This post provides you a complete solution of Chapter 8 Class 9th for Rajasthan Board of Secondary Education.
This post contains
all the main and important topics which have complete and detailed description.
Force and Laws of Motion Questions- Answers
This
chapter explains –
1. Balanced and Unbalanced Forces
2. First Law of Motion
3. Inertia and Mass
4. Second Law of Motion
5. Third Law of Motion
6. Conservation of Momentum
Intext Questions
Answers of Questions on Page 91
Q.1 Which of the following has more inertia:
(a) A rubber ball and a stone of the same size?
(b) A bicycle and a train?
(c) A five rupee coin and a one rupee coin?
Answer
(a)Stone has more inertia
than the rubber ball as the mass
of the stone is more than
the mass of the ball.
(b)A train has more
inertia than a bicycle.
(c) A five rupee coin has more inertia than a
one-rupee coin.
Q.2 In the following
example try to identify the number of
times the
velocity of the ball changes:
“A
football player kicks a football to another player of his team who kicks the
football towards the goal. The goalkeeper of the opposite team collects the
football and kicks it towards a player of his own team”.
Also
identify the agent supplying the force in each case.
Answer
(i)When first player kicks the balls towards another
player of his team, Velocity of the football changes
(ii) When another player kicks the football towards
the goal, Velocity of the football also changes
(iii) When the goalkeeper of the opposite team stops
the football by collecting it, Velocity of the football also changes (velocity
become zero)
(iv) When the goalkeeper kicks it towards a player
of his team. Velocity of the football changes thus, the velocity of the ball
changes four times in this case.
In first and
second case, the force is supplied by the foot of players. In third case, force
is supplied by the hands of the goalkeeper. In fourth case, as the goalkeeper
hits the football with his foot. So the foot of the goalkeeper supplies the
force.
Q.3 Explain why some of the leaves may get detached
from a tree if we vigorously shake its branch.
Answer
Leaves are at rest before shaking the branches, when
branches are shaken vigorously, they come in motion
while the leaves tend to remain at rest due to inertia of
rest. As a result, leaves get detached from the branches
and fall down.
Q.4 Why do you fall in the forward direction when a
moving bus brakes to stop and fall backwards when it
accelerates from rest?
Answer
(i) Our body is in motion when the bus is moving, when
a moving bus brakes to stop, the lower portion of our
body also comes to rest but the upper part of our body
remain in motion due to inertia of motion. Hence, we fall
in forward direction.
(ii) Our body is at rest when the bus is stationary, when a
bus accelerates from rest, the lower portion of our body
also comes in motion with bus buts the upper part of our
body remain at rest due to inertia of rest. Hence, we fall
backwards.
Answers of End Exercise Questions on Page
Q.1 An object experience a net zero external force. It is
possible for the object to be travelling with a non-zero
velocity? If, yes state the conditions that must be
placedon the magnitude and direction of the velocity. If
no, provide a reason.
Answer
Yes, an object may travel with a non-zero velocity even
when the net external force on it is zero.
Conditions are:
(i) If any object already moving with a uniform speed
along a straight line.
(ii) Magnitude of velocity and direction of motion should
not be changed.
(iii) There should be no friction between the object and
the
ground.
(iv) Air resistance must be zero. If any of the above
conditions is not fulfilled, then the answer will be ‘No’.
The reason is that external unbalanced force is needed for
initiating the motion and also for any changes in its
velocity.
Q.2 When a carpet is beaten with a stick, dust comes out
of it. Explain.
Answer
When we beat the carpet with a stick, it comes into
motion, but the dust particles continue to be at rests due
to inertia of rests and get detached from the carpet due to
gravity.
Q.3 Why is it advised to tie any luggage kept on the roof
of a bus with a rope?
Answer
The motion in the moving bus is not uniform and Due to
sudden jerks or due to bus taking sharp turn on the road,
the luggage may fall down from the roof because of its
tendency to resist any change in its sate of rest or motion
so it fall down. To avoid this, the luggage is tied with a
rope on the roof.
Q.4 A batsman hits a cricket ball which then rolls on a
level ground. After covering a short distance, the ball
comes to rest. The ball slows to a stop because:
(a) The
batsman did not hit the ball hard enough.
(b) Velocity is proportional to the force exerted on the
ball.
