How do you build a pendulum that swings freely and stays together?

Build a rigid stand, let the arm turn in a round pivot hole, and retain both the pivot axle and the wheel bob with seated stops. That gives the pendulum one intended motion: swinging around its pivot. Moving the bob changes the mass distribution, so compare timing by measuring complete swings rather than assuming the animation predicts seconds.

A swing needs support before it needs timing

The Pendulum Metronome is a free-swing experiment with 28 compatible construction parts and ten assembly steps. Its yellow arm hangs below a short axle in a braced stand. A wheel supplies part of the swinging mass. There is no motor, wind-up mechanism or escapement supplying energy after you release it. Friction eventually reduces the swing.

A mechanism can look convincing on screen while a real wheel quietly slides off its axle. This guide checks the smaller details that make the whole experiment possible: two posts connected firmly to a foot, a top brace, a retained pivot, and stops touching the actual bob hub. The tyre is wider than its hub, so a bush placed beside the tyre outline can leave a hidden axial gap. Inspect the centre where the axle passes through.

The independent video above is a useful comparison by 2in1 Bricking. It includes its own building instructions and a wind-up demonstration in the Experimenting chapter. The creator's powered assembly and our simple stand are different designs. Watch for where energy enters that demonstration, then ask what happens when a pendulum receives only one initial release. We credit that teaching comparison; this page does not reproduce the creator's model or claim to provide their instructions.

Assemble the real guide in order

Open the free animated guide and follow each insertion before adding the next part. Its booklet contains the same authored assembly. The overview below helps you recognize each assembly's job; use the guide for the exact locations and orientations.

1. Lay the 5×7 frame flat as the foot. Add the two corner pins and cross blocks.
2. Connect the upright blocks and cross blocks with the base axle. Add two pins to each upright block.
3. Attach both 11-hole posts using their first and third holes. Join the top holes with the red seven-hole brace.
4. Retain the three-long pivot axle with a half bush at each end. Hang the seven-hole arm by its top round hole so it can turn.
5. Add the six-long bob axle through the arm's bottom hole. Fit the rear stop, spacer and wheel, seating the rear hub face on the spacer.
6. Seat the front half bush on the front hub face. Add the blue swing stoppers and check the arm's travel gently.

Do not squeeze the pivot into a friction lock. A round bearing lets the arm turn; its retainers prevent sideways escape. At the bob, the stop faces instead need to sit against the hub so the wheel cannot walk along its shaft. Read pins and axles alongside the guide if these two jobs seem similar. The frames idea explains why the upper brace and paired attachment pins matter.

Predict the direction of gravity first

Raise the hanging arm a little to one side. Gravity tends to turn it back toward the lowest position. Once it reaches that position, it can still have speed and continue to the other side. Zero turning torque at the bottom does not mean zero speed. Turning tendency and current motion are different questions.

Try the prediction lab below. Choose hanging mode, select an angle, predict the initial turning direction, and then reveal the result. Compare the two pivot choices at the same angle and chosen mass. Reset before trying the opposite side. The distances and mass in this lab are deliberately chosen virtual values; they are not measured properties of our wheel or the six-module bob position.

The lab reports ideal gravity torque in millinewton metres and potential energy. It models the signed turning tendency from mass, gravity, centre-of-mass distance and angle. It does not calculate swing period, bearing friction or a powered escapement. Upright mode is a virtual counterexample: gravity drives that displaced arrangement farther away from its upper balance point. Do not infer that our hanging stand supports an upright wheel experiment.

For a hands-on comparison of pivot location and balance, try the Pivot Position Bench. Use the balance idea to explain why moving the centre of mass changes the turning tendency. A mass distribution question is useful at every level, but it should not become an unsupported timing claim.

Measure complete swings fairly

One full swing returns to the same position while travelling in the same direction. Two end ticks make one full swing. Timing ten full swings usually makes a start or stop error a smaller fraction of the total than timing one. Divide the elapsed seconds by ten to report seconds per swing.

Keep the same bob, arm and pivot for the first comparison. Start with a small release angle, about ten degrees or less, and release without pushing. Run three trials at the lower bob position. Then move its axle and all retaining bushes two holes up the arm, seat both hub stops again, and repeat. Check clearance before every trial. Keep the stand on the same surface and count consistently.

Predict before measuring. Moving the wheel changes both the combined centre of mass and the rotating body's resistance to angular acceleration. This is a rigid arm and wheel, not an ideal point mass on a weightless string. Avoid using bob-hole distance alone to promise an exact period. If you later change the wheel itself, record that as a separate experiment rather than quietly changing two variables at once.

The model's one-second animation loop is illustrative timing with a 25-degree swing. It demonstrates clearance in its authored range; it is not a physical measurement, a prediction of your particular parts, or proof that a clock will keep time. Our geometry checks are also not a claim that a physical prototype was tested.

Diagnose the quiet failures

If the swing fades very quickly, inspect rubbing at the pivot before adding a larger push. If the bob moves sideways, stop and reseat its hub bushes; an animation cannot supply the missing retainer. If the posts twist, check both attachment pins per post and the top brace. If a wide release hits a stopper, return to the small release used for timing. Do not turn a collision into a supposed calibration method.

Finish by explaining three different things: what keeps the assembly together, what makes gravity restore the hanging arm, and what your stopwatch actually measured. Explore the free builds for the next mechanism, or print this guide's free booklet for another experimenter. A useful observation includes the setup and units, even when its result differs from your prediction.