When we try to push a very heavy box kept on ground, it does not move at all. Which force is preventing this box to move forward? Where does this force act?

Suppose your writing table (or desk) is tilted a little. A book kept on the table starts sliding down. Draw a diagram to show the direction of force of friction acting on the book.

Which will cause more friction: a rough surface or a smooth surface? Why?

Explain why sliding friction is less than static friction.

What is meant by 'rolling friction'?

Iqbal has to push a lighter box and Seema has to push a similar heavier box on the same floor. Who will have to apply a larger force and why?

Why does a man slip when he steps on a banana peel thrown on the road?

Car wheels often spin on icy roads. Explain why.

Explain why:

(a) A pencil will write on paper but not on glass.

(b) climbing a greasy pole is very difficult.

Why does a matchstick light when we strike it on a rough surface ?

Why is it difficult to light a matchstick by striking it on a smooth surface ?

Which parts wear away first in shoes? Give a reason for your answer.

Why do brake pads of bicycles have to be replaced quite often?

A pencil eraser loses tiny pieces of rubber each time you use it. Why does this happen ?

What happens when you rub your hands vigorously for a few seconds ? Why does this happen ?

Explain how, friction enables us to walk without slipping.

Which is easier to hold in hand: a kulhar (earthen pot) or a glass tumbler? Why?

How does a bicycle stop when its brakes are applied ?

Explain why, the soles of our shoes wear out gradually.

Why do tyres of cars wear out gradually ?

State two advantages and two disadvantages of friction.

Explain why, sportsmen use shoes with spikes.

How will you reduce friction between those machine parts which rub against each other? Give the simplest method.

What is meant by lubrication ? Why is it important?

Explain why, wheels are so useful.

Why are lubricants (oil or grease) applied to rubbing surfaces of machines?

Explain with the help of diagrams, how the use of oil reduces friction between two surfaces in contact with each other.

Why are cars, aeroplanes and rockets streamlined ?

Explain why, a speedboat has a streamlined shape.

What are fluids? Name two common fluids.