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
placed on 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?
Answer
(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
(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.
Q.15 An object of mass 1kg travelling in a straight line 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.
Answer
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
Related Topics
RBSE Class 9 Science All Chapters Solutions
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