Moving Things, People & Ideas (VI–VIII) • Topic 3 of 3

Friction

CTET recap: friction is the force that opposes the relative motion (or tendency of motion) between two surfaces in contact — it always acts opposite to the direction of motion. It is caused by the interlocking of tiny irregularities (bumps) on the two surfaces, so rougher surfaces produce more friction and smoother/polished surfaces less. Friction increases when the surfaces are pressed together harder (more weight). There are three kinds, and their order of strength must be known: static friction (acts before an object starts to move) is the largest; sliding friction (when a body slides) is smaller — which is why it is harder to start moving a heavy box than to keep it sliding; and rolling friction (when a body rolls) is the smallest of all, which is the whole point of wheels and ball bearings. Friction is a 'friend and foe': useful (we can walk, write, hold things, brakes work, a matchstick lights) but also wasteful (wears out shoe soles and machine parts, produces heat, lowers efficiency). We reduce friction with lubricants (oil, grease), polishing, ball bearings, and streamlined shapes; we increase it with treads/grooves on tyres and shoe soles. Pedagogy & misconceptions: the classic child error is 'a moving ball stops on its own because its force/energy is used up' — in reality friction (and air resistance) is the external force bringing it to rest; remove friction and it would keep going. Another is 'friction is always bad'. How it's tested: identify why friction acts, rank static/sliding/rolling, pick the method to reduce or increase friction, or spot the misconception in a child's explanation.

✅ Solved examples

1. Arrange in increasing order of magnitude: rolling, static and sliding friction.
Rolling < Sliding < Static. Rolling friction is the smallest and static friction the largest — that is why wheels (rolling) move loads easily and why a box is hardest to get moving from rest.
2. Why is it easier to roll a heavy drum than to slide it across the floor?
Rolling friction is much smaller than sliding friction, so rolling needs less force. This is the principle behind wheels and ball bearings.
3. Oil or grease is applied to the moving parts of a machine in order to:
Reduce friction (and the heat and wear it causes). Oil and grease act as lubricants, forming a thin layer that keeps the surfaces from rubbing directly.
4. A child explains 'the rolling marble stopped because the force inside it finished'. What should the teacher correct?
There is no 'force inside' a moving object that runs out; the marble slows and stops because friction (an external opposing force) acts on it. On a frictionless surface it would keep moving — this corrects a very common misconception.

✏️ Practice — try these, take hints as needed

1. Friction always acts in which direction relative to motion?
It resists movement.
Think of pushing a box.
Opposite to the direction of motion
2. Which type of friction is the smallest in magnitude?
The reason wheels are used.
Ball bearings exploit it.
Rolling friction
3. Treads (grooves) are made on tyres and shoe soles in order to:
Rougher grip.
Opposite of using a lubricant.
Increase friction (better grip, to prevent slipping)
4. Friction is caused mainly by:
Look closely at the surfaces.
Tiny bumps interlock.
The interlocking of irregularities on the two surfaces in contact

📝 Topic test — 8 questions

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