Laws of Motion | Chapter 3 | Class 9 Science | ICSE

Laws of Motion

Q1. A ball is hanging by a string from the ceiling of the roof. Draw a neat labeled diagram showing the forces acting on the ball and the string.

Ans: The diagram shows the forces acting on the ball and the string

Q2. State one factor on which the magnitude of a noncontact force depends. How does it depend on the factor stated by you?

Ans: The magnitude of non-contact force depends on the distance.

Distance and magnitude of force are inversely related. The magnitude of force decreases as the distance increases.

Q3. State Newton’s first law of motion.

Ans: Newton’s first law of motion states that a body remains in the state of rest or uniform motion in a straight line unless and until an external force acts on it.

Q4. Define inertia. Classify it and define each class with an example. On which factor does inertia depend and how is it related to inertia?

Ans: Inertia is defined as a property of matter by which it remains at the state of rest or in uniform motion in the same straight line unless acted upon by some external force.

Following are the three types of inertia

a. Inertia of Rest: When the resistance is offered by the body to continue in the state of rest unless an external force acts on it.
b. Inertia of Motion: When the resistance is offered by the body to continue to be in the uniform motion unless an external force acts on it.

The inertia of a body depends on its mass. Inertia of a body increases with the increase in mass of the body.

Q5. Why does a coin, placed on a card, drop into the tumbler when the card is rapidly flicked with a finger?

Ans: This is because of the first law of motion because the coin was at rest so it remained at rest for some time because no external force was applied to it and on the card, the force is applied therefore It moved while before the coin could start moving.

Q6. Define linear momentum and state its SI unit.

Ans: Linear momentum of a body is defined as the product of its mass and velocity.

S.I. unit of linear momentum is Kgms−1.

Q7. State Newton’s 2nd Law of Motion. Derive F=ma.

Q8. Two bodies A and B of the same mass are moving with velocities v and 2v respectively. Compare their:

i) Inertia and

ii) momentum

Q9. Name the SI and CGS units of force. How are they related?

Ans: S.I. unit of force is newton, C.G.S. unit of force is dyne.

1newton = 10dyne

Q10. Use Newton’s second law of motion to explain the following instances:

i) You pull your hands back while catching a fast-moving cricket ball

Ans: 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.

ii) You prefer to land on sand instead of a hard floor while taking a high jump.

Ans: Athlete is made to land on the sand after a long jump so as to increase the time of impact. This reduces the momentum and force by which he is landing on the ground. Thus, the athlete is saved from getting injured.

Q11. A force acts for 10m s on a stationary body of mass 100 kg, after which the force ceases to act. The body moves through a distance of 100 m in the next 5 s.

Calculate:

i) The velocity acquired by the body,

ii) The acceleration produced by the force and

iii) The magnitude of the force.