NCERT Solution
Friction Chap 9 for Class 8
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REPRODUCTION IN ANIMALS NIMALS
Chap 6 |
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 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 smaller 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.
1. rolling, static, sliding
2. rolling, sliding, static
3. static, sliding, rolling
4. sliding, static, rolling
Answer
static, sliding, rolling
Reason —
- Static friction is usually the highest among the
three types because it's the force required to overcome the initial
inertia and get an object moving from rest.
- Once the object is in motion, the contact points
on the object do not get enough time to lock into the contact points of
the other surface. So, the sliding friction is slightly smaller than the
static friction.
- Rolling friction is least as rolling motion
reduces the contact area between the object and the surface, thus reducing
the friction. Hence, the decreasing order of friction 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 :
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
wet marble floor, dry marble floor,
newspaper and towel.
Reason — The force
of friction depends on several factors, including the roughness of the surfaces
and presence of moisture.
- Movement on towel offers high friction because
its surface is usually rough, providing significant resistance to motion.
- Newspaper is smoother than a towel but still
provides some resistance due to its texture.
- Dry marble is smoother than newspaper, so it has
less friction compared to the previous two surfaces.
- Wet marble has the least friction among the
listed surfaces because water acts as a lubricant, reducing the friction
between the car's tires and the surface of the wet marble.
Hence, the order of increasing
friction will be: wet marble floor < dry
marble floor < newspaper < towel.
Question 4
Suppose you’re 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 the book slides down on the
desk, the frictional force acts opposite to the direction of motion of book
(i.e., it acts in the upward direction).
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
Spilling soapy water on a marble
floor would make it more difficult to walk on the floor. When we push the floor
in the backward direction with our feet, the force of friction pushes it in the
forward direction and allows us to walk. The soap in the water creates a
slippery surface, reducing friction between our feet and the marble floor. As a
result, walking becomes more difficult on the slippery marble floor.
Question 6
Explain why sportsmen use shoes
with spikes.
Answer
Sportsmen use shoes with spikes to
run faster, improve their stability and avoid injury. By digging into the
ground, the spikes increase the surface area contacting the playing surface,
leading to greater friction. This increased friction enables faster running and
reduces the risk of slipping.
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 greater force
than Iqbal because her box is heavier and heavier object will have greater
frictional force with the surface of contact. This happens because friction is
caused by the interlocking of irregularities in the two surfaces. The force of
friction will increase if the two surfaces are pressed harder. Hence, more
force is required to push the heavier box.
Question 8
Explain why sliding friction is
less than static friction.
Answer
Static friction is the force
required to overcome the initial inertia and get an object moving from rest.
Sliding friction comes into action when the object is in motion. Here, the
contact points on the object do not get enough time to lock into the contact
points of the other surface. So, the sliding friction is smaller than the
static friction.
Question 9
Give examples to show that friction
is both a friend and a foe.
Answer
Friction as a friend:
1. We are able to walk on floor due to friction between floor
and our feet.
2. We are able to write due to friction between pen and paper.
3. An object in motion will never stop if there is no friction.
4. Without friction between tyres of automobiles and roads, they
could not be started, stopped or turned to change direction of motion.
5. Friction helps us to tie a knot.
Friction as a foe:
1. It wears out the materials like screws, ball bearing or soles
of shoes.
2. Friction produces heat and so the jar of mixer becomes hot
when it runs for a few minutes.
3. Friction slows down moving objects, hence, more force is
required.
4. Due to friction blades of scissors or knives become blunt.
5. Due to increased friction between tire and the road surface,
when the tire deflates, it becomes difficult to move the vehicle.
Question 10
Explain why objects moving in
fluids must have special shapes.
Answer
Objects moving through fluids
experience drag, a frictional force exerted by the fluids. Special shapes, such
as streamlined designs, are essential to minimize drag. These shapes reduce
disturbance and redirect fluid flow around the object, improving efficiency and
helping the object overcome resistance as it moves through the fluid.
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