Levers and Moments: Balance the Beam
Understand that the turning effect of a weight, called its moment, is its force multiplied by its distance from the pivot, and that a beam balances when the clockwise and anticlockwise moments are equal.
Before you try
What if a small child sat far from the middle of a see-saw and a big adult sat close in? Who goes down?
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What happened?
Move a control to see what changes and why.
The beam has no weight of its own and the pivot is frictionless. A real beam’s own mass changes where it balances.
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Try these ideas
- Balance a 4 kg weight at 30 cm using a 2 kg weight. How far out must it go?
- Halve one mass and see what you must do to its distance to keep the balance.
- Press Balance it, then adjust only weight B until the beam is level.
Why it works
A lever turns about a pivot. How much a weight turns it depends on two things: how heavy the weight is and how far it sits from the pivot. Multiply the two and you get the moment, the turning effect, measured in newton metres. Weights on the left turn the beam anticlockwise and weights on the right turn it clockwise. When the two totals match exactly, nothing turns and the beam balances.
In this simulator you change the mass and the distance of each weight, and can drag them along the beam too. Bars compare the clockwise and anticlockwise totals so you can see which side wins and by how much. The beam itself is treated as weightless and balanced, and gravity is 9.8 m/s², so the numbers stay simple.
Check your understanding
Question 1 of 4
A 4 kg mass sits 20 cm from the pivot. Where must a 2 kg mass go on the other side to balance it?
Two masses are equal but one sits twice as far from the pivot. What happens?
What is a moment?
Why is a long spanner easier to use than a short one?
Explain it in your own words
What changed? Why did it happen? Where might you notice this in everyday life?
For parents & teachers
- Try a see-saw in a park and find where each person must sit to balance.
- Ask which everyday tools are levers: scissors, a bottle opener, a wheelbarrow.
- Balance a ruler on a pencil with coins and predict each move before making it.