Force and laws of motion NCERT in-text questions
Hello, readers ! Here are NCERT in-text questions and solutions from Chapter 9: Force and Laws of Motion. Force and laws of motion NCERT in-text questions.
NCERT in-text questions Solution page 118
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?
Ans: (a) a stone ball has more inertia as comparison with a rubber ball of the same size because stone ball has greater mass than rubber ball. As inertia of a body is directly proportional to mass of body.
(b) a train has more inertia as comparison with a bicycle because train has greater mass than cycle. As inertia of a body is directly proportional to mass of body.
(c) a five rupees coin has more inertia as comparison with a one-rupee coin because five rupees coin has greater mass than one-rupee coin. As inertia of a body is directly proportional to mass of body.
Question 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.
Ans: The velocity of football changes four times in the above example.
First, when a football player kicks a football to another football player on his own team,
2nd-When that player kicks the football to the goalkeeper.
3rd-When the goalkeeper of opposite team stops the football.
4th—When the goalkeeper kicks the football towards a player of his own team.
In the preceding cases, the Agent supplying the force
1st case The first player on his team to kick a football to another player on his team.
2nd case The second player, who kicks the football to the goalkeeper.
3rd case goalkeeper while collecting the football.
4th case Goalkeeper, when he kicks the football towards a player of his own team.
Question 3:
Explain why some of the leaves may get detached from a tree if we vigorously shake its branch.
Ans: Due to the inertia of rest. When we vigorously shake a branch of a tree, it starts moving suddenly but the leaves remain in resting condition. This leads to tension between the leaves and the tree branch. As a result, leaves may get detached from the tree.
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?
Ans: When a moving bus brakes to a stop: When the bus is in removing condition due to inertia of motion, the whole body part of the passenger is in moving condition, but as the bus stops, the lower part of the body stops suddenly as it is in contact with the bus, while the upper part, due to inertia of motion, is in moving condition. That’s why passengers are thrown forward from their seats when a bus stops suddenly
When bus accelerates from rest : When the bus suddenly starts moving, the lower part of the passenger’s body, which is in contact with the bus, begins to move while the upper part of the passenger’s body remains at rest due to inertia of rest. Since the lower part moves forward as the bus moves, passengers fall backward.
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NCERT in-text questions Solution page 126-27
Question 2:
Explain, why is it difficult for a fireman to hold a hose, which ejects large amounts of water at a high velocity.
Ans: According to Newton’s third law Hose apply large amount of force on fireman that’s why it is difficult for a fireman to hold a hose, which ejects large amounts of water at a high velocity.
Question 3:
From a rifle of mass 4 kg, a bullet of mass 50 g is fired with an initial velocity of 35 m/s. Calculate the initial recoil velocity of the rifle.
Ans: Mass of rifle m1= 4 kg Mass of bullet m2= 50g = 0.05 kg
Initial velocity of rifle (before firing) u1 = 0
Initial velocity of bullet (before firing) u2 = 0
From conservation of momentum
m1u1 + m2u2 = m1v1+ m2v2
0 = 4 × v1+ 0.05 × 35
v1= 0.4375 m/s
Question 4:
Two objects of masses 100 g and 200 g are moving along the same line and direction with velocities of 2 m/s and 1 m/s, respectively. They collide and after the collision, the first object moves at a velocity of 1.67 m/s. Determine the velocity of the second object.
Ans: m1 = 0.100kg m2 = 0.2kg
u1 = 2m/s u2 = 1m/s
v1 = 1.67m/s v2 = ?
From conservation of momentum
m1v1+ m2u2 = m1 v1+ m2 v2
0.1× 2 + 0.2 × 1 = 0.1 × 1.67 + 0.2 v2
0.4 = 0.167 + 0.2 × v2
0.4 – 0.167 = 0.2 v2
0.233 = 0.2 v2
v2 = 1.165 m/s