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Hang brass weights on a ruler. Can a light weight far out balance a heavy one close in?
Ruler See-saw lab · Last updated: 16 October 2026
A ruler balances on a pivot like a see-saw. You hang a heavy weight close to the middle on one side and a lighter weight further out on the other, then slide them until the ruler sits level.
What you'll discover
- A weight's turning effect depends on both how heavy it is and how far it is from the pivot.
- A light weight far from the pivot can balance a heavy weight close to it.
- The ruler balances when weight times distance is the same on both sides.
The science in plain words
A force can make something turn as well as move. The turning effect of a force is called its moment, or torque. It depends on two things: the size of the force and how far from the pivot it acts. A small force a long way out can turn something as much as a big force close in.
For the ruler see-saw, each weight tries to turn the ruler one way. The ruler balances when the two turning effects are equal. Multiply each weight by its distance from the pivot; when the two answers match, the ruler stays level. So 2 newtons at 30 centimetres balances 3 newtons at 20 centimetres, because both give 60.
This rule is called the principle of moments, and Archimedes described it over two thousand years ago. It explains why a heavier child sits nearer the middle of a see-saw, why door handles sit far from the hinges, and why a long spanner loosens a stiff nut more easily than a short one.
Levers like crowbars, scissors and wheelbarrows all use this idea: a small push far from the pivot makes a big push close to it.
Where it fits
- India, NCERT: The turning effect of forces is not a separate topic in the current NCERT classes 6 to 10 books; it builds on Class 8 Chapter 5: Exploring Forces.
- United States: Grades 6 to 8
Common questions
How can a light weight balance a heavy one?
By sitting further from the pivot. The turning effect is weight multiplied by distance, so a weight half as heavy balances when it is twice as far from the middle.
What is the principle of moments?
It says that an object is balanced when the total turning effect clockwise equals the total turning effect anticlockwise, with each turning effect equal to force times distance from the pivot.
Why are door handles far from the hinges?
Pushing far from the hinge gives a bigger turning effect for the same push, so the door opens easily. Try pushing near the hinge and you will feel how much harder it is.
Related labs
- Bridge Stress: How can a bridge of thin sticks hold a heavy load? Which sticks are pulled, and which are squashed?
- Pulleys and a Newton Meter: Lift a brass weight with one wheel, then with more. How hard do you have to pull?
- Gear Mechanisms: Big gear, small gear: which one turns faster, and why?
See all physics labs or every Taz science lab.
Try it yourself
The Ruler See-saw lab is one of the interactive 3D science labs inside Taz. Change one thing, predict what will happen, then run it and watch. Labs open after you sign in.