# Exercise-3C Laws of Motion Concise ICSE Class-9 Selina Publishers

Exercise-3C Laws of Motion Concise ICSE Class-9 Selina Publishers Solutions Chapter-3 . Step By Step ICSE Selina Concise Solutions of Chapter-3 **Laws of Motion** with Exercise-3(A), Exercise-3(B) , Exercise-3(C) ,Exercise-3(D) and Exercise-3(E) including Numerical and MCQ Questions Solved **. **Visit official Website CISCE for detail information about ICSE Board Class-9.

Board | ICSE |

Publications | Selina Publication |

Subject | Physics |

Class | 9th |

Chapter-3 | Laws of Motion Exe-3(C) |

Book Name | Concise |

Topics | Solution of Exercise-3(A), MCQ-3(A), Exercise-3(B), MCQ-3(B), Exercise-3(C), MCQ-3(C), Numericals-3(C), Exercise-3(D), MCQ-3(D) , Numericals-3(D), Exercise-3(E), MCQ-3(E) , Numericals-3(E), |

Academic Session | 2021-2022 |

## Exercise-3C Laws of Motion Concise ICSE Class-9 Selina Publishers Physics Chapter-3

–: Select Topics :–

** Exercise-3(C), **** MCQ-3(C)**** , ****Numericals-3(C),**

**Exercise-3(D), ****MCQ-3(D) , **** Numericals-3(D),**

**Exercise-3(E), ****MCQ-3(E) , ****Numericals-3(E),**

Note :- Before Viewing Selina Concise Physics Solutions of Chapter-3 **Laws of Motion Class-9 Physics** . Read the whole chapter carefully and Solved all example of Exercise-3 **Laws of Motion Class-9 Physics****. **

**EXERCISE-3(C) Chapter-3 Laws of Motion ICSE Class-9 Physics Selina Revised Concise Solutions**

**Page 69**

**Question 1**

Name the two* *factors on which the force needed to stop a moving body in a given time depends.

**Answer 1**

Force needed to stop a moving body in a given time depends on its mass and velocity.

**Question 2**

Define linear momentum and state its S.I. unit.

**Answer 2**

Linear momentum of a body is the product of its mass and velocity.

Its SI unit is kgms^{-1}.

**Question 3**

A body of mass *m *moving with a velocity *v *is acted upon by a force. Write an expression for change in momentum in each of the following cases: (i) When *v * <<c, (ii) When *v →*c and (iii) When *v <<* c but *m *does not remain constant. Here, c is the speed of light.

**Answer 3**

(i) When *v * <<c,

p = (mv) = mv

(ii) When *v →*c

p = (mv)

(iii) When *v <<* c but *m *does not remain constant.

p = (mv)

**Question 4**

Show that the rate of change of momentum = mass × acceleration. Under what condition does this relation hold?

**Answer 4**

Let a force ‘F’ be applied on a body of mass m for a time ‘t’ due to which its velocity changes from u to v. Then,

Initial momentum of body = mu

Final momentum of body = mv

Change in momentum of the body in ‘t’ seconds = mv mu = m (vu)

Rate of change of momentum = Change in momentum/time

= [m (vu)]/t

However, acceleration a = Change in velocity/time = (vu)/t

Therefore, rate of change of momentum = ma = mass × acceleration

This relation holds true when the mass of the body remains constant.

**Question 5 Exercise-3C Laws of Motion Concise ICSE **

Two bodies *A *and *B *of same mass are moving with velocities *v *and *2v, *respectively. Compare their (i) inertia and (ii) momentum.

**Answer 5**

(i) Mass is the measure of inertia.

Let ‘m’ be the mass of the two bodies.

Inertia of body A:Inertia of body B :: m:m

Or, Inertia of body A:Inertia of body B :: 1:1

(ii)

Momentum of body A = m (v)

while Momentum of body B = m (2v) = 2mv

hence Momentum of body A:Momentum of body B :: mv:2mv

Or, Momentum of body A:Momentum of body B :: 1:2.

**Question 6**

Two balls *A *and *B *of masses *m *and 2 *m *are in motion with velocities *2v *and *v, *respectively. Compare:

(i) Their inertia.

(ii) Their momentum.

(iii) The force needed to stop them in the same time.

**Answer 6**

(i) Inertia of body A:Inertia of body B :: m:2m

Or, Inertia of body A:Inertia of body B :: 1:2.

(ii)

Momentum of body A = m ( 2v) = 2mv

and Momentum of body B = (2m) v = 2mv

Hence Momentum of body A:Momentum of body B :: 2 mv:2mv

Or, Momentum of body A:Momentum of body B :: 1:1.

(iii) According to Newton’s 2^{nd} law of motion, rate of change of momentum is directly proportional to the force applied on it. Therefore,

Force needed to stop A:Force needed to stop B :: 1:1.

**Question 7**

State the Newton’s second law of motion. What information do you get from it?

**Answer 7**

According to Newton’s second law of motion, the rate of change of momentum is directly proportional to the force applied on it and the change of momentum takes place in the direction in which the force is applied.

It gives the quantitative value of force, i.e. it relates the force to the measurable quantities such as acceleration and mass.

**Question 8 Exercise-3C Laws of Motion Concise ICSE **

How does Newton’s second law of motion differ from the first law of motion?

**Answer 8**

Newton’s first law of motion gives the qualitative definition of force. It explains the force as the cause of acceleration only qualitatively but Newton’s second law of motion gives the quantitative value of force. It states force as the product of mass and acceleration. Thus, it relates force to the measurable quantities such as acceleration and mass.

**Question 9**

Write the mathematical form of Newton’s second law of motion. State the conditions if any.

**Answer 9**

Mathematical expression of Newton’s second law of motion is as shown below:

Force = Mass × Acceleration

Above relation holds for the following conditions:

(i) When the velocity of the body is much smaller than the velocity of light.

(ii) When the mass remains constant.

**Question 10**

State Newton’s second law of motion. Under what condition does it take the form F* *= ma?

**Answer 10**

According to Newton’s second law of motion, the rate of change of momentum is directly proportional to the force applied on it, and the change of momentum takes place in the direction in which the force is applied.

The relation F=ma holds for the following conditions:

(i) When the velocity of the body is much smaller than the velocity of light.

(ii) When the mass remains constant.

**Question 11**

How can Newton’s first law of motion be obtained from the second law of motion?

**Answer 11**

From Newton’s second law of motion, F = ma.

If F = 0, then a = 0.

This means that if no force is applied on the body, its acceleration will be zero. If the body is at rest, then it will remain in the state of rest and if it is moving, then it will remain moving in the same direction with the same speed. Thus, a body not acted upon by an external force, does not change its state of rest or motion. This is the statement of Newton’s first law of motion.

**Question 12**

Draw graphs to show the dependence of

(i) acceleration on force for a constant mass and

(ii) force on mass for a constant acceleration.

**Answer 12**

**(i) acceleration on force for a constant mass and**

**(ii) force on mass for a constant acceleration.**

**Question 13**

How does the acceleration produced by a given force depend on the mass of the body? Draw a graph to show it.

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