The left side is 60 g × 20 cm, the right is 40 g × 30 cm. The right side is lighter and it balances. Move a slider and "left minus right" leaves zero; the card on the far right gives the distance that would bring it back.
How to read this board
Each side computes weight × distance, and the two are compared. The cards are called "turning effect" because the quantity is neither a weight nor a length but a third thing. When it is equal on both sides, the lever balances. (In physics force × distance is the moment of a force; this board keeps to the school version, using the weight of the load and its distance from the pivot.)
There is a difference card because balance is a statement of equality. The question is not which side is bigger but whether the difference is zero.
Where it trips you up
The places that are easy to get wrong, next to how to check each one on the board.
| Where it trips you up | Why that happens | How the board shows it |
|---|---|---|
| Thinking the weight decides it | Every seesaw you have been on says so. In fact the distance counts for exactly as much. | 60 g on the left, 40 g on the right, and it balances. The weights alone cannot explain that. |
| Distance from the pivot, not from the end | It is easy to measure from the end of the bar in the diagram. | The distance cards are measured from the pivot. There is no move-the-pivot control on the board, so that part has to be held in your head. |
| Double the distance, halve the weight | It looks like a proportion, so both get doubled. | Take the right distance from 30 to 60 and the balancing weight goes from 40 to 20. They are inversely proportional. |
| Feeling like a lever gives you something for nothing | You move it with less force, and in exchange your end travels further. | The board only looks at the balance of forces. The distance moved is not in it. |
| When the pivot is off-centre | The weight of the bar itself starts to matter. School problems usually ignore it. | This board treats the bar as weightless too, which is where it parts company with a real one. |
| Three or more loads hanging | Once there is more than one per side, the turning effects have to be added up. | The board has one per side. To add more, open it in CalcAnyway, add cards and total them. |
A lever's balance is not settled by the weights. 60 × 20 and 40 × 30 both come to 1200. Comparing weight × distance-from-the-pivot on the two sides is what shows why a lighter load can hold its own by sitting further out.
Try this
- Double the right-hand distance — 30 to 60 cm, and the balancing weight halves to 20 g. The product is held constant, which is what inverse proportion means.
- Raise only the left weight — the difference opens up on the positive side, and extending the right-hand distance closes it again. Being able to fix it without adding weight is the whole point of a lever.
- Set both distances equal — only then does the heavier side go down. "The heavier side goes down" was a rule about one special case.