is lost to friction at the support and to air resistance — correct. Yes!
• Its mechanical energy turns into new potential energy. The loss is due to friction at the support and air resistance.
• The Earth stops pulling on it. The Earth keeps pulling on the bob. The pendulum stops because of friction and air resistance.
• Much higher than it started. It cannot gain energy. It reaches almost the same height.
• Almost the same height it started from — correct. Yes!
• Only half of the way up. The bob regains its potential energy and reaches almost the same height as at P.