EXERCISE
A. Objective Questions
1. Write TRUE or FALSE for each statement.
a) The frictional force acts in the direction of motion of
body. FALSE
b) The unit of weight is kilogram. FALSE
c) A force can change the direction of motion of a moving
body. TRUE
d) A force increases the mass of the body when applied on it.
FALSE
e) The force of friction is always disadvantageous. FALSE
f) The sliding friction is more than the rolling friction. FALSE
g) Liquids offer more friction than the gases. TRUE
h) A wet oily road offers more friction than a dry rough
road. FALSE
2. Fill in the
blanks
a) Force
is applied as push or pull.
b) On squeezing a gum tube, its shape changes.
c) On pulling a string its length
increases.
d) A moving football when
kicked, its direction of motion changes.
e) On applying brakes on a
moving car its speed slows down.
f) We use ball bearing to reduce the
friction.
g) Friction opposes
motion.
h) Lubricants are used to reduce
friction.
3. Match the following columns:
Column A Column B
a) Non contact force i) repel
b) Like poles ii) kg
c) Contact force iii) Gravitational force
d) Mass iv) wear and tear
e) Weight v) force of friction
f) Friction vi) kgf
Ans. a-(iii),
b-(i), c-(v), d-(ii), e-(vi), f-(iv)
4. Select the
correct alternative.
i) The body falls
downwards because of
a) electrical force b)
gravitational force
c) mechanical force d) magnetic force
ii) A force does not
change
a) mass b)
length c) shape d) state of motion
iii) A force is to be expressed correctly requires
a) only the magnitude b)
only the direction
c) Both d)
None
iv) Friction
a) promotes motion b) opposes motion
c) acts in the direction of motion d) it always a nuisance
v) Friction is
reduced by
a) making the surface
weight b) making the surface dry
c) making the surface rough d)
sprinkling sand on the surface
vi) Friction is
increased if
a) an oil is sprayed b)
the surface are made wet
c) the surface are made
dry d) the surface are polished
B. Short/Long answer
questions
1. Name the
term used for push or pull.
The push or pull is termed as force.
2. Give one example each of a force as (i) a
push, (ii) a pull, (iii) a stretch
and (iv)
a squeeze.
(i) A push: When we
move a trolley the force applied as a push.
(ii) A pull: When we open a door the force applied as a
pull.
(iii) A stretch: When a rubber expands the force applied as a
stretch.
(iv) A squeeze: When we use a tooth paste the force applied
as a squeeze
3. Explain the meaning of the term force.
Force is the cause which changes the state of motion or the
state of rest of an object. It can also changes the size and shape of a body.
The force is expressed by both its magnitude and direction. The S.I unit of
force is Newton.
4. What effect can a force have on a stationary
body?
When a force applied on a stationary body the body begins to
move.
For example a car originally at rest when pushed, begins to
move.
5. What effect can a force have on a moving
body?
The force applied on a moving body can have the following
effects.
a) It can stop or slow down the moving body if applied in the
direction
opposite to the motion of the moving body.
b) It can make the moving body move faster if applied in the
direction
of motion of the moving body.
c) It can change the direction of the moving body if applied
in a
direction other than the direction of the moving body.
6. What effect can a force
produce on a body which is not allowed to
move?
When a force applied on a body which cannot move, it gets
deformed.
The size and shape of the body changes. For example, when we
squeeze a piece of rubber, its shape changes. Also when we
stretch
a rubber string its length increases.
7. Give one example each to indicate that the
application of a force
a) produces motion b) stops motion c) slows down motion
d) changes the direction of motion e) deforms a body
a) Produces motion: A car which is originally at rest when
pushed, it
begins to move.
b) Stops motion: A moving bicycle stops, when the brake is
applied.
c) Slows down motion: The speed of a moving vehicle is slowed
down by applying brakes.
d) Changes the direction of motion: When a player kicks a
moving
football its direction changes.
e) Deforms a body: On stretching a rubber string its length
increases.
8. State the
effect produced by a force in the following cases.
a) The sling of
a rubber catapult is stretched.
b) A man pushes
a heavy cart
c) A player
uses his tricks to deflect the ball.
d) A cyclist
applies brakes.
e) A spring is
compressed.
a) The shape and size of the
catapult changes. The length of the
rubber increases.
b) The cart begins to move.
c) The direction of movement
of the ball changes.
d) The moving cycle stops or
its speed slows down.
e) There is a change in the
shape and size of the spring.
