⚡ Force and Laws of Motion – Class 9 Science MCQs

Section 2: Newton’s Second Law & Momentum (Q26–Q50)

Q26. Newton’s second law gives a definition of:

A. Force
B. Acceleration
C. Momentum
D. Inertia

✅ Correct Answer: A. Force

Explanation:

• A. Correct, Newton’s second law defines force as rate of change of momentum.
• B. Acceleration is change of velocity.
• C. Momentum is mass × velocity.
• D. Inertia is resistance to motion.

Q27. The SI unit of momentum is:

A. Newton
B. kg m/s
C. Joule
D. Watt

✅ Correct Answer: B. kg m/s

Explanation:

• Momentum = mass × velocity → kg × m/s = kg m/s.
• Other units are unrelated.

Q28. Which equation expresses Newton’s second law?

A. F=maF = ma
B. F=mvF = mv
C. F=mgF = mg
D. F=u+atF = u + at

✅ Correct Answer: A. F=maF = ma

Explanation:

• A. Correct, force = mass × acceleration.
• B. Wrong.
• C. Special case (weight).
• D. First equation of motion.

Q29. Momentum of a body is product of:

A. Mass × acceleration
B. Mass × velocity
C. Force × time
D. Mass × displacement

✅ Correct Answer: B. Mass × velocity

Explanation:

• B. Correct definition of momentum.
• C. Force × time = change in momentum (impulse).

Q30. Impulse is equal to:

A. Force × acceleration
B. Mass × velocity
C. Force × time
D. Mass × acceleration

✅ Correct Answer: C. Force × time

Explanation:

• Impulse = change in momentum = F × t.

Q31. A body of mass 2 kg moving with velocity 5 m/s. Its momentum = ?

A. 2.5 kg m/s
B. 5 kg m/s
C. 7 kg m/s
D. 10 kg m/s

✅ Correct Answer: D. 10 kg m/s

Explanation:

• p = m × v = 2 × 5 = 10.

Q32. A force of 20 N acts on mass 4 kg. Acceleration = ?

A. 2 m/s²
B. 4 m/s²
C. 5 m/s²
D. 10 m/s²

✅ Correct Answer: C. 5 m/s²

Explanation:

• a=F/m=20/4=5a = F/m = 20/4 = 5.

Q33. Which physical quantity gives the measure of force applied in collision?

A. Work
B. Momentum
C. Impulse
D. Energy

✅ Correct Answer: C. Impulse

Q34. Which law of motion explains recoil of a gun?

A. First law
B. Second law
C. Third law
D. Law of gravitation

✅ Correct Answer: C. Third law

Explanation:

• Recoil = action-reaction pair.

Q35. A fast-moving cricket ball is stopped by a player by moving his hands backward. This increases:

A. Time of contact
B. Momentum
C. Mass of ball
D. Velocity

✅ Correct Answer: A. Time of contact

Explanation:

• Increased time reduces force (F = Δp/Δt).

Q36. A force of 50 N acts on 2 kg body at rest. Acceleration = ?

A. 10 m/s²
B. 20 m/s²
C. 25 m/s²
D. 30 m/s²

✅ Correct Answer: C. 25 m/s²

Explanation:

• a=F/m=50/2=25a = F/m = 50/2 = 25.

Q37. Momentum is a:

A. Scalar quantity
B. Vector quantity
C. Dimensionless
D. None

✅ Correct Answer: B. Vector quantity

Explanation:

• Momentum has magnitude & direction (same as velocity).

Q38. Law of conservation of momentum is based on:

A. Newton’s first law
B. Newton’s second law
C. Newton’s third law
D. Law of gravitation

✅ Correct Answer: C. Newton’s third law

Q39. A bullet of mass 20 g is fired with velocity 100 m/s. Momentum = ?

A. 2 kg m/s
B. 20 kg m/s
C. 200 kg m/s
D. 0.2 kg m/s

✅ Correct Answer: A. 2 kg m/s

Explanation:

• m = 0.02 kg, v = 100 → p = 0.02 × 100 = 2.

Q40. Which formula relates force and momentum?

A. F=Δp/ΔtF = Δp/Δt
B. F=mvF = mv
C. F=ma2F = ma^2
D. F=p/tF = p/t

✅ Correct Answer: A. F=Δp/ΔtF = Δp/Δt

Q41. A 1000 kg car moves with velocity 20 m/s. Momentum = ?

A. 20000 kg m/s
B. 1000 kg m/s
C. 2000 kg m/s
D. 500 kg m/s

✅ Correct Answer: A. 20000 kg m/s

Explanation:

• p = m × v = 1000 × 20 = 20000.

Q42. When net external force on system = 0, momentum is:

A. Zero
B. Constant
C. Increasing
D. Decreasing

✅ Correct Answer: B. Constant

Explanation:

• Conservation of momentum.

Q43. The SI unit of impulse is same as that of:

A. Force
B. Energy
C. Momentum
D. Acceleration

✅ Correct Answer: C. Momentum

Explanation:

• Impulse = F × t = Δp → same as momentum (kg m/s).

Q44. A ball of mass 0.5 kg moving with velocity 10 m/s. Momentum = ?

A. 2.5 kg m/s
B. 5 kg m/s
C. 10 kg m/s
D. 20 kg m/s

✅ Correct Answer: B. 5 kg m/s

Explanation:

• p = 0.5 × 10 = 5.

Q45. A force of 100 N acts on mass 20 kg. Find acceleration.

A. 2 m/s²
B. 5 m/s²
C. 10 m/s²
D. 20 m/s²

✅ Correct Answer: B. 5 m/s²

Explanation:

• a = F/m = 100/20 = 5.

Q46. According to Newton’s second law, force is proportional to:

A. Mass only
B. Acceleration only
C. Rate of change of momentum
D. Velocity

✅ Correct Answer: C. Rate of change of momentum

Q47. A truck of mass 2000 kg moving with velocity 20 m/s. Momentum = ?

A. 20000 kg m/s
B. 40000 kg m/s
C. 200000 kg m/s
D. 4000 kg m/s

✅ Correct Answer: B. 40000 kg m/s

Explanation:

• p = 2000 × 20 = 40000.

Q48. Momentum of a body at rest is:

A. Zero
B. Infinity
C. Constant
D. Cannot be defined

✅ Correct Answer: A. Zero

Explanation:

• v = 0 → p = m × 0 = 0.

Q49. Which of these statements is NOT true about momentum?

A. It is a vector quantity
B. It is conserved in all interactions
C. It depends on mass and velocity
D. It is equal to force

✅ Correct Answer: D. It is equal to force

Explanation:

• Momentum ≠ force; force = rate of change of momentum.

Q50. Which factor increases momentum of body most effectively?

A. Decrease mass, keep velocity constant
B. Decrease velocity, keep mass constant
C. Increase both mass and velocity
D. Keep mass constant, velocity zero

✅ Correct Answer: C. Increase both mass and velocity

Explanation:

• Momentum = m × v, so increasing both m and v maximizes p.

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