Class 8 Science

# Friction

## NCERT Solution

### Question 1: Fill in the blanks:

1. Friction opposes the _____________ between the surfaces in contact with each other.
2. Friction depends on the _____________ of surfaces.
3. Friction produces __________.
4. Sprinkling of powder on the carrom board ________ friction.
5. Sliding friction is ___________ than the static friction.

Answer: (a) movement, (b) irregularities, (c) heat, (d) reduce, (e) less

Question 2: 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.

1. Rolling, static, sliding
2. Rolling, sliding, static
3. Static, sliding, rolling
4. Sliding, static, 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:

1. Wet marble floor, dry marble floor, newspaper and towel.
2. Newspaper, towel, dry marble floor, wet marble floor.
3. Towel, newspaper, dry marble floor, wet marble floor.
4. Wet marble floor, dry marble floor, towel, newspaper

Answer: (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.

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?

Answer: The soapy water would make a thin layer over the marble floor. This will reduce friction. Hence, walking on the floor shall be difficult in that case.

Question 6: Explain why sportsmen use shoes with spikes.

Answer: Shoes with spikes offer more friction because of more irregularities in the surface. This gives better grip to sportsmen while running.

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?

Answer: Seema has to apply larger force, because friction increases with increase in weight.

Question 8: Explain why the sliding friction is less than the static friction.

Answer: When something is static, a greater force is required to break the interlocking between two surfaces. When something is in motion, there is no need to break the interlocking and a smaller force is required the keep the object in motion. Hence, sliding friction is less than static friction.

Question 9: Give examples to show that friction is both a friend and a foe.

Answer: Following two examples show that friction is both a friend and a foe:

• Because of friction a vehicle can be stopped and we can walk, thus friction is a friend.
• Friction wears out the parts of machine, thus friction is a foe.

Question 10: Explain why objects moving in fluids must have special shapes.

Answer: The streamlined shape encounters less resistance while moving through a fluid. This is not true for other shapes. So, objects moving in the fluids must have special shapes like boat or fish.