Lesson–4 : Laws of Motion

Stage–3 : Important Questions & Numericals (Intermediate)

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🔹 PART–A : VERY SHORT ANSWER QUESTIONS (1–2 Marks)

1. Define force.

Force is an external agent which can change the state of rest, state of motion, direction of motion, or shape of a body.

2. What is inertia?

Inertia is the tendency of a body to resist any change in its state of rest, motion, or direction.

3. Which physical quantity measures inertia?

Mass.

4. State Newton’s First Law of Motion.

A body continues in its state of rest or uniform motion in a straight line unless acted upon by an external unbalanced force.

5. Write the formula for momentum.

p = mv

6. Is force a scalar or vector quantity?

Force is a vector quantity.

7. What is impulse?

Impulse is the product of force and time and is equal to change in momentum.

8. State Newton’s Third Law of Motion.

For every action, there is an equal and opposite reaction.

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🔹 PART–B : SHORT ANSWER QUESTIONS (3–4 Marks)

1. Explain inertia with examples.

Inertia is the property of a body by virtue of which it resists any change in its state of rest or motion.

• Inertia of rest: Passenger falls backward when bus starts suddenly
• Inertia of motion: Passenger falls forward when bus stops suddenly
• Inertia of direction: Stone released from circular motion moves tangentially

2. Define momentum and give its SI unit.

Momentum is the quantity of motion possessed by a body and is given by p = mv.

SI unit of momentum is kg m s⁻¹.

3. Derive F = ma.

According to Newton’s Second Law, force is proportional to the rate of change of momentum.

Rate of change of momentum = m(v − u)/t = ma.

Hence, F = ma.

4. Explain impulse with examples.

Impulse is the effect of a force acting for a short time.

Examples:

• Catching a ball by pulling hands backward
• Airbags in vehicles

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🔹 PART–C : LONG ANSWER QUESTIONS (5–6 Marks)

1. State and explain Newton’s three laws of motion.

First Law: Explains inertia.

Second Law: Gives relation between force, mass, and acceleration.

Third Law: Explains action–reaction forces.

These three laws together explain all mechanical motions.

2. Explain Newton’s Third Law with examples.

Action and reaction are equal and opposite forces acting on different bodies.

Examples include walking, swimming, gun recoil, and rocket motion.

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🔹 PART–D : NUMERICAL PROBLEMS (VERY IMPORTANT)

1. Find the force required to accelerate a body of mass 5 kg at 2 m/s².

Given:
m = 5 kg, a = 2 m/s²

Solution:

F = ma = 5 × 2 = 10 N

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2. A force of 20 N acts on a body of mass 4 kg. Find the acceleration produced.

Solution:

a = F/m = 20/4 = 5 m/s²

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3. A body of mass 2 kg is moving with velocity 10 m/s. Find its momentum.

Solution:

p = mv = 2 × 10 = 20 kg m s⁻¹

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4. A force of 50 N acts for 0.2 s on a body. Find the impulse.

Solution:

Impulse = F × t = 50 × 0.2 = 10 N s

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5. A man of mass 60 kg stands in a lift moving upward with acceleration 2 m/s². Find the normal reaction. (g = 10 m/s²)

Solution:

N = m(g + a) = 60(10 + 2) = 720 N

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🔹 PART–E : EXAM-ORIENTED TIPS

• Always draw Free Body Diagram in numericals
• Write formulas before substitution
• Use correct SI units
• Present answers neatly with steps

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Stage–3 Completed ✔
Next: Stage–4 (IIT JEE Concepts – Advanced Level)