9. Name the two kinds of force in nature.
The two kinds of force in nature are
(i) contact force, Example: Muscular force and Frictional
force etc.
(ii) Non contact force, Example: Gravitational force,
Magnetic force
and electrostatic force etc.
10. Name
the types of force which acts in the following cases:
a) A coolie lifts a luggage.
Muscular force
b) A bicycle comes to rest slowly when the
cyclist stops pedaling.
Frictional force
c) A stone falls from a roof
Gravitational force
d) A comb rubbed with silk attracts bits of
paper
Electrostatic force
e) A string hangs with a load
Force of tension
f) A horse moves a cart
Muscular force
g) A magnet attracts an iron pin
Magnetic force
h) A boy opens a door
Muscular force
i) An apples fall from a tree
Gravitational force
j) A man rows a boat
Muscular force
11. What do you mean by the gravitational force? Give an example
to illustrate it.
Gravitational force is the force of attraction between two
bodies at a
distance, such as the force of attraction between two
heavenly bodies.
Example: When we through a ball upward it goes to a certain
height
and then returns back to the ground.
12. Define the term “weight of a body”.
The weight of a body is the force with which the earth pulls
the
body. It is expressed in gravitational unit kgf or gf.
13. What do you
understand by the term friction?
Friction is the force that opposes the relative motion
between the two
surfaces in contact with each other. The word friction is
derived from
the Latin word “fricare” which means to “rub”.
14. Give an
example to illustrate the existence of force of friction.
If we stop pedalling a bicycle, it slows down and finally
stops after
travelling a certain
distance. It stops because of the force of friction
between the bicycle tyre and the road. Again to start the
bicycle we
have to apply a force on the pedal greater than the force of
friction
between the tyres and the road. This proves the existence of
force of
friction.
15. What is the cause of friction?
The interlocking of the irregular projections on the two
surfaces in
contact causes friction.
16. State two factors which directly affect the
force of friction.
The two factors that directly affect the force of friction
are
i) The smoothness of the surface
ii) The weight of the moving body on the surface
17. In which case there will be more friction
between the truck and the
road: when the truck is empty or when it is
loaded?
When the truck is loaded.
18. Which offers more friction on a body: a
glass surface or a wooden
surface?
A wooden surface.
19. Name the three kinds of friction.
The three kinds of friction are
i) Static friction
ii) Sliding friction
iii) Rolling friction
20. List three disadvantages of friction.
Three disadvantages of friction are
i) Friction decreases the efficiency, because it opposes the
motion
of a body.
ii) Friction causes wear and tear in the moving parts.
iii) Friction produces heat.
21. When you
apply the brakes, the bicycle stops and the rim of the
wheel becomes hot. Explain the reason.
When we apply the brakes of a bicycle, the brakes come in
contact
with the wheel rim and friction takes place between the
brakes and
the rim. Due to this frictional force heat is produced and
the rim
becomes hot.
22. The eraser gets smaller and smaller as you
use it more and more.
Explain the reason.
When we use an eraser it’s surface gets in contact with the
surface
we erase and friction occurs. This friction causes wear and
tear of
the eraser. Due to this the eraser gets smaller and smaller
as we use
it more and more.
23. List three ways of reducing friction
Three ways to reduce friction are
i) By making the surface smooth
ii) By using lubricants
iii) By using ball bearings
24. It is difficult to open an inkpot with
greasy or oily hands. Explain.
When the hands are greasy or oily, then the oil or grease act
as a
lubricant which help in reducing the friction. As the
frictional force
become less, it is difficult to open the inkpot.
25. It is difficult to walk on a wet road.
Explain.
When a road becomes wet, the frictional force reduced due to
lubrication and the road becomes slippery. Due to this it is
difficult to
walk on a wet road.
26. Give three examples to illustrate that
friction is a necessary evil.
Most of our daily activities depend on the existence of
friction. The
friction less world will not be at all convenient. Following
examples
show that friction is a necessary evil.
Example 1: Friction helps us to stop a vehicle
by applying brakes.
Without friction a moving vehicle cannot be stopped and will
lead to
disaster.
Example 2:
In the absence of friction the roads will become slippery
and we would not be able to
walk on the road.
Example 3:
Friction is necessary to burn a matchstick.
27. Define (i)
static friction (ii) sliding friction and (iii) rolling friction.
