Equal moments from three gear weights do not guarantee that a complete see-saw sits level. The beam, pointer and connectors also have weight, and pivot friction can hide a small imbalance. Predict the gear-only turning effects first, then observe the whole supported model rather than calling a calculated equality a measured result.
The Balance See-Saw is our original retained lever bench. This reviewed construction-part guide replaces the earlier fabrication topic at this URL. It reuses a catalogue model honestly and corrects its balance explanation. The credited grohl666 demonstration provides an independent lever introduction; our compact model and instructions are separate from that creator's design.
What distance changes
A moment is a turning effect about a pivot. For identical gear weights hanging from a level beam, compare the number of weights multiplied by their perpendicular distance from the pivot. One gear six holes to the left contributes six gear-distance units. Gears two and four holes to the right contribute two plus four, also six.
This is a useful equality for the added weights. It omits the beam's own mass distribution and the offset pointer. As the beam tilts, horizontal distances also change. With all these hanging positions on the same straight plank, the simple comparison can still teach the lever principle, but it is not a complete friction or mass model. Review torque before translating a turning effect into a force claim.
Assemble a pivot that stays put
The free animated steps begin with a flat frame and paired pinned blocks. The front and rear posts use two separated attachment points. Their round holes support the pivot axle; the beam rotates around it. The pointer is joined to the plank at two positions, so it follows the beam instead of twisting independently on a single pin.
Fit the bushes and front post before adding the gear weights or testing motion. The bushes are real axial stops. A round bearing hole allows an axle to slide as well as turn, so the hole alone is not complete retention. A beam that escapes along its pivot can change the weight planes and bind against another part. Bushes and spacers explain why this is a different problem from ordinary turning friction.
The published animation shows a gentle chosen rocking excursion of plus or minus twelve degrees. It is an illustration of the retained pivot's permitted motion, not a prediction of how gravity will oscillate the real plank or proof of a stable balance point. The free booklet uses the same assembly and explicitly describes that limit.
Predict a virtual balance, then question it
Place one virtual weight at six holes on the left and two at two and four on the right. Predict the gear-only result before releasing the virtual beam. Move only the outer right weight one hole inward and explain which side now has the larger moment.
Interactive: Lever balance simulator. Hang gear weights on holes 1–7 each side of a see-saw pivot, predict whether it tips left, right or balances, then let go. Eight scored challenges include the Balance See-Saw puzzle: can one gear beat two?
The lab treats the displayed weights using its stated ideal assumptions. Use it to practice moment comparisons. It does not measure the masses of your particular pieces or the pointer on the physical bench. Do not add a compensating weight in the simulation and assume its location is automatically a validated physical attachment.
Make the physical test fair
Set the base on a flat surface. Before hanging the gears, release the empty moving body gently and record its tendency. Add the three equal gears in their authored locations, release again, and record whether it tilts left, tilts right or stays near level. Repeat three times without holding the pointer sideways.
| Trial | Empty-body tendency | Three-gear result | Rubbing or sticking noticed |
|---|---|---|---|
| 1 | Record | Record | Record |
| 2 | Record | Record | Record |
| 3 | Record | Record | Record |
Do not fill this table from the animation. Those are physical observations that you make. Keep the pivot retention, posts and pointer unchanged between trials. When investigating position, change one gear location at a time and use an actual compatible connection from the step guide. A gear placed near the frame may have a smaller permitted swing than one farther away; check the entire excursion slowly.
When the prediction and model disagree
A repeatable empty-body tilt can explain why equal gear moments do not produce a level complete assembly. A result that depends on which direction you approached can suggest friction or sticking. Neither observation establishes a precise mass or friction coefficient. Check for contact at the posts and for a bush squeezed so tightly that the beam cannot rock freely.
Removing a stop to cure rubbing creates an escape problem. Refit the retained arrangement and correct seating instead. Compare the pivot-position bench, which changes where a supported beam pivots, and read balance. The free catalogue offers other experiments when you can explain both the moment calculation and the support path. A good lever result distinguishes a useful simplified prediction from an honestly recorded physical balance.
Interactive: Check yourself. A few quick questions on the ideas in this guide, each with an explanation.
- What does 6 = 2 + 4 compare here?
- Why record the empty-body tendency?
- Does the rocking animation prove physical balance?
