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 __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 :
(c) static, sliding, rolling
Friction comes
into play when irregularities present in the surfaces of two objects in contact
get interlocked with each other. Static friction comes into play when we try to
move an object which is at rest. Sliding friction comes into play when an
object slides over the surface of another object. In sliding friction, the time
given for interlocking is very small. Hence, interlocking is not strong.
Therefore, less force is required to overcome this interlocking. Because of
this reason, sliding friction is less than static friction. Similarly, the area
of contact in case of rolling friction is smallest as compared to static or
sliding friction. This area of contact changes gradually because of rolling.
Hence, rolling friction is lesser than both static and sliding friction. Thus,
the correct sequence is—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
Force of
friction depends on the nature of surfaces in contact. The rougher the surface,
the more is the friction between the surfaces in contact and vice-versa.
Roughness present in the given surfaces can be arranged in an ascending order
as wet marble floor, dry marble floor, newspaper and towel. Hence, the correct
sequence of these surfaces when arranged according to the increase in the force
of friction acting on the car is—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 a
book slides on the writing desk, a frictional force acts between the book and
the surface of the desk. The direction of frictional force on the book is
opposite to the direction of its motion and acts in upward direction, as shown
in the following figure.
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 :
We are
able to walk because of the friction present between our feet and the ground.
In order to walk, we push the ground in the backward direction with our feet.
The force of friction pushes it in the forward direction and allows us to walk.
The force of friction between the ground and feet decreases when there is soapy
water spilled on the floor. Hence, it becomes difficult to walk on the soapy
floor.
Question 6:
Explain why sportsmen use shoes with spikes.
Answer :
Sportsmen use shoes with spikes
because these shoes give them a better grip while running. This is because the
force of friction between the shoes and the ground increases with the help of
spikes.
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 :
Force of
friction arises because of interlocking of irregularities on the two surfaces
in contact. When a heavy object is placed on the floor, the interlocking of
irregularities on the surfaces of box and floor become strong. This is because
the two surfaces in contact are pressed harder. Hence, more force is required
to overcome the interlocking. Thus, to push the heavier box, Seema has to apply
a greater force than Iqbal.
Question 8:
Explain why sliding friction is less than static friction.
Answer :
Friction comes into play when
irregularities present in the surfaces of two objects in contact get interlocked
with each other. In sliding, the time given for interlocking is very small.
Hence, interlocking is not strong. Therefore, less force is required to
overcome this interlocking. Because of this reason, sliding friction is less
than static friction.
Question 9:
Give examples to show that friction is both a friend and a
foe.
Answer :
Advantages of friction:
(i) We are able to walk because of
friction.
(ii) Friction between the tip
of the pen and a paper allows us to write.
Disadvantages of friction:
(i) Tyres and soles of shoes
wear out because of friction.
(ii) Friction between the
different parts of machines produces heat. This can damage the machines.
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 the drag force. This
frictional force depends on the shape of the body. By giving objects a special
shape, the force of friction acting on it can be minimised. Hence, it becomes
easier for the body to move through the fluid.
SOURCE- NCERTHELP.IN