(i) Static friction: The
maximum opposing force between the object
and the surface in contact, which makes the object remain
stationary even on applying the external force is known as
static
friction. It is self-adjusting.
(ii) Sliding friction: When
an object begins to slide on a surface, the
force exerted by the surface on the object is called the
sliding
friction. It is the force required to keep the object in
motion over
the surface. It is less than the static friction.
(iii) Rolling friction: When
an object rolls over a surface, the force which
opposes the rolling motion of the object is called the
rolling friction.
Rolling friction is less than the sliding friction.
28. Arrange the
following in descending order of magnitude.
(i) static
friction (ii) sliding friction (iii) rolling friction
Static friction > Sliding friction > Rolling friction
29. A body
needs a force F1 just to start motion on a surface, a force F2
to continue its
motion and a force F3 to roll on the surface. What is
(i) the static
friction (ii) sliding friction and (iii) rolling friction?
State whether F2
is equal, less than or greater than (i) F1 and (ii) F3.
(i) Static friction is F1
(ii) Sliding friction is F2
(iii) Rolling friction is F3
F2 is less than F1 and F2 is
greater than F3
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Extra
Questions and Answers
A. Objective Questions
1. Fill in the
blanks
(a) Friction depends on the weight
of the moving object.
(b) Shoes and tyres have treads on the surface to increase the friction
with the road.
(c) Rolling friction offers less
resistance than sliding friction.
(d) Movement of a body is obtained as a result of a push or a pull.
(e) Force is the
cause which tends to result in the movement of a body.
(f) When a force applied on a body which is not allowed to
move. It
gets deformed.
(g) A force is expressed by stating both its magnitude and direction.
(h) Two forces acting on a body in the same direction get
added to give
the resultant force.
(i) Two forces acting on a body in the opposite direction
get subtracted
to give the resultant force.
(j) The S.I unit of force is Newton
(N)
(k) The gravitational units of force are kgf and gf.
(l) 1kgf = 1000 gf and 1 kgf = 10N
(nearly).
(m) Depending upon the application of forces they are
classified in to
Contact forces and non-contact
forces.
(n) The force of attraction between two heavenly bodies is
known as
gravitational force.
(o) Like poles repel
each other while the unlike poles attracts
each other.
(p) Gravitational force
is always attractive.
(q) The electrostatic
and magnetic forces can be attractive
as well as
repulsive.
(r) When the distance of separation is doubled the magnitude
of
Non-contact force becomes one
fourth.
(s) The magnitude of non-contact
forces depends on the distance of
separation .
(t) Friction is the
force of contact.
(u) Greater the weight of the moving body more is the force of friction.
(v) The three kinds of friction are static friction, sliding
friction and
rolling friction.
(w) We use solid lubricants in the form of powders.
(x) When two forces act on a body which are equal in
magnitude, but
opposite in direction, the net force on the body is Zero.
(y) When two forces act in opposite directions, the net force
is equal to
the difference of
these forces, in the direction of the bigger
force.
(z) The sliding friction is less
than the static friction but more
than the
rolling friction.
2. Name the following:
a) The force which acts on a body by making an actual contact
–
Contact force
b) The force applied as a push or pull – Muscular force
c) The force acting on a string when a load is suspended from
it –
Tension force
d) Force between two charged bodies at a distance – Electrostatic force
e) Force between the poles of a magnet at a distance – Magnetic force
f) The force that opposes the relative motion between two
surfaces in
contact with each other – Friction
g) In machines the moving parts gradually wear out due to – Friction
h) The process of giving shape with pointed ends is called – Streamlining
i) A body not changing
its position with time with respect to a nearby
fixed object is said to be – At rest.
j) A body changing its position with time with respect to a
nearby
fixed object is said to
be – In
motion.
3. Correct the underlined words.
a) Force of friction acting between two surfaces can be
zero.
Force of friction acting between two surfaces
cannot be zero.
b) A force changes the mass of the body on which it is
applied.
A force does not changes the mass of the body
on which it is applied.
c) We can see a force.
We cannot see a force, we can see or feel the
effect of force.
d) The frictional force is a non-contact force.
The frictional force is a contact force.
e) Unlike charges repel each other
Unlike charges attracts each other.
f) Gravitational force can be attractive or repulsive.
The electrostatic and magnetic forces can be
attractive or repulsive.
g) The magnitude of frictional forces depends upon the
distance of
separation.
