Friction — NCERT Solutions, AI Tutor & Practice

Understand: When one object moves or tends to move over the surface of another object, a force acts against this motion.

Definition: Friction is the force that opposes the relative motion between two surfaces that are in contact with each other.

Where it acts: Friction acts at the surface of contact between the two objects—for example, between the sole of your shoe and the floor, or between a book and a tabletop.

Key point: Friction always acts in a direction opposite to the direction of motion (or the tendency of motion).

Friction depends on two main factors:

1. The nature (smoothness or roughness) of the two surfaces in contact.

   • Rougher surfaces → more friction.
   • Smoother surfaces → less friction.

2. How hard the two surfaces are pressed together (the force pressing them).

   • Greater pressing force → more friction.
   • For example, a heavier object experiences more friction than a lighter one on the same surface.

Conclusion: Friction is caused by the interlocking of tiny irregularities present on the surfaces. The more the irregularities interlock, the greater the friction.

Understand: To walk, our foot pushes backward on the ground, and friction lets the ground push us forward.

Analyze:

• A wet, smooth marble floor has very few surface irregularities, and the water fills the tiny gaps, reducing the interlocking further.
• This makes the friction between our foot and the floor very small.

Conclusion: With very little friction, our foot cannot get a proper grip. It slips backward instead of pushing us forward, so we are likely to slip and fall. This is why walking on a wet, smooth floor is difficult.

Definitions:

1. Static friction – The friction that acts on an object at rest when a force is applied but the object has not yet started moving. It balances the applied force until motion just begins.

2. Sliding friction – The friction that acts when one object is actually sliding over the surface of another.

3. Rolling friction – The friction that acts when one object rolls over the surface of another (like a ball or wheel).

Order of magnitude (smallest to largest):

$\text{Rolling friction} < \text{Sliding friction} < \text{Static friction}$

Key point: Sliding friction is slightly smaller than static friction—that is why it is easier to keep a body moving than to start it moving. Rolling friction is the smallest, which is why wheels and ball bearings are used to reduce friction.

Understand: Rolling friction is much smaller than sliding friction.

Analyze:

• Ball bearings contain small metal balls placed between the moving parts.
• These balls roll instead of sliding, converting sliding friction into rolling friction.

Conclusion:

• Because rolling friction is the least, ball bearings greatly reduce friction between the moving parts.
• This makes the machines and wheels turn smoothly, reduces wear and tear, and saves energy that would otherwise be lost in overcoming friction.

Understand: Friction has both advantages and disadvantages, so it is called a necessary evil.

Why it is 'necessary' (advantages):

• It allows us to walk without slipping.
• It lets vehicles move and stop using brakes.
• We can write with a pen or pencil because of friction.
• We can hold objects, tie knots, and light a matchstick because of friction.

Why it is an 'evil' (disadvantages):

• It wears out the soles of shoes, tyres, and machine parts.
• It produces heat and causes energy loss in machines.
• It slows down moving objects.

Conclusion: Since we cannot live without friction but it also causes damage and waste, friction is rightly called a 'necessary evil'.

Methods to reduce friction:

1. Using lubricants – Applying oil, grease, or graphite between moving parts forms a thin layer that stops the surfaces from directly rubbing, reducing friction.

2. Using ball bearings / rollers – Converting sliding friction into rolling friction (which is smaller).

3. Polishing surfaces – Making surfaces smoother reduces interlocking of irregularities.

4. Streamlining shapes – Giving vehicles, aeroplanes, and ships a special shape reduces friction due to air or water (fluid friction).

5. Using air cushions – In some machines, a layer of air is used between surfaces so they do not touch directly.

Conclusion: These methods lower friction, making machines efficient and reducing wear.

Definition: The frictional force exerted by fluids (liquids and gases) on objects moving through them is called fluid friction or drag.

Examples:

• A swimmer feels resistance from water.
• An aeroplane and birds feel resistance from air.

Fluid friction depends on:

1. The speed of the object with respect to the fluid (higher speed → more drag).
2. The shape of the object (a streamlined shape reduces drag).
3. The nature of the fluid (its density and how thick/thin it is).

Conclusion: To minimise drag, objects such as boats, aeroplanes, cars, and even the bodies of fish and birds are given a streamlined shape.

Understand: Grooves and treads make the surface rougher and increase gripping.

Analyze:

• Grooves on tyres and treads on shoe soles increase the friction between them and the ground.
• More friction means a firmer grip.

Conclusion:

• Tyres: The treads give a better grip on the road, especially on wet or slippery surfaces. This prevents skidding and helps in braking and controlling the vehicle.
• Shoe soles: The grooves increase friction so that we do not slip while walking or running.

This is an example of a case where friction is deliberately increased for safety.

Understand: The box is at rest even though a force is applied, so a friction force must be balancing the push.

Analyze:

• The friction acting on a body that is at rest but a force is applied to it is called static friction.
• Since the box does not move, the static friction must be equal in magnitude and opposite in direction to the applied force.

Conclusion: $\text{Static friction} = \text{Applied force (as long as the box does not move)}$

As the boy increases his push, the static friction also increases to keep the box in place—until the push exceeds the maximum static friction, and then the box just begins to move.

Understand: When two surfaces rub against each other, friction between them produces heat.

Analyze:

• The side of the matchbox is deliberately made rough to increase friction.
• When the matchstick head is rubbed quickly against this rough surface, a large amount of heat is generated due to friction.

Conclusion:

• This heat raises the temperature of the chemicals on the matchstick head to their ignition point.
• As a result, the chemicals catch fire and the matchstick lights up.

This shows a useful application of friction: producing heat to start a fire.

Understand: Walking depends on sufficient friction between our feet and the floor.

Analyze:

• Oil is a lubricant. When spilled, it fills the tiny irregularities on the floor and on the sole of the shoe.
• This forms a thin slippery layer that prevents the two surfaces from interlocking.
• As a result, the friction between the foot and the floor becomes very small.

Conclusion:

• With such low friction, the foot cannot grip the floor properly and slips easily.
• Therefore, walking on an oil-spilled floor is risky, and one may fall. This is why spilled oil should be cleaned immediately.