FORCE AND LAWS OF MOTION
Question 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-rupees coin and a one-rupee coin?
Answer:
(a) A stone of the same size
(b) a train
(c) a five-rupees coin
As the mass of an object is a measure of its inertia, objects with more mass have
more inertia.
Question 2. Why do you fall in the forward direction when a moving bus brakes
to a stop and fall backwards when it accelerates from rest?
Answer: When a moving bus brakes-to a stop: When the bus is moving, our body is
also in motion, but due to sudden brakes, the lower part of our body comes to rest as
soon as the bus stops. But the upper part of our body continues to be in motion and
hence we fall in forward direction due to inertia of motion.
When the bus accelerates from rest we fall backwards: When the bus’ is stationary
our body is at rest but when the bus accelerates, the lower part of our body being in
contact with the floor of the bus comes in motion, but the upper part of our body
remains at rest due to inertia of rest. Hence we fall in backward direction.
Question 3. Explain, why is it difficult for a fireman to hold a hose, which ejects
a large amount of water at a high velocity.
Answer: The water that is ejected out from the hose in the forward direction comes
out with a large momentum and equal amount of momentum is developed in the
hose in the opposite direction and hence the hose is pushed backward. It becomes
difficult for a fireman to hold a hose which experiences this large momentum.
Question 4. What is inertia?
Answer: The natural tendency of an object to resist a change in their state of
rest or of uniform motion is called inertia.
Question 5. State Newton’s first law of motion.
Answer: An object remains in a state of rest or of uniform motion in a straight
line unless acted upon by an external unbalanced force.
Question 6. State Newton’s second law of motion.
Answer: The rate of change of momentum of an object is proportional to the
applied unbalanced force in the direction of the force.
, Question 7. What is momentum?
Answer: The momentum of an object is the product of its mass and velocity and
has the same direction as that of the velocity. The S. I. unit is kg m/s. (p = mv)
Question 8. State Newton’s III law of motion.
Answer: To every action, there is an equal and opposite reaction and they act
on two different bodies.
Question 9. Why do fielders pull their hand gradually with the moving ball
while holding a catch?
Answer: While catching a. fast moving cricket ball, a fielder on the ground gradually
pulls his hands backwards with the moving ball. This is done so that the fielder
increases the time during which the high velocity of the moving ball decreases to
zero. Thus, the acceleration of the ball is decreased and therefore the impact of
catching the fast moving ball is reduced.
Question 10. Explain how momentum gets conserved in collision of two
bodies.
Answer: Consider two bodies i.e., balls A and B, the mass and initial velocities are
mAuA and mBuB respectively before collision. The two bodies collide and force is
exerted by each body. There is change in their velocities due to collision.
Question 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-rupees coin and a one-rupee coin?
Answer:
(a) A stone of the same size
(b) a train
(c) a five-rupees coin
As the mass of an object is a measure of its inertia, objects with more mass have
more inertia.
Question 2. Why do you fall in the forward direction when a moving bus brakes
to a stop and fall backwards when it accelerates from rest?
Answer: When a moving bus brakes-to a stop: When the bus is moving, our body is
also in motion, but due to sudden brakes, the lower part of our body comes to rest as
soon as the bus stops. But the upper part of our body continues to be in motion and
hence we fall in forward direction due to inertia of motion.
When the bus accelerates from rest we fall backwards: When the bus’ is stationary
our body is at rest but when the bus accelerates, the lower part of our body being in
contact with the floor of the bus comes in motion, but the upper part of our body
remains at rest due to inertia of rest. Hence we fall in backward direction.
Question 3. Explain, why is it difficult for a fireman to hold a hose, which ejects
a large amount of water at a high velocity.
Answer: The water that is ejected out from the hose in the forward direction comes
out with a large momentum and equal amount of momentum is developed in the
hose in the opposite direction and hence the hose is pushed backward. It becomes
difficult for a fireman to hold a hose which experiences this large momentum.
Question 4. What is inertia?
Answer: The natural tendency of an object to resist a change in their state of
rest or of uniform motion is called inertia.
Question 5. State Newton’s first law of motion.
Answer: An object remains in a state of rest or of uniform motion in a straight
line unless acted upon by an external unbalanced force.
Question 6. State Newton’s second law of motion.
Answer: The rate of change of momentum of an object is proportional to the
applied unbalanced force in the direction of the force.
, Question 7. What is momentum?
Answer: The momentum of an object is the product of its mass and velocity and
has the same direction as that of the velocity. The S. I. unit is kg m/s. (p = mv)
Question 8. State Newton’s III law of motion.
Answer: To every action, there is an equal and opposite reaction and they act
on two different bodies.
Question 9. Why do fielders pull their hand gradually with the moving ball
while holding a catch?
Answer: While catching a. fast moving cricket ball, a fielder on the ground gradually
pulls his hands backwards with the moving ball. This is done so that the fielder
increases the time during which the high velocity of the moving ball decreases to
zero. Thus, the acceleration of the ball is decreased and therefore the impact of
catching the fast moving ball is reduced.
Question 10. Explain how momentum gets conserved in collision of two
bodies.
Answer: Consider two bodies i.e., balls A and B, the mass and initial velocities are
mAuA and mBuB respectively before collision. The two bodies collide and force is
exerted by each body. There is change in their velocities due to collision.
