Pendulum clock
The different parts of this pendulum clock - the pendulum, the escapement wheel and the other gear wheels - are each composed of rigidly bound groups of balls, each able to rotate around a fixed ball (drawn with cross-hatching). The "top" view of the clock shows how the gears mesh together.

The key to how the pendulum clock works lies in the escapement mechanism - the particular arrangement of the two balls at the top of the pendulum and the green escapement wheel below. (The name "escapement" describes the gear teeth periodically escaping the restraint of the time keeping element.)

The motive force driving the clock is provided by the small beige gear wheel (which in a real clock could be driven by weights or a coiled spring). Follow how this force is applied to the smaller of the (ridigly bound) blue gears, and how the outer blue gear then drives the green escapement wheel.

1. How do the pendulum and the escapement wheel affect each other's movements? How is this vital to the working of the clock?

2. What happens when you move the pendulum up, out of engagement with escapement wheel? (Grab the smaller hatched ball of the pendulum to do this. To move the pendulum back down later - either move it down or press rewind.)

3. What happens when you delete the (beige) driving wheel?
(To do this, pause the simulation, select the wheel with the mouse and press the Delete key. Press play to restart the simulation.)

4. Describe and explain the movement of the blue gear wheels (in normal operation).

5. What makes the purple and large green gear wheels move?
How many times does the blue gear wheel rotate for one revolution of the large green gear wheel?


The evolution of the escapement
The evolution of the escapement2

14 escapement mechanisms in motion (scroll to bottom of page for links to animations)

Lego clock1 , Lego clock2

A brief history of timekeeping

Clock parts

The evolution of the quartz crystal clock

History of timekeeping

About the simulation:
Red arrows show forces on an object. Thin headed arrows show the individual forces that combine together to give the triangle headed resultant force.
Blue arrows show the direction and speed of an object's velocity.
The longer an arrow, the greater the force and velocity it represents. Beyond a certain length - indicated by a bar the arrow - the arrow grows logarithmically; so that a doubling in force or velocity is shown as a fixed increase in length beyond the bar. This allows a much greater range of magnitudes of force and velocity to be displayed side-by-side on the screen.

Experimentors with slower computers can speed up the simulation by pausing it, selecting the purple and large green gear wheels, and deleting them by pressing the delete key. With fewer objects interacting the simulation can now run faster.

Pressing the  rewind button returns the simulation to its initial state.