# Laws of Motion Exe-3C Short Answer Physics Class-9 ICSE Selina Publishers

Laws of Motion Exe-3C Linear Momentum Newton’s Second Law Short Answer Type for Class-9 ICSE Concise Physics. There is the solutions of short Answer type Questions of your latest textbook which is applicable in 2023-24 academic sessionVisit official Website CISCE for detail information about ICSE Board Class-9.

## Laws of Motion Exe-3C Linear Momentum Newton’s Second Law Short Answer

(ICSE Class – 9 Physics Concise Selina Publishers)

 Board ICSE Class 9th Subject Physics Writer / Publication Concise selina Publishers Chapter-3 Laws of Motion Exe – 3C Linear Momentum Newton’s Second Law Topics Solution of Exe-3(C) Short Answer Type Academic Session 2023-2024

### Exe-3C Linear Momentum Newton’s Second Law Short Answer Type

Ch-3 Laws of Motion Physics Class-9 ICSE Concise

Page 77

#### Question 1. Name the twofactors on which the force needed to stop a moving body in a given time depends.

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

#### Question 2. Show that the rate of change of momentum = mass × acceleration. Under what condition does this relation hold?

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

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.

(Laws of Motion Exe-3C Short Class-9 ICSE)

#### Question 4. How does Newton’s second law of motion differ from the first law of motion?

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

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

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

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.

(Laws of Motion Exe-3C Short Class-9 ICSE)

#### Question 8. Why does a glass vessel break when it falls on a hard floor, but it does not break when it falls on a carpet?

When a glass vessel falls from a height on a hard floor, it comes to rest almost instantaneously, i.e. in a very short time, so the floor exerts a large force on the vessel and it breaks. However, if it falls on a carpet, then the time duration, in which the vessel comes to rest, increases, so the carpet exerts less force on the vessel and it does not break.

#### Question 9. Use Newton’s second law of motion to explain the following:

(a) A cricketer pull your hands back while catching a fast moving cricket ball.

(b) A athlete prefer to land on sand instead of hard floor while taking a high jump.

(a) pull your hands back while catching a fast moving cricket ball because

We pull our hands back while catching a fast moving cricket ball, because by doing so, we increase the time of catch, i.e. increase the time to bring about a given change in momentum, and hence, the rate of change of momentum decreases. Thus, a small force is exerted on our hands by the ball.

(b)prefer to land on sand instead of hard floor while taking a high jump because

When an athlete lands from a height on a hard floor, his feet comes to rest instantaneously, so a very large force is exerted by the floor on his feet, but if he lands on sand, his feet push the sand for some distance; therefore, the time duration in which his feet comes to rest increases. As a result, the force exerted on his feet decreases and he is saved from getting hurt.

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