Class 8 - Physics - Friction

Question1.

Fill in the blanks:

(a) Friction opposes the _____________ between the surfaces in contact with each other.

(b) Friction depends on the _____________ of surfaces.

(c) Friction produces __________.

(d) Sprinkling of powder on the carom board ________ friction.

(e) Sliding friction is ___________ than the static friction.

(a) Friction opposes the motion between the surfaces in contact with each other.

(b) Friction depends on the nature of surfaces.

(c) Friction produces heat.

(d) Sprinkling of powder on the carom board reduces friction.

(e) Sliding friction is lesser than the static friction.

Question2.

Four children were asked to arrange forces due to rolling, static and sliding frictions in a decreasing order.

Their arrangements are given below.

Choose the correct arrangement.

(a) Rolling, static, sliding

(b) Rolling, sliding, static

(c) Static, sliding, rolling

(d) Sliding, static, rolling

Correct option is: - (c) Static, sliding, rolling.

Question 3.

Alida runs her toy car on dry marble floor, wet marble floor, newspaper and towel spread on the floor.

The force of friction acting on the car on different surfaces in increasing order will be:

(a) Wet marble floor, dry marble floor, newspaper and towel.

(b) Newspaper, towel, dry marble floor, wet marble floor.

(c) Towel, newspaper, dry marble floor, wet marble floor.

(d) Wet marble floor, dry marble floor, towel, newspaper

Friction will be more if the surface is rough and it will be less if the surface is smooth.

Correct option is: - (a) wet marble floor, dry marble floor, newspaper and towel.

Question 4.

Suppose your writing desk is tilted a little. A book kept on it starts sliding down. Show the direction of frictional force acting on it.

Frictional force always acts always opposite to the direction of motion.

Force of friction will be always parallel to the surfaces in contact.

Question 5.

You spill a bucket of soapy water on a marble floor accidently. Would it make it easier or more difficult for you to walk on the floor? Why?

When soapy water falls on floor then the friction reduces therefore the chances to slip increases,

as a result it becomes difficult to walk. When there was no soapy water then there will be interlocking between the foot and the ground.

When the soapy water spills the interlocking gets spoilt as a result friction reduces.

Question 6.

Explain why sportsmen use shoes with spikes.

Spikes make the surface of the shoes rough. This results in increase of friction.

As friction increases it becomes easy for them to walk or run as their grip on the ground increases.

Question 7.

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?

Friction is directly proportional to mass. Heavier an object more force to be applied to move it.

Therefore heavier box will press floor surface harder as compared to the lighter box.

This implies heavier box will experience more friction, so more force have to be applied to overcome friction.

So Seema has to apply more force as compared to Iqbal.

Question 8.

Explain why the sliding friction is less than the static friction.

During sliding, contact points do not get enough time to get interlocked properly.

Therefore, less interlocking and less friction.

But, when the object is at rest, the surfaces interlock well, therefore friction is more.

This shows sliding friction is less than static friction.

Question 9.

Give examples to show that friction is both a friend and a foe.

Friction is a friend

1. Helps us to walk.
2. Helps us to hold things.
3. Stop a moving object.
4. Helps to start /stop a vehicle
5. Helps in grinding
6. Helps in construction purposes.
7. Helps us to write.
8. Makes matchstick ignition possible.

Friction as a foe

1. Opposes motion.
2. Friction produces lot of heat, results in wastage of energy.
3. Wear & tear of machinery parts.
4. More power to be exerted in machines.
5. Noise produced in machines.

Question 10.

Explain why objects moving in fluids must have special shapes.

Special shapes experiences less fluid friction therefore can move easily through fluids.

Foreg: - In case of aeroplane when it is moving through the air, the air will exert some friction.

This frictional force will be opposed by the motion of the aeroplane.

In order to overcome this friction aeroplane have to apply a lot of force to overcome it.

This will result in waste of energy.

Keeping all these things in consideration the shape of the aeroplane is in such a way that it experiences lesser fluid friction.