9
Solution
Every object has inertia, that is, it resists a change in its
state of rest or in state of motion. If the luggage kept on the roof of the bus
is not tied to it with a rope then due to inertia, it may fall off the roof (i)
in the backward direction if the bus is accelerated or suddenly started
(ii) in the forward direction if the bus is retarded by applying breaks
(iii) in the outward direction if the bus takes a sharp turn at high speed.So
to avoid this it is advised to tie any luggage kept on the roof of the bus with
a rope.
Q 2.
Identify whether it is balanced or unbalanced force that causes the following
types of movements. (i) A person resting in an armchair. (ii) A cyclist
braking. (iii) A lorry travelling at a constant speed on a straight road. (iv)
A car that has a deceleration of 10 m/s2
Solution
A person in the armchair is at rest, that is not moving. Hence,
the forces are balanced. When the brakes are applied, the speed of cyclist
decreases. Thus, there is an unbalanced force acting. The speed of lorry is
constant and the direction in which it is travelling does not change.
Therefore, the forces acting on the lorry must be balanced. A car that has a
deceleration of 10 m/s2 has an unbalanced force acting on it,
because unbalanced forces are necessary for acceleration or deceleration to be
produced.
Q 3.
What is the momentum of a boy of mass 30 kg when he walks with a uniform
velocity of 2m/s?
Solution
Given mass of the boy = 30 kg Velocity = 2 m/s Momentum, p = m x v
= 30 x 2
p = 60 kg.m/s
Q 4. If
action is always equal to the reaction, explain how a horse can pull a cart.
Solution
According to the Newton's third law, the horse exerts some force
on the cart and the cart exerts equal and opposite force on the horse.
Therefore, to make the cart move, the horse bends forward and pushes the ground
with its feet. When the forward reaction to the backward push of the horse is
greater than the opposing frictional forces of the wheels, the cart moves.
Q 5.
What is Galileo's law of inertia?
Solution
Gallileo's law of inertia states that "A body moving on a
level surface will continue in the same direction at a constant speed when no
force acts on the body".
Q 6.
Explain why it is dangerous to jump out of a moving bus?
Solution
While jumping from a moving bus, our feet comes to rest on
touching the ground, whereas the upper part of our body continues to move
forward due to inertia of motion. As a result we may fall with our face
downwards.
Q 7.
Two similar trucks are moving with a same velocity on a road. One of them is
loaded while the other is empty. Which of the two will require a larger force
to stop it?
Solution
Force is directly proportionally to mass. Therefore, the loaded
truck will have more momentum then the empty truck and hence a large force will
be required by the loaded truck to stop it.
Q 8. Is
it possible for an object to be travelling with a non-zero velocity if the
object experiences a net zero external unbalanced force? If yes, what is the
magnitude and direction of the velocity of the object?
Solution
Yes it is possible for an object to be travelling with a non-zero
velocity if the object experiences a net zero external unbalanced force. If the
object was initially travelling with a non zero velocity, then the magnitude of
velocity remains constant and the direction remains unchanged.
Q 9.
What are the changes possible on an object at rest if we apply on it: (a) a
balanced force. (b) an unbalanced force.
Solution
(a) Object may change its shape or size. (b) Object changes its
velocity, that is it accelerates.
Q 10.
When two spring balances joined at their free ends are pulled apart, both show
the same reading. Explain.
Solution
Couple two spring balances A and B as shown in the figure. When we
pull the balance B, both the balances show the same reading indicating that
both the action and reaction forces are equal and opposite. In this case, the
pull of either of the two spring balances can be regarded as action and that of
the other balance as the reaction.
Q 11. A
8000 kg engine pulls a train of 5 wagons, each of 2000 kg, along a horizontal
tra ck. If the engine exerts a force of 40,000 N and the track offers a
frictional force of 5,000 N, then calculate : (a) Net accelerating force. (b)
Acceleration of the train. (c) For ce of the 1st wagon on the 2nd wagon.
