Q1.  Fill in the blanks.

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

Answer-relative motion

(b) Friction depends on the __________of surfaces.

Answer- nature

(c) Friction produces _________.

Answer-heat

(d) Sprinkling of powder on the carrom board __________friction.

Answer-reduce

(e) Sliding friction is _________than the static friction.

Answer-less

Q2. Four children were asked to arrange forces due to rolling, static and sliding friction 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

Answer- (c) static, sliding, rolling

Q3. Alida runs her toy car on a 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

Answer- (a) wet marble floor, dry marble floor, newspaper and towel.

Q4. 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.

Answer-

When my writing desk is tilted a little and my book starts sliding down kept on it then there will act a frictional force between the book and the surface of the desk. The direction of the frictional force will act opposite to the motion of the sliding book. If the book is sliding down then the frictional force will act oppositely upward.

Q5. You spill a bucket of soapy water on a marble floor accidentally. Would it make it easier or more difficult for you to walk on the floor? Why?

Answer-It is difficult to walk on a soapy marble floor because the friction between the feet and the marble floor is reduced due to the layer of soap which makes the floor smooth.

Q6. Explain why sportsmen use shoes with spikes.

Answer- Sportsmen use shoes with spikes because it provides better grip while running due to the increase in the force of friction between the shoes and ground. Hence, it helps sportsmen to run and play without getting a slip.

Q7. 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?

Answer- Seema will have to apply a larger force to push the similar heavier box because it produces more friction between the surface of the box and floor because of interlocking of both surfaces with each other due to presence of irregularities of surfaces in contact.

Therefore, when the heavier object is placed on the floor then the force of interlocking due to irregularities between the surfaces in contact become stronger. So, to overcome this force, Seema has to apply a larger force to push that box.

Q8. Explain why sliding friction is less than static friction.

Answer– The sliding friction is less than static friction because interlocking due to irregularities of two surfaces is less when an object is in motion (sliding). The irregularities on the surfaces don’t get enough time to be locked with each other. Thus, interlocking is not much stronger here as compared to static friction case. Hence, sliding friction is less than static friction.

Q9. Give examples to show that friction is both a friend and a foe.

Answer- Friction is both a friend and a foe.

Examples to show that friction behaves as a friend-

• It helps us to walk.
• It helps us to write.
• It helps to stop the moving car while applying a brake.

Examples to show that friction behaves as a foe-

• It causes wear and tear.
• It produces heat in moving part of machines which can damage the machine.

Q10. Explain why objects moving in fluids must have special shapes.

Answer- When an object moves in a fluid, it encounters an opposing force on it which wants to oppose the motion of the moving object. This frictional force is known as a drag force. Also, the frictional force depends on the shape of the object. Hence, giving some special shapes to that object minimises the frictional force acting on it. Thus, that special-shaped object can travel easily through a fluid.

Example- Streamlined shape- This is a special shape of an object in which front is pointed and the middle portion is little broader which further gets tapered at the back.