(c) There
is a force on the ball opposing the motion,
(d) There is no unbalanced force on the ball, so the bal
would want to
come to rest.
Answer
(c) There
is a force on the ball opposing the motion.
Q.5 A trucks starts from rests and rolls down a hill with a
constant acceleration. It travels a distance of 400 m in
20s. Find its acceleration. Find the force acting on it if its
mass is 7 metric tons.
Answer
Q.6 A stone of 1 kg is
thrown with a velocity of 20 ms-1
across
the frozen surface of a lake and comes to rests after travelling a distance of
50 m. What is the force of friction between the stone and the ice?
(a) The
net accelerating force.
(b) The
acceleration of the train and
(c) The
force of wagon 1 on wagon 2
Answer
Q.8 An automobile vehicle has a mass of 1500 kg. What
must be the force between the vehicle and the road if the
vehicle is to be stopped with a negative acceleration of
1.7 ms-2?
Answer
moving with velocity v?
(a) (mv)2 (b) mv2+ (c) 
(d) mv.
Answer
(d) mv.
Q.10 Using a horizontal force of 200 N, we intend to
move a wooden cabinet across a floor at constant
velocity. What is the frictional force that will be exerted
on the cabinet?
Answer
The cabinet will move with constant velocity only when
the net force on it is zero
∴Force of friction on the cabinet = 200 N. in a direction
opposite to the direction of motion of the cabinet.
The wooden cabinet moves across the floor at a constant
velocity and the force applied is 200 N so the frictional
force will be extorted on the wooden cabinet will be less
than 200N.
Q.11 According to the third law of motion when we
push an object, the object pushes back on us with an
equal and opposite force. If the object is a massive truck
parked along the roadside, it will probably not move. A
student justifies this by answering that the two opposite
and equal forces cancel each other. Comment on this
logic
and explain why the truck do not move?
Answer
NO, Student’s justification is not correct. Two equal and
opposite forces cancel each other if they act on the same
body. According to the third law of motion, action and
reaction forces are equal and opposite but they both act
on different bodies. Hence, they cannot cancel each
other.
When we push a massive truck, then the applied force
on the truck is not sufficient to overcome the force of
friction between the tyres of truck and ground, that’s
why, truck does not move.
Q.13 A hockey ball of mass 20 g travelling at 10 ms-1 is
struck by a hockey stick so as to return it along its
original path with a velocity of 5 ms-1. Calculate the
change of momentum occurred in the motion of the
hockey ball by the force
applied by the hockey stick.
Answer
Q.14 A bullet of mass 10 g travelling horizontally with a
velocity of 150 ms-1 strikes a stationary wooden
block and comes to rest in 0.03 s. Calculate the distance
of penetration of the bullet into the block. Also calculate
the magnitude of the force exerted by the wooden block
on the bullet.
with a velocity of 10 ms-1 collies with and sticks to a
stationary wooden block of mass 5 kg, then they both
move off together in the same straight line. Calculate the
total momentum just before the impact and just after the
impact. Also calculate the velocity of the combined object.
Q.16 Akhtar, Kiran and Rahul were riding in a motorcar
that was moving a high velocity. On an expressway when
an insect hit the windshield and got stuck on the wind
screen, Akhtar and Kiran started pondering over the
situation. Kran suggested that the insect suffered a
greater change in momentum as compared to the change
in momentum of the motor car (because the change in
the velocity of the insect was much more than that of the
motorcar). Akhtar said that since the motorcar was
moving with a larger velocity, it exerted a larger force on
the insect and as a result, the insect died. Rahul while
putting an entirely new explanation said that both the
motorcar and the insect experienced the same force and
change in their momentum. Comment on these
suggestions.
Answer
Both the motorcar and insect experience the equal force
and hence, a same change in their momentum. So
we agreed with Rahul. According to the law of
conservation of momentum, when two bodies collide
Initial momentum before collision = Final momentum
after collision
So the equal force is exerted on both the bodies but
because the mass of insect is very small that’s why it
suffer greater change in velocity.
Q.17 How much momentum will a dump-bell of mass
10 kg transfer to the floor if it falls from a height of 80
cm? Take its downward acceleration to be 10 ms-2 .
Answer
.jpg)
.jpg)
.jpg)
No comments:
Post a Comment