Riya gives a gentle push to her science book on a table. The book moves forward for some distance and then slowly comes to rest, even though nobody touches it after the initial push. She wonders why the book does not go on moving forever.

1. (a) Which force acts on the moving book to bring it to rest?
2. (b) In which direction does this force act with respect to the motion of the book?
3. (c) Is this force a contact force or a non-contact force? Why?

During cleaning, the marble floor in Aman’s house becomes wet and very smooth. Aman tries to walk quickly on it and slips. He realises that it is much harder to walk safely on the wet floor than on a dry, rough floor.

1. (a) Why does Aman slip more easily on the wet marble floor?
2. (b) What would happen to the friction between his feet and the floor when it is wet?
3. (c) Suggest one safety measure to reduce the chances of slipping on such a floor.

Two children try to push a heavy almirah lying at rest. They push it gently but it does not move. When their father joins and all three push together with greater force, the almirah finally starts moving slowly across the floor.

1. (a) Which type of friction acts on the almirah before it starts moving?
2. (b) Why does the almirah finally move when a larger force is applied?
3. (c) What happens to friction once the almirah starts sliding on the floor?

Kavya wears the same pair of shoes on three different surfaces: smooth tiles, a rough playground, and a carpeted floor. She notices that it is easier to run without slipping on the rough playground and carpet than on the smooth tiles.

1. (a) On which surface is friction the least? How do you know?
2. (b) How does roughness of a surface affect friction?
3. (c) Why are sports shoes usually made with rough soles?

Mohan pushes a wooden box lying on the floor. Initially he finds it very difficult to start the motion. After the box starts moving, it becomes slightly easier to keep it in motion with the same force.

1. (a) Which type of friction resists the motion before the box starts to move?
2. (b) Why does it become easier to move the box once it is already sliding?
3. (c) Write the correct order: static friction, sliding friction, rolling friction (from highest to lowest).

At the railway station, Ankit first tries to drag his suitcase without using its wheels and finds it very hard to pull. Then he opens the handle and pulls the suitcase on its small wheels. Now it becomes much easier to move the same suitcase.

1. (a) When the suitcase is dragged without using the wheels, which type of friction is involved?
2. (b) Which type of friction acts when the suitcase is pulled on its wheels?
3. (c) Why is it easier to move the suitcase on wheels than to drag it?

A wooden block is kept on a rough table. A spring balance is used to pull the block. The reading gradually increases as the pulling force is increased, but the block does not move. At a certain reading, the block just begins to move and then starts sliding with a nearly constant reading.

1. (a) What is the frictional force equal to before the block starts moving?
2. (b) What is the name of the maximum value of static friction just before motion starts?
3. (c) Why is the reading on the spring balance slightly less while the block is sliding than at the instant it just starts to move?

To move a very heavy machine in a workshop, workers place several cylindrical iron rods under it. The machine is then pushed, and it rolls over the rods. Whenever a rod comes out behind, it is shifted again to the front.

1. (a) Which type of friction is used in this method of moving the machine?
2. (b) Why is this method better than dragging the machine without rollers?
3. (c) Name another device used in machines to convert sliding friction into rolling friction.

During a sports meet, athletes wear spiked shoes while running. On the other hand, players playing carrom sprinkle powder on the board to make the coins move smoothly. In both situations, friction plays an important role, but in different ways.

1. (a) Why do athletes wear spiked shoes while running?
2. (b) Why is powder sprinkled on a carrom board before playing?
3. (c) In which of the two cases is friction being increased and in which is it being decreased?

Meera writes with a ball pen in her notebook and also writes with a piece of chalk on the blackboard. In both cases, marks are left behind, forming letters and diagrams. She realises that without this behaviour, writing would not be possible.

1. (a) Which force helps in leaving marks on the paper or blackboard?
2. (b) How does friction help a pen to write on paper?
3. (c) How does friction help chalk to leave marks on the blackboard?

A mechanic uses a spanner with a rough handle to tighten a nut. A student uses a pencil with a rubber grip to write for a long time. Both find that they can hold these objects firmly without slipping.

1. (a) Why are tool handles and grips often made rough?
2. (b) How does friction help in holding a tool or pencil firmly?
3. (c) What might happen if the handle or grip becomes smooth and oily?

A car mechanic shows Rahul two tyres – one new and one old. The old tyre has almost smooth surface with worn-out treads, while the new tyre has deep grooves. The mechanic advises the car owner to replace the old tyres for safety.

1. (a) Why do tyres become smooth after long use?
2. (b) How does friction cause this change in tyres?
3. (c) Why are smooth, worn-out tyres unsafe on the road?

In a small workshop, a machine is run for a long time without proper lubrication. After some time, the worker notices that its moving metal parts become very hot and produce a burning smell. The machine stops working smoothly.

1. (a) Why do the machine parts become hot when used for a long time without lubrication?
2. (b) Which form of energy is produced due to friction between the machine parts?
3. (c) How can this problem be reduced or prevented?

On a cold winter morning, students in a school assembly rub their hands together to feel warm. After rubbing for some time, they feel that their palms have become hotter than before.

1. (a) Which force acts between the two palms while rubbing?
2. (b) Why do the palms become warm after rubbing?
3. (c) Is this heating effect of friction always useful? Give one example where it is harmful.

Ravi notices that his bicycle chain and gear make a loud noise and the pedalling feels hard. A mechanic cleans the chain and applies oil to it. After this, the bicycle runs more smoothly and quietly.

1. (a) Why was pedalling hard and noisy before oiling the chain?
2. (b) How does oil reduce friction between the moving parts?
3. (c) Mention one more place where lubrication is used to reduce friction.

Gymnasts and weightlifters often apply chalk powder on their hands before performing. This helps them hold the bar or weights firmly and prevents slipping, even when their hands are sweaty.

1. (a) What is the purpose of applying chalk powder on the hands?
2. (b) Does chalk powder increase or decrease friction? How?
3. (c) Give one more example where friction is increased for safety or better grip.

Inside the ceiling fan, the rotating shaft is fitted with ball bearings. These bearings contain small steel balls between two rings. This arrangement allows the fan to rotate smoothly for a long time with very little noise.

1. (a) Why are ball bearings used in the fan instead of having the shaft directly rubbing against the casing?
2. (b) How do ball bearings reduce friction?
3. (c) Name one more machine where ball bearings are commonly used.

A cyclist riding fast on a highway feels a strong force of air against his body. To reduce this effect, he bends forward and keeps his body close to the bicycle, which helps him cycle faster with the same effort.

1. (a) What is the name of the force of air acting against the motion of the cyclist?
2. (b) Why does bending forward help the cyclist to move faster?
3. (c) Which type of friction is involved here – solid friction or fluid friction?

A science teacher shows pictures of a fish and a ship. Both have bodies that are narrow in the front, broader in the middle and tapering at the back. The teacher explains that this shape helps them move easily through water.

1. (a) What is the name of this special shape?
2. (b) How does this shape help in reducing fluid friction?
3. (c) Give one example of a man-made vehicle (other than a ship) that uses this shape to reduce friction.

During an air show, a parachutist jumps from an aeroplane. At first he falls quickly, but when he opens his parachute, his speed decreases and he comes down slowly and safely towards the ground.

1. (a) Why does the parachutist fall quickly at first before opening the parachute?
2. (b) What change occurs when the parachute is opened in terms of air resistance?
3. (c) How is fluid friction (air resistance) useful in this situation?