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CodeVID-P9-02-TLM-01
Newton's Third Law & Conservation of Momentum — Assignment
Name: ____________________
Roll No.: __________
Date: ____________
General Instructions
- All questions are compulsory.
- Section A carries 1 mark each, Section B 2 marks, Section C 3 marks and Section D 5 marks.
- Show full working with correct SI units and state directions in numerical answers.
Section A — Multiple Choice Questions
5 × 1 = 5 marks
1.
Action and reaction forces act on:
- A.the same body
- B.two different bodies
- C.no body
- D.three bodies
2.
The recoil of a gun is an example of conservation of:
- A.energy
- B.momentum
- C.mass
- D.charge
3.
A rocket moves forward by pushing gases:
- A.upward
- B.downward
- C.sideways
- D.forward
4.
Momentum is conserