The Balancing Act simulation from PhET lets you place objects on a teeter-totter (seesaw) and learn what makes it balance. You can discover how mass and distance from the pivot work together, write your own rules for which way the plank will tilt, and test them in a balance challenge game. It is free, runs in any browser on a phone, tablet or laptop, and it is a great fit for the levers, moments and torque lessons in Tamil Nadu State Board (Samacheer Kalvi) Science and Physics.
What is the Balancing Act simulation?
This interactive simulation lets you explore balance and rotation with a plank on a pivot. You can place objects of different masses at different positions and see which way the plank tips. Because it runs on HTML5, there is nothing to install, and it works on phones, tablets, Chromebooks and computers.
Elementary to university
Balance, torque, levers and ratios
Phone, tablet, Chromebook, PC
English
PhET’s learning goals are to predict how objects of different masses can balance a plank, to predict how moving masses along the plank changes its motion, to write rules for which way the plank will tilt, and to apply those rules to solve balancing puzzles.
Launch the Balancing Act simulation online
Tap or click inside the player and start exploring. Use the full-screen button in the player for a bigger view.
Credit: University of Colorado Boulder
How to use the Balancing Act simulation, step by step
- Predict first. Before you place an object, say which way you think the plank will tilt.
- Place equal masses. Put the same mass on both sides at the same distance from the pivot and see that the plank balances.
- Move a mass. Slide one mass closer to or farther from the pivot and watch how the tilt changes.
- Use different masses. Balance a heavy mass with a lighter one by changing the distances.
- Write a rule and test it. Write your own rule for which way the plank tips, then try the balance challenge game to check it.
Tip for students: change only one thing at a time, either the mass or the distance. That is how you find out what each one does.
The idea behind the simulation: torque = force × distance
Torque = m × g × d
Here m is the mass in kg, g is the pull of gravity (about 9.8 m/s²) and d is the distance from the pivot in metres. The turning effect of each mass is called its torque, or moment, measured in newton-metres (N·m). A plank balances when the clockwise torque equals the anticlockwise torque. Because g is the same on both sides, you can simply compare mass × distance.
Worked example: A 40 kg mass sits 2 m to the left of the pivot, and a 20 kg mass sits 4 m to the right. Left: 40 × 2 = 80. Right: 20 × 4 = 80. They are equal, so the plank balances. In newton-metres, each torque is 80 × 9.8 = 784 N·m.
Balance and torque key terms explained
These are the words you will meet while using the Balancing Act simulation, with simple meanings.
Torque (moment)
The turning effect of a force. It equals force times the distance from the pivot.
Fulcrum (pivot)
The point the plank turns around.
Lever arm
The distance from the pivot to where the force acts.
Balance
The state where the plank does not turn because the torques on both sides are equal.
Rotational equilibrium
When the total clockwise torque equals the total anticlockwise torque.
Mass
The amount of matter in an object, in kilograms (kg).
Lever
A simple machine, such as a seesaw, that turns around a pivot.
Ratio
A comparison of two amounts. Balancing uses the ratio of mass to distance.
Common mistakes: Thinking that only the mass decides the tilt, measuring the distance from the end of the plank instead of from the pivot, and forgetting that the heavier mass can balance a lighter one if it is closer to the pivot.
Try the seesaw balance calculator (practice what you explore)
Enter a mass and a distance for the left and right sides to see which way the seesaw tips. Then tap a “what if” button.
What happens when you change a variable
| Change | Effect on the plank |
|---|---|
| Increase a mass | That side tips down more |
| Move a mass farther from the pivot | That side tips down more |
| Move a mass closer to the pivot | That side tips down less |
| Double the mass and halve the distance | The torque stays the same |
| Equal torques on both sides | The plank balances |
Classroom activities for teachers
The Balancing Act simulation fits a single 40-minute period.
- Predict and test (10 min): students write a prediction before each change, then check it in the simulation.
- Write the rule (10 min): in pairs, students write a rule for which way the plank tips and share it with the class.
- Balance puzzles (10 min): students balance different masses by changing the distances and record mass × distance on each side.
- Exit ticket (10 min): students explain in two sentences why a child can balance a heavier adult on a seesaw.
Looking for the matching chapters? Browse the Science and Physics books on TN SchoolBooks and download the Samacheer Kalvi PDFs, then use this simulation to make the moments and levers lessons visual. More free science simulations and textbook PDFs are on the home page.
Quick quiz: test your understanding of balance
Score: 0 / 5
Balancing Act simulation: frequently asked questions
Is the Balancing Act simulation free?
Yes. PhET simulations are free to run online in a browser. Check PhET’s licensing page for reuse terms.
Does it work on a mobile phone?
Yes. It is built with HTML5 and works on phones, tablets and Chromebooks. Use landscape mode or the player’s full-screen button for a bigger view.
What makes a seesaw balance?
A seesaw balances when the clockwise torque equals the anticlockwise torque. Torque depends on both the mass and the distance from the pivot.
How can a light mass balance a heavy one?
Place the light mass farther from the pivot. If you halve the mass, you need to double the distance to get the same torque.
The Balancing Act simulation turns the idea of torque into something you can try, test and see. Explore it, take the quiz above, and keep experimenting until predicting the balance feels easy.