Answer:
1. Answer : (b) always act on different bodies in opposite directions
Explanation: As per Newton's third law, whenever one object exerts a force on another object, the second object exerts an equal and opposite on the first. If object A exerts a force on object B, object B also exerts an equal and opposite force on object A.
2. Answer : (d) decrease the rate of change of momentum
Explanation: A goalkeeper in a game of football pulls his hands backward after holding the ball shot at the goal. This enables the goalkeeper to decrease the rate of change of momentum and hence reduce the force exerted by the ball on hands.
3. Answer : (c) to resist any change in its state of motion
Explanation: Inertia is the physical property of a body of an object that tends to cause the object to resist any change in its state of rest or motion. The object will stay at rest if it is at rest or will stay in motion if the object is already in motion.
4. Answer : (a) accelerated
Explanation: A passenger in a moving train tosses a coin upward which falls behind him. It implies that the motion of the train is accelerated. It happens because by the time the coin remains in the air, its velocity reduces and it falls behind the passenger.
5. Answer : (c) momentum
Explanation: The rocket works because of the law of conservation of linear momentum. The law of conservation of linear momentum is very important in physics. Momentum is defined as the mass of object times its velocity.
6. Answer : (a) inertia of motion
Explanation: It is because the body of the passengers tries for resisting the change exactly in the side of inertia. Hence, the passengers will slip for getting to one side of a car. When a motor makes a sharp turn at a high speed we tend to get thrown to one side due to the inertia of motion.
7. Answer : (a) inertia of motion
Explanation: Inertia is a habit of an object to resist any change in its state of motion. In case of a running athlete, the body is in motion but when the athlete stop immediately then his legs will come to rest while a body is in moving forward.
8. Answer : (b) Constant
Explanation: The relationship also implies the conservation of momentum: when the net force on the body is zero, the momentum of the body is constant. Any net force is equal to the rate of change of momentum.
9. Answer : (b) rate of change of momentum
Explanation: Newton actually stated his second law of motion in terms of momentum: The net external force equals the change in momentum of a system divided by the time over which it changes. The change in momentum is the difference between the final and initial values of momentum.
10. Answer : (c) Increase the time to stop
Explanation: This is done to prevent injury to the athlete when he falls down after making a high jump. When the high jumper falls on a soft landing site, such as a cushion or a heap of sand, Then the jumper takes a longer time to come to a stop. Thus cushion or sand, being soft reduces the athlete's Momentum more gently.
11. Answer : (d) Frictional force
Explanation: The property by virtue of which a resisting force is created between two rough bodies that resist the sliding of one body over the other is known as friction. The force that always opposes the motion of one body over the other body in contact with it is known as the force of friction or frictional force.
12. Answer : (c) Four times
Explanation: The velocity of football changes four times. First, when a football player kicks to another player, second when that player kicks the football to the goalkeeper. Third when the goalkeeper stops the football. Fourth when the goalkeeper kicks the football towards a player of his own team.
13. Answer : (a) Resultant force on it is zero
Explanation: The mass of the object is constant and cannot be zero. So, we can say an object will continue accelerating until the resultant force on it is zero.
14. Answer : (a) In the direction opposite to the direction of motion of the bus.
Explanation: The passengers in a bus tend to fall back when it starts suddenly due to inertia as the passengers tend to remain in a state of rest while the bus starts to move.
15. Answer : (d) All the above
Explanation: When a force is exerted on an object, it can change its state, position, and also its shape.
16. Answer : (c) Must experience acceleration
Explanation: The Second Law of Motion states that if an unbalanced force acts on a body, that body will experience acceleration ( or deceleration), that is, a change of speed. One can say that a body at rest is considered to have zero speed, ( a constant speed). So any force that causes a body to move is an unbalanced force.
17. Answer : (d) 80 N
Explanation: Mass = 20 kg, Acceleration = 4 m/s2.
According to the second law of motion,
Force = mass \(\times\) acceleration
F = 20 \(\times\) 4
F = 80 N
18. Answer : (b) Move forward
Explanation: A water tanker filled up to 2/3 of its height is moving at a uniform speed. On sudden application of the brake, the water in the tank would. On sudden application of brakes, water in the tank would move forward due to inertia of motion.
19. Answer : (a) Balanced forces act on the balloon
Explanation: Balanced forces are responsible for bringing about a change in the shape of an object.
20. Answer : (b) More volume than that of an ideal gas under identical conditions
Explanation: When there is high pressure, the distance between the adjacent gas molecules is relatively less. As a result, it occupies a higher volume as compared to an ideal gas subjected to high pressure due to the absence of repulsion force.
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