The magnitude of non-contact forces depends
upon the distance of
separation.
h) Friction always acts in the same direction of
motion.
Friction always acts in the opposite direction
of motion.
i) The force of friction is more between smooth surfaces.
The force of friction is more between rough
surfaces.
j) The force of friction is still less between a solid and
a solid.
The force of friction is still less between a
solid and a gas.
k) Static friction is less than rolling friction
but more than sliding friction.
Sliding friction is less than static friction
but more than rolling friction.
l) The force of friction depends on the area of the
surface in contact.
The force of friction depends on the smoothness
of the surface in
contact.
4. Match the following columns.
Column A Column B
i) S.I. unit of force a) Contact force
ii) A force can not change b)
Reduce friction
iii) Force of tension c) Newton
iv) Unlike poles d) Produces heat
v) Gravitational force e) mass of the body
vi) Friction
f) Attract
each other
vii) Rolling motion g) Is always attractive
viii) Zero friction h) kgf
ix) Lubricants i) Practically not possible
x) Weight j) Ball
bearings
Ans.
i-c, ii-e, iii-a, iv-f, v-g, vi-d, vii-j, viii-i, ix-b, x-h
B. Answer in brief
1. Give three examples of force as push.
Examples of force as push are
a) To open a door we push it.
b) To move a trolley, it is push it.
c) If a car does not start by its engine we have to push it
to move.
2. Give three examples of force as pull.
Examples of force as pull are
a) A gardener pulls the grass roller to move it.
b) To move a goods trolley a coolie needs to pull it.
c) We pull a door to open it.
3. What is a balanced force?
If a force applied does not change the state of the body from
rest to
motion or from motion to rest, it is called a balanced force,
4. What is an unbalanced force?
If the force applied changes the state of the body from rest
to motion
or from motion to rest, it is called an unbalanced force.
5. If two arrows one with a sharp or pointed
tip and other with a blunt or
flat tip are thrown in to air, then which will
cover larger distance and
why?
The arrow with sharp or pointed tip will cover a larger
distance,
because it will experience less friction than the arrow with
blunt tip.
6. Explain the types of friction with the help
of a flow chart.
7. Explain with example that a force can move a
body originally at rest.
When a force is applied on a body at rest it begins to move.
Example 1: When we push a car originally at rest it begins to
move.
Example 2: When we pull a grass roller originally at rest it
begins to move.
Example 3: When we kick a ball lying on the ground it begins
to move.
8. Explain with example that a force can stop a
moving body.
When we apply force on a moving body it stops.
Example 1: A moving bicycle, car or bus stop when we apply
the brakes.
Example 2: A moving cricket ball stops when a fielder catches
it.
9. Explain with example that a force can make a
moving body move faster.
When a force is applied on a moving body in the same
direction of
motion the body begins to move faster.
Example 1: The speed of the bicycle increases when we apply
more
force on the pedal.
Example 2: A swing moves faster when we push it.
10. Explain with example that a force can
change the direction of a
moving body.
When a force is applied on a moving body in a direction other
than its
direction of motion, the direction of motion of the body
changes.
Example 1: A player kicks a moving ball to change its
direction.
Example 2: The direction of a cricket or tennis ball is
changed by hitting
it in opposite direction.
11. Explain with example that a force can
change the shape and size of a
body.
When a force is applied on a body which is not allowed to
move, it
gets deformed.
Example 1: If we will squeeze a piece of rubber its shape
changes.
Example 2: On stretching a robber its length changes.
12. Define magnetic force.
The force between the poles of two magnets at a distance is
called
magnetic force. Like poles of the magnet repel each other and
the
unlike poles of the magnet attract each other.
13. Explain with example that friction produces
heat.
Friction produces heat can be explained by the following
examples.
Example 1: When we rub our palms they become worm due to
friction.
Example 2: When a matchstick is rubbed on a surface it burns
due to
the friction.
14. Explain with example that friction causes
wear and tear.
Friction causes wear and tear, which can be explained by the
following examples.
Example 1: Tyres of different vehicles wear out when they are
continuously used. This is because of the friction of the
tyre
with the road.
Example 2: In machines the moving parts gradually wear out when
they are continuously used due to friction.
C. Long Questions and Answers
1. Give some advantages of friction.
Most of our daily activities are depending upon the friction.
