Chapter 12 - Friction
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 opposes the relative 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 smaller/lesser 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: 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
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.
Answer: When book slides down on the desk, friction acts between the book and surface of the desk. The direction of friction on the book is opposite to the direction of its motion and act in an upward direction.
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 layer of soap makes the floor smooth due to which the friction is reduced. This makes the slippery and the foot cannot make a proper grip on the floor. Therefore, it is difficult to walk on the floor. We may slip on the floor.
Question 6: Explain why sportsmen use shoes with spikes.
Answer: Sportsmen use shoes with spikes to increase friction between shoes and the surface. So the shoes with spikes do not slip while sportsmen play.
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: Due to interlocking of the irregularities on the two surfaces in contact the force of friction takes place. On floor, when a heavy object is placed, the interlocking of irregularities on the surface of the box and the floor becomes strong. This because the two surfaces in contact are pressed harder. Due to this, force is required to overcome the interlocking. Therefore, to push the heavier box, Seema has to apply more force than Iqbal.
Question 8: Explain why sliding friction is less than static friction.
Answer: When irregularities present in surfaces of two objects in contact get interlocked with each other, friction comes into play. The time given in sliding for interlocking is very small. Therefore, less force is required to overcome this interlocking. And hence, sliding friction is less than static friction.
Question 9: Give examples to show that friction is both a friend and a foe.
Answer:
• Friction is a friend because:
→ It helps us to walk.
→ We can write because of friction.
→ It helps us to light matchstick.
→ We can catch or grip anything because of friction.
→ We can slow down the speed of our vehicles because of friction.
→ It helps to generate heat.
• Friction is a foe because:
→ It causes wear and tear of machines.
→ Due to the friction, the soles of shoes get wear out.
→ Friction acts in opposite direction, which causes the speed of object to decrease.
→ When a machine is operated, heat generated causes much wastage of energy.
→ Two rough surfaces when rubbed against each other cause noise pollution.
→ Due to excessive friction, more fuel is required to run vehicles.
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. This opposing force is known as drag force. This frictional force depends on the shape of the body. By giving the objects a special shape, the friction acting on it can be minimised. Therefore, it becomes easier for a body to move through the fluid.
Question 11: Define:
a) Drag
b) Friction
c) Lubricants
d) Streamlined
Answer:
a) The frictional force exerted by fluids is called drag.
b) It is a force which opposes the motion of object over a surface.
c) The substances which reduce friction are called lubricants.
d) It is a shape in which the body is pointed at the ends and broad in the middle.
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