Solution
The track offers a frictional force on each wagon. Thus,Net
accelerating force = Total force exerted - Frictional force on each wagon
= 40000 – (5 × 5000) N = 15000 N
Acceleration of the train:Total mass = Mass of 5 wagons + Mass of engine
= 5 x 2000 kg + 8000 kg = 18000 kg
F = m a→Acceleration =
F/m = 15000 N/18000 kg = 0.83 m/s2
Force F exerted by the first wagon is opposite to the friction force of 2nd wagon. Thus, the net force is F – 5000 N. Now, the 2nd wagon is moving with an acceleration of 0.83 m/s2. Thus, we haveF – 5000 = 2000 × 0.83F = 5000 + 1660F = 6660 N
= 40000 – (5 × 5000) N = 15000 N
Acceleration of the train:Total mass = Mass of 5 wagons + Mass of engine
= 5 x 2000 kg + 8000 kg = 18000 kg
F = m a→Acceleration =
F/m = 15000 N/18000 kg = 0.83 m/s2
Force F exerted by the first wagon is opposite to the friction force of 2nd wagon. Thus, the net force is F – 5000 N. Now, the 2nd wagon is moving with an acceleration of 0.83 m/s2. Thus, we haveF – 5000 = 2000 × 0.83F = 5000 + 1660F = 6660 N
Q 12.
It is difficult to balance our body when we accidentally slip on a peel of
banana. Explain why?
Solution
When we walk on the ground our foot pushes the ground in the
backward direction (action) and the ground pushes our foot in the forward
direction (reaction). This reaction helps us to move forward. But, when our
foot falls on a peel of banana, the peel cannot push the ground in the backward
direction. Consequently no reaction force acts on our foot and we lose balance.
Q 13.
Based on Newton's laws of motion, explain the action of flying of a bird.
Solution
The flying of a bird is based on action and reaction force, that
is, on Newton's third law. While flying, the bird pushes the air down with its
wings to get an equal and opposite reaction in the upward direction, which
helps the bird in flying up.
Q 14.
Give any two examples of force in each case:-
a)Lifts b)
pushes c) pulls d) Attracts e)
changes the shape of object.
Solution
a) Lifting a bucket filled with water; lifting the stone lying on
a ground b) Pushing a table; pushing a handcart c) Pulling a drawer; pulling a
chair d) Use of magnet to attract iron nails; rubbing a plastic scale and
attracting bits of paper with it e) Stretching a rubber band to change its
shape; pressing a rubber ball to change its shape
Q 15. A
large truck and a car, both moving with a velocity of magnitude v, have a
head-on collision and both of them come to a halt after that. If the collision
lasts for 1 s: a. Which vehicle experiences greater force of impact?
b. Which vehicle experiences greater change in momentum? c. Which
vehicle experiences greater acceleration? d. Which vehicle is likely to
suffer more damage? Why?
Solution
Le t the mass of the truck be M and that of the car be m and their
initial velocity be v. → M > m Let the final velocity of both vehicles be
v′. Given that both vehicles come to rest after collision, i.e. v’ = 0 Time of
impact, t = 10 s a. From Newton's second law of motion, the net force
experienced by each vehicle is given by the relation: Fcar =
m (v′ − v)/t = mv/10 Ftrack = M (v′ − v)/t =
−Mv/10since the mass of the truck is greater than that of the car, it will
experience a greater force of impact. b. Initial momentum of the car = mv
Final momentum of the car = 0 Change in momentum = mv
Initial momentum of the truck = Mv Final momentum of the
truck = 0 Change in momentum = MvMass of the truck is greater than
that of the car. Thus, the truck will experience a greater change in momentum
as compared to the car. c. According to the first equation of motion,
acceleration produced in a system is independent of the mass of the system. The
initial velocity, final velocity and time of impact remain the same in both
cases. Hence, the car and truck experience the same amount of acceleration. d.
According to Newton's third law of motion, for every action, there is an equal
and opposite reaction which acts on different bodies. Since the truck
experiences a greater force of impact (the action force), this large impact
force is also experienced by the car as a reaction force. Thus, the car is
likely to suffer more damage than the truck.
Q 16.
Illustrate an example of each of the three laws of motion
Solution
Example of 1st law of motion: When we are
travelling in a bus, our body is also travelling with the velocity equal to
that of the bus. If the bus stops suddenly our body tends to continue in the
same state of motion due to inertia and we tend to fall forward. Example of 2nd law
of motion: When a glass vessel falls from a height on a hard floor, it comes to
rest instantaneously (i.e., in a very short time) so the floor exerts a large
force on the vessel and it breaks. If it falls on a carpet (or sand), the time
duration in which the vessel comes to rest, increases and so the carpet (or
sand) exerts a less force on the vessel and it does not break. Example of 3rd law
of motion: When a fish swims in water, the fish pushes the water back and the
water pushes the fish with equal force but in opposite direction. The force
applied by the water makes the fish to move forward.