The major
advantages of friction are:-
a) In the absence of friction we will be not able to walk on
the road.
b) Due to friction it is possible to write on a board by
chalk.
c) Friction helps to burn a matchstick.
d) Without friction it will become difficult to brush our
teeth.
e) A moving vehicle stops
due to friction when we apply brakes.
2. Describe different ways to increase
friction.
Friction can be increased by following ways.
By making the surface rough: A rough surface provides more
friction
than a smooth surface.
So we can achieve more friction by making
the surface rough.
By using dry surface: A dry surface provides more friction
than a wet
surface. So surface can be dried to increase friction.
By increasing the weight: Friction increases as the weight of
a moving
objects increases. This develops a better grip to the moving
object.
3. What are the commonly used lubricants? How they help in reducing
the friction? Give an example of
solid lubricant and its application.
Oil and grease are the commonly used lubricants. When oil and
grease are applied between the moving parts of a machine it
fills the
fine pores in the surface of the moving parts and thus
reduces friction.
Solid lubricants are used in the form of powders. We spread
fine powder
on the carom board to reduce friction between the coins and
board.
4. Describe different ways to reduce friction.
Friction can be reduced by the following ways.
a) By making the surface smooth: Friction can be reduced by
making
the surface smooth. Roads are cemented to make them smooth.
b) By use of lubricants: Lubricants are applied between the
moving
parts of a machine. It fills the fine pores in the surface of
the moving
parts and thus reduces friction.
c) By the use of ball bearings: As rolling friction is much
less than the
sliding friction, we can use ball bearings to reduce
friction. The ball
bearings are used in place of wheels and axles to reduce
friction.
d) By streamlining: Friction can be reduced by giving a
special shape
to the body. For this the front portion of the body is made
pointed
as compared to its rear portion. For example the shape of
boat, car
and aeroplane etc. is designed to be streamlined.
5. What is streamlining? How it is helpful show with examples?
The friction experienced by a body moving in water or air can
be
reduced by designing a special shape of the body. For this
the front
portion of the body is made pointed as compared to its rear
portion.
This process of giving shapes with pointed ends is called
streamlining.
Streamlining helps in reducing the friction and makes the
body to move
faster.
Example 1: An arrow with a pointed tip moves faster and
covers a
larger distance than the arrow with a blunt tip.
Example 2: The front portion of a boat or an aeroplane is
made pointed
which helps them to move faster due to the reduced
friction.
6. What are the effects of force?
When a force is applied on a body it can have the following
effects.
i) A force can move a body originally at rest.
ii) A force can stop a moving body.
iii) A force can make a moving body move faster.
iv) A force can slow down a moving body.
v) A force can change the direction of motion of a moving
body.
vi) A force can change the shape and size of a body.
7. What are the different kinds of forces? Explain with example.
There are two kinds of forces. They are contact forces and
non-contact
forces.
Contact forces: These are the forces acting on a body by
making an
actual contact.
Examples: The muscular force applied as push and pull, the
force of
friction or frictional force and the force of tension in a
string
when a load is suspended from it.
Non-contact forces: These are the forces acting on bodies
without
making any contact with them. These are also
called forces from a distance.
Examples: Gravitational force, Electrostatic force and
Magnetic force.
8. Differentiate between the Electrostatic force
and the Magnetic force.
Electrostatic force
|
Magnetic force
|
This is the force between two charged bodies at a distance.
|
This is the force between the poles of two magnets at a distance.
|
Like charges repel each other and the unlike charges attract each
other.
|
Like poles repel each other and the unlike poles attract each other.
|
9. Differentiate between the Gravitational
force and the Magnetic force.
Gravitational force
|
Magnetic force
|
This is the force between two bodies at a distance. For example the
force between two heavenly bodies.
|
This is the force between the poles of two magnets at a distance.
|
Gravitational force is always attractive.
|
Magnetic force is attractive as well as repulsive.
Like poles repel each other and the unlike poles attract each other.
|
10. Give reasons
i)Why rollers are used to move heavy objects on
the ground?
ii) Why The vehicle like cars, trucks. Scooters, bicycles and
luggage trolleys
etc. are provided with wheels?
i)
Rolling friction is lesser than the sliding friction on a body of same mass.
Because
of this rollers are used to move heavy objects on the ground.
ii)The
vehicle like cars, trucks. Scooters, bicycles and luggage trolleys etc.
are
provided with wheels to reduce the friction as Rolling friction is lesser
than
the sliding friction.
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