The Friction simulation from PhET lets you move a Chemistry book across another and observe what happens at the molecular level. You can connect rubbing, heat and temperature, and see why surfaces that look smooth still grip each other. It is free, runs in any browser on a phone, tablet or laptop, and it is a great fit for friction and heat lessons in Tamil Nadu State Board (Samacheer Kalvi) Science and Physics.
What is the Friction simulation?
This interactive simulation asks you to move the Chemistry book and watch what happens. Instead of just showing a force arrow, it links friction to the tiny motion of the particles that make up the surfaces, and to the temperature of the objects. Because it runs on HTML5, there is nothing to install, and it works on phones, tablets, Chromebooks and computers.
Elementary school to university
Friction, heat and temperature
Phone, tablet, Chromebook, PC
English
PhET’s learning goal is to build a molecular-level model of friction and to describe matter in terms of molecular motion. This includes how temperature affects the picture, how the motion of particles in solids, liquids and gases compares, and how the size and speed of gas molecules relate to everyday objects.
Launch the Friction simulation online
Tap or click inside the player and move the book. Use the full-screen button in the player for a bigger view.
Credit: University of Colorado Boulder
How to use the Friction simulation, step by step
- Predict first. Before you move anything, say what you think will happen when two surfaces rub together.
- Move the Chemistry book. Rub it across the other book and watch what changes on screen.
- Look at the particles. Notice how the motion of the particles at the surfaces changes while you rub.
- Watch the temperature. See what happens to the temperature as you keep rubbing, and what happens when you stop.
- Explain it in your own words. Describe how rubbing turns movement into heat, using what you saw at the particle level.
Tip for students: rub your hands together for ten seconds, then open the simulation. You will feel the same idea you are about to see.
The idea behind the simulation: friction force and heat
f = μ × N | Heat ≈ f × d
Here f is the friction force in newtons (N), μ (mu) is the coefficient of friction, which has no unit, and N is the normal force, the push of the surface on the object. On a flat surface N = m × g, where m is the mass in kg and g is about 9.8 m/s². The work done against friction over a distance d, in metres, ends up as heat in joules (J).
Worked example: A 5 kg box is dragged along the floor. The coefficient of friction is 0.3. The normal force is 5 × 9.8 = 49 N, so the friction force is 0.3 × 49 = 14.7 N. If the box slides 2 m at a steady speed, about 14.7 × 2 = 29.4 J of the work you did becomes heat in the box and the floor.
Friction key terms explained
These are the words you will meet while using the Friction simulation, with simple meanings.
Friction
A force that opposes the motion between two surfaces that touch.
Static friction
The friction that holds an object still. It must be overcome to start the motion.
Kinetic friction
The friction that acts on an object that is sliding. It is usually smaller than static friction.
Normal force
The push of a surface on an object, at right angles to the surface. A heavier object gives a larger normal force.
Coefficient of friction (μ)
A number that tells how rough or grippy a pair of surfaces is. A larger μ means more friction.
Heat
Energy that flows from a hotter object to a colder one. Rubbing produces heat.
Temperature
A measure of how fast, on average, the particles of an object are moving.
Molecular motion
The constant jiggling of the particles in matter. It is stronger in a hotter object.
Lubricant
A substance such as oil that reduces friction between surfaces.
Common mistakes: Thinking friction only happens on rough surfaces (even smooth surfaces have tiny bumps at the molecular level), forgetting that friction always opposes the motion, and not realising that the motion energy lost to friction becomes heat.
Try the friction calculator (practice what you explore)
Enter the mass, the coefficient of friction and the sliding distance to see the friction force and the heat produced on a flat surface. Then tap a “what if” button.
What you will observe
| Situation | What happens |
|---|---|
| Surfaces rub together | Particle motion increases and the temperature rises |
| Rubbing stops | The extra heat spreads out and the object cools |
| Heavier object | Larger normal force, so more friction |
| Rougher surface (higher μ) | More friction and more heat for the same distance |
| Lubricant added | Less friction and less heat |
Classroom activities for teachers
The Friction simulation fits a single 40-minute period.
- Predict and test (10 min): students write a prediction before moving the book, then check it in the simulation.
- Rub and feel (10 min): students rub their hands, then a coin on a desk, and explain the warmth they feel.
- Draw the particles (10 min): students sketch the particles before rubbing and while rubbing, and label the temperature change.
- Exit ticket (10 min): students explain in two sentences why rubbing two surfaces makes them warm.
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 friction and heat lessons visual. More free science simulations and textbook PDFs are on the home page.
Quick quiz: test your understanding of friction
Score: 0 / 5
Friction simulation: frequently asked questions
Is the Friction 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.
Why does rubbing make things warm?
When two surfaces rub, the particles at the surfaces are pushed and made to move faster. Faster particle motion means a higher temperature, so the motion energy you put in becomes heat.
What does friction depend on?
Friction depends on how hard the surfaces are pressed together (the normal force) and on the type of surfaces, which is described by the coefficient of friction. It always acts against the motion.
The Friction simulation turns an everyday force into something you can move, observe and explain. Explore it, take the quiz above, and keep experimenting until linking friction, motion and heat feels easy.