Friction
Class 8th Science CBSE Solution
- Fill in the blanks. (a) Friction opposes the ________ between the surfaces in contact with…
- Four children were asked to arrange forces due to rolling, static and sliding frictions in…
- Alida runs her toy car on dry marble floor, wet marble floor, newspaper and towel spread…
- Suppose your writing desk is tilted a little. A book kept on it starts sliding down. Show…
- You spill a bucket of soapy water on a marble floor accidentally. Would it make it easier…
- Explain why sportsmen use shoes with spikes.
- Iqbal has to push a lighter box and Seema has to push a similar heavier box on the same…
- Explain why sliding friction is less than static friction.
- Give examples to show that friction is both a friend and a foe.
- Explain why objects moving in fluids must have special shapes.
Exercises
Question 1.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 carrom board ________ friction.
(e) Sliding friction is ________ than the static friction.
Answer:
(a) Friction always opposes 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 carrom board reduces friction.
(e) Sliding friction is less than the static friction
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.
(a) rolling, static, sliding
(b) rolling, sliding, static
(c) static, sliding, rolling
(d) sliding, static, rolling
Answer:
Option (c)
The static friction > Sliding Friction > Rolling friction.
The sliding friction is less than static friction because in sliding friction, the interlinking of the two surfaces is for very short time. Thus, the two surfaces could not properly come in contact with each other. But this is not the case for static friction, as the two surfaces in static friction form the contact properly. And, Rolling friction is the friction or drag which opposes the motion when a body rolls. In the rolling motion, the interlinking of the two surfaces is for the least time among all three.
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 orderwill 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:
Option (a) i.e. wet marble floor, dry marble floor, newspaper and towel.
As the roughness of the surface increases, the friction force increases.
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.
Answer:
The frictional force is opposing force generated by two surfaces that contacts and slide against each other. The frictional force will act in a direction opposite to the direction of the motion the book i.e. downwards. Hence, frictional force will exert in upward direction when book is sliding downward.
Question 5.
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 would make it more difficult for us to walk on a soapy floor because layer of soap on the floor decreases the friction of the floor and we are more likely to slip on the smooth floor rather than walking properly.
Question 6.
Explain why sportsmen use shoes with spikes.
Answer:
The sports people use shoes with spikes, because spikes increase the friction with the ground and does not let the person slip or fall when running fast.
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 will have to apply larger force than Iqbal.
Because Seema’s box is heavier and Seema will have to apply greater force to overcome the larger friction in case of the heavier box.
Question 8.
Explain why sliding friction is less than static friction.
Answer:
The sliding friction is less than static friction because in sliding friction, the interlinking of the two surfaces is for very short time. Thus, the two surfaces could not properly come in contact with each other. But this is not the case for static friction, as the two surfaces in static friction form the contact properly.
Question 9.
Give examples to show that friction is both a friend and a foe.
Answer:
Friction acts friend:
1) If friction would not have been there, we would not have been able to walk.
2) Friction between the tip of the pen/pencil and a paper allows us to write.
3) It is necessary when you want to hold items and prevent slipping or sliding.
Friction as foe:
1) Tyres of cars and soles of shoes wear and tear out because of friction.
2) Friction between the different parts of machines produces heat. This can damage the machines.
3) Friction causes wearing of over-bridges at railway stations.
Question 10.
Explain why objects moving in fluids must have special shapes.
Answer:
When a body moves through a fluid, it experiences an opposing force which tries to oppose its motion through the fluid. Example, aeroplane in air, birds, ships in water etc.
This opposing force is known as the drag force. By giving objects a special shape, as shown in figure.
The force of friction acting on it can be minimized. Hence, it becomes easier for the body to move through the fluid.