Wave Motion Pendulum Model

Quick Answer: The Wave Motion Pendulum Model is a row of pendulums of gradually increasing length hung from one frame. Released together, the bobs fall out of step in an orderly way and trace travelling waves, snakes and seemingly random patterns before lining up again, showing how small differences in period build up into phase patterns.

Patterns From Pendulums of Different Lengths

The listing image of the wave motion pendulum model shows a horizontal bar on two uprights fixed to a wooden base. From hooks along the bar hang orange spherical bobs on threads, and each thread is a little longer than the one before, so the bobs form a gently sloping line from one end of the frame to the other.

The period of a simple pendulum depends on its length, T = 2π√(L/g), so every pendulum in the row swings at a slightly different rate. When all the bobs are pulled back together and released, they begin as a straight line. Within a few swings the shorter pendulums pull ahead, each one a small step in front of its neighbour, and the line curls into a travelling wave. Later the row breaks into two or three interleaved waves, then into two groups swinging in opposite directions, and in a well-tuned set all the bobs return to a straight line at the end of the cycle before the sequence repeats.

The waves are a pattern of phase, not a transfer of energy: each bob hangs on its own thread from a rigid bar and swings independently. That makes this model a useful contrast with the Coupled Pendulum Experiment Kit, where a coupling does pass energy from one pendulum to the other. The number of pendulums, and whether the thread lengths can be adjusted for tuning, should be confirmed at enquiry.

Specifications

Applications

• Showing that the period of a pendulum depends on its length and not its mass
• Illustrating phase difference and how wave shapes arise from it
• Introducing repeating cycles and beats in oscillation topics
• Contrasting independent oscillators with coupled systems

Care & Handling

• Pull all the bobs back together against a flat ruler or board and remove it smoothly to release them in step.
• Keep the swings small, since large amplitudes alter each period slightly and blur the pattern.
• Untangle the threads before putting the frame away, and store it where the bobs cannot knot together.
• Check hooks and knots before use, and replace a worn thread with one of exactly the same length.

Why Choose LabEquip

Physics teachers use the pendulum wave in lessons on oscillations and waves, and schools often set it up for open days because the patterns hold attention from across a room. LabEquip supplies it as part of the STEM kits range; send questions about tuning or pendulum count through our contact page.

Frequently Asked Questions

Why do the pendulums drift out of line?

Each thread has a different length, so each pendulum has a slightly different period. The shorter ones swing a little faster, and those small time differences add up swing after swing, spreading the bobs into a wave shape.

Does energy travel along the row of bobs?

No. Each pendulum hangs from the rigid bar on its own thread and swings independently. The travelling wave is a pattern made by the changing phase of neighbouring pendulums, not energy passing from one bob to the next.

Why do the bobs line up again?

In a tuned set the lengths are chosen so that over one full cycle each pendulum completes a whole number of swings, one more than its longer neighbour. At the end of that cycle every bob is back at the start together, and the pattern repeats.

Why must all the bobs be released at once?

The pattern depends on every pendulum starting in phase. Releasing them one after another adds random delays that blur the waves, so holding them back with a flat board and sliding it away gives a clean start.

Does the mass of the bobs matter?

For small swings a pendulum’s period depends on its length and on gravity, not on its mass. Identical bobs are used so the row looks even and air resistance acts alike on each, but changing one bob would barely change its period.

How long does one full pattern last?

That depends on the chosen lengths; many classroom pendulum waves are tuned to repeat in about a minute. Timing a complete cycle and counting the swings of the shortest pendulum makes a good extension activity.

Last Updated: September 2026