Q 17.
How can you explain the movement of rocket in the upward direction with the
help of law of conservation of momentum?
Solution
The momentum of a rocket before it is fired is zero. When the
rocket is fired, gases are produced in the combustion chamber of the rocket due
to the burning of the fuel. These gases come out from the rocket with high
speed and the direction of momentum of the gases coming out is downward. Thus,
to conserve the momentum of the rocket and gases ,the rocket moves upwards and
continues to move as long as the gases are ejected out of the rocket.
Q 18.
Define force and state its any two effects.
Solution
A force is a push or pull acting upon an object as a result of its
interaction with another object. Effects of force: (Any two) 1. It may move a
body at rest 2. Stop a moving body 3. Change the speed of a body 4. Change the
direction of a moving body 5. Change the size and shape of a body.
Q 19.
Why does a gunman get a jerk on firing a bullet?
Solution
When a bullet is fired from a gun, the force sending the bullet
forward is equal to the force sending the gun backward (Newton’s third law of
motion). But, due to the high mass of the gun, it moves only by a small
distance backwards and gives a backward jerk to the shoulder of the gunman.
Q 20.
State Newton's second law of motion.
Solution
Newton's second law of motion states that the rate of change of
momentum of a body is directly proportional to the applied unbalanced force and
takes place in the same direction in which the force acts.
Q 21.
Why do passengers tend to fall sideways when the bus takes a sharp turn?
Solution
When a bus runs along a straight line path, the passengers also
move with the same speed in a straight line but when the bus takes a sharp
turn, the upper portion of the body of passengers still continues to be in
straight line motion. Thus, the passengers tend to fall sideways when the bus
takes a sharp turn.
Q 22. A
book is placed on a table. Find the action and reaction force in this case.
Solution
The book exerts action force on the table equal to its weight in
the downward direction. The table exerts an equal force (reaction force) on the
book in the upward direction.
Q 23.
When you jump on a concrete surface, your feet hurt more than when you jump on
sand. Why?
Solution
When you jump onto the concrete surface, your feet are brought to
rest almost instantaneously because the rate of change of momentum is very
high. Hence, you get injured due to a large force on your body on account of
the hard floor. But, when you jump on sand, you come to rest in a longer period
of time. So, the change in momentum takes place in longer interval of time.
Hence, a small force is exerted on your body and you don’t get injured.
Q 24.
Give reason: A person alighting suddenly from a moving bus falls down.
Solution
A person alighting suddenly from a moving bus falls down due to
inertia of motion. This is because his body still tends to move forward with
the velocity of the bus but his feet are stationary.
Q 25.
Explain why it is difficult to walk on sand?
Solution
As we press against the sandy ground in the backward direction,
the sand gets pushed away and as a result we get very small reaction from the
ground, making it difficult to walk.
Q 26.
What are the effects seen on a body when external force acts on it?
Solution
Effects of force seen on a body are listed below: (i) Change in
shape (ii) Change in position (iii) Change in the state of motion (iv) Change
in the velocity of a moving body
Q 27.
Calculate the momentum of a toy car of mass 300 g moving with a speed of
18 kmh-1.
Solution
Momentum = Mass × Velocity Given: Mass = 300 g = 0.3 kg
Q
28. Explain why it is easier to push an empty box than a box full of books?
Solution
Heavier objects have more inertia than lighter objects. So,
inertia of an object depends on its mass. Thus, the box filled with books has
more inertia than the empty box due to which it it is easier to push an
empty box than a box full of books.
Q 29.
Identify the type of forces acting in the following case: 'A toy is moving over
a horizontal smooth surface with a constant velocity'.
Solution
The toy is moving with a constant velocity. Hence, no net
unbalanced forces are acting on the toy. Thus, balanced forces are acting on
the toy.
Q 30. A
force of 100 N acts on a body moving with a velocity of 20 m/s on a floor in a
straight line. What is the force of friction between the body and the floor?
Solution
Since body is moving with constant velocity, so no net force acts
on the body. Hence, the force of 100 N acting on the body must be balanced by
the force of friction between the body and the floor in contact. Therefore, the
force of friction between the body and the floor is equal to 100 N.
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