CBSE Board - Class 9, Force and Laws of Motion
Chapter 9, NCERT Science (Physics)
NCERT (CBSE) solutions of Science Textbook Chapter In-text Questions - Answers
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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 rupee coin and a one
rupee coin.
Answer: (a) stone
(b) train (c) five rupee coin.
Question.2: In the following
example, try to identify the number of times the velocity of 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: The
velocity of the ball changes three times. First time, the velocity changes when
the football player of one team kicks the ball. Second time the velocity
changes when another player of the same team kicks the football. Third
time the velocity changes when the goalkeeper of the opposite team kicks
the football.
The agent
supplying the force in each case, have been underlined.
Question.3: Explain why some of
the leaves may get detached from a tree if we vigorously shake its branch.
Answer: Before
shaking the branches the leaves are at rest. When the branches are shaken, they
come in motion at once 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.
Question.4: 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, the lower part of our body in contact with bus
comes to rest while the upper part of our body tends to keep moving due to
inertia of motion. Hence we fall forwards. When the bus accelerates from rest,
the lower part of our body comes into motion along with the bus while the upper
part of body tends to remain at rest due to inertia of motion and as a result
which we fall backwards.
Question.5: If action is always
equal to reaction, explain how a horse can pull cart.
Answer: The horse
pulls the cart with a force (action) in the forward direction. Since every
action has an equal and opposite reaction so, the cart also pulls the horse
with an equal force (reaction) in the backward direction. As a result of which
the two forces get balanced. But while pulling the cart the horse also pushes
the ground with its feet in the backward direction. The reaction of the earth
of the earth makes it forward direction along with the cart. This is how the
horse applies force and pulls the cart.
Question.7: Explain why it is
difficult for a fireman to hold a hose, which ejects large amount of water at a
high velocity.
Answer: Water is
ejected with a large forward force (action). As we know by Newton’s third law
of motion that every action has an equal and opposite reaction so, because of
this action fireman experiences a large backward force or reaction. That is why
he feels difficulty in holding the hose.
Question.9: From a rifle of
mass 4 kg a bullet of mass 50 gm is fired with an initial velocity of 35 ms-1.
Calculate the initial recoil velocity of the rifle.
Answer:
Mass of
bullet, m1 = 50 gm = 0.05 kg.
Mass of
rifle, m2 = 4 kg.
Initial
velocity of bullet, u1 = 0
Initial
velocity of rifle, u2 = 0
Final
velocity of bullet, v1 = 35 ms-1
Final
velocity of rifle, v2 = ?
According to
the law of conservation of momentum,
Total
momentum after firing = Total momentum before firing,
Or, m1v1
+ m2v2 = m1u1 + m2u2
Or, 0.05 x
35 + 4v2 = 0 + 0
Or, v2
= – 0.44 ms-1
The negative
sign indicates the direction of recoil (backward).
Question.10: Two objects of
masses of 100 gm and 200 gm are moving in along the same line and direction
with velocities of 2 ms-1 and 1 ms-1 respectively. They
collide and after collision, the first object moves at a velocity of 1.67 ms-1.
Determine the velocity of the second object.
Answer:
m1
= 100 gm = 0.1 kg, m2 = 200 gm = 0.2 kg,
u1 =
2 ms-1, u2 = 1 ms-1, v1 = 1.67 ms-1,
v2 = ?
By the law
of conservation of momentum,
m1v1
+ m2v2 = m1u1 + m2u2
Or, 0.1 x
1.67 + 0.2 v2 = 0.1 x 2+ 0.23 x 1
Or, v2
= 1.165 ms–1.
It will move
in the same direction after collision.
Class IX, Ncert Science - Force and Laws of Motion - Must Read
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- Class 9 Ncert Cbse Science Solutions - Lesson 9 Physics, Force and Laws of Motion
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