You are here
๐ฌ Think about it
Push a heavy box and sometimes it just won't budge until you push harder. Stop pedalling and a cycle slowly stops on its own. An invisible force is at work — friction. Let us learn more about it.
What is the force of friction?
- It acts between surfaces in contact, opposite to the direction of the applied force or motion.
- An object moves only when the applied force is larger than friction, giving a net force.
- On a moving object, friction gradually slows it down and stops it.
Example:
Stop pedalling a bicycle and friction brings it to rest after some distance.
What other forces act on a pushed object?
- The gravitational force (weight) acts downward.
- The normal force from the surface acts upward, perpendicular to it.
- These two are balanced.
- Air also exerts friction, usually small enough to neglect.
๐ Note
- Multiple forces may act on an object, but its motion depends only on the net force.
In this Activity, we will release a coin stack with a stretched rubber band on different surfaces to see how friction changes the distance it travels.
Procedure
1. Collect four โน10 coins, a strong rubber band and adhesive tape. Find level horizontal surfaces of different materials (wooden, laminated, marble or tiled).
2. Stack the four coins and tape them together.
3. Hold the rubber band slightly stretched on the wooden top; mark points A and B at its ends, and a mark C up to which you will stretch it (Fig. 6.12).
4. Place the coin stack in the middle of A and B, pull it back to mark C, release it, and measure the distance it travels before stopping. Repeat twice.
5. Repeat on a laminated top (same A, B, C distances). Does the stack travel farther and slow down more gently?
6. Repeat on polished marble or tile. Does it travel even farther? What do you conclude?
2. Stack the four coins and tape them together.
3. Hold the rubber band slightly stretched on the wooden top; mark points A and B at its ends, and a mark C up to which you will stretch it (Fig. 6.12).
4. Place the coin stack in the middle of A and B, pull it back to mark C, release it, and measure the distance it travels before stopping. Repeat twice.
5. Repeat on a laminated top (same A, B, C distances). Does the stack travel farther and slow down more gently?
6. Repeat on polished marble or tile. Does it travel even farther? What do you conclude?
Observation
The smoother the surface, the farther the stack travels and the more slowly its velocity decreases. On rough wood it stops soon; on smooth marble it slides much farther.
Conclusion
The distance changes with the surface, so the force of friction is different on different surfaces — smaller friction means a longer slide.
โ Summary
- Launch a coin stack with a rubber band.
- Try different surfaces.
- Measure how far it slides.
- Smoother = farther = less friction.
What happens to the coin stack after release?
- The rubber band's forward force is larger than friction, so a net force accelerates it forward.
- Once it leaves the band, only friction acts, opposite to its motion.
- Friction gradually reduces its velocity and brings it to rest.
In this Activity, we will use a spring balance to measure and compare the force of friction on different surfaces.
Procedure
1. Take a spring balance and a wooden block.
2. Place the spring balance horizontally on a surface from Activity 6.1 and check its reading is zero. Attach the block to the hook (Fig. 6.14).
3. Pull with gradually increasing force and note the reading when the block just starts moving. When the block's velocity is steady, the net force is zero, so this reading equals the force of friction.
4. Repeat on the other three surfaces.
5. Compare the readings. Is the reading smallest for the surface on which the coin stack travelled farthest?
2. Place the spring balance horizontally on a surface from Activity 6.1 and check its reading is zero. Attach the block to the hook (Fig. 6.14).
3. Pull with gradually increasing force and note the reading when the block just starts moving. When the block's velocity is steady, the net force is zero, so this reading equals the force of friction.
4. Repeat on the other three surfaces.
5. Compare the readings. Is the reading smallest for the surface on which the coin stack travelled farthest?
Conclusion
A smaller reading means smaller friction. The reading is smallest for the surface on which the coin stack slid the farthest, confirming that smoother surfaces have less friction.
โ Summary
- Pull a block with a spring balance.
- Read the value as it just moves.
- That reading is the friction.
- Compare across surfaces.
Important Points
- Friction opposes motion and depends on the nature of the surfaces.
- Weight (down) and normal force (up) on a resting object are balanced.
- Smoother surfaces have less friction, so objects slide farther.
๐งช Think as a Scientist
-
Imagine an object and a floor so smooth that friction is zero. If you repeat Activity 6.1, will the object slow down? Will it ever stop?
With no friction there is no force to oppose its motion, so its velocity would not decrease — it would keep moving forever at a constant velocity.
โ What if …
-
What if the force of friction disappeared from the world? How would motion be impacted?
Once moving, objects would never stop on their own; we could not walk, grip or hold anything, and vehicles could not start or brake.
โ Test Yourself
-
In which direction does friction act?
View Answer
Opposite to the direction of motion or applied force. -
What balances the weight of a block resting on a surface?
View Answer
The normal force from the surface. -
On which surface does an object slide farthest?
View Answer
The smoothest one, which has the least friction.
Important Definitions
- Force of friction — a force between surfaces in contact that opposes motion.
- Normal force — the upward force a surface exerts, perpendicular to it, on an object resting on it.
๐ NCERT Question 1 — Using a horizontal force
A table is moved at constant velocity by a horizontal force F — how much is the frictional force?
View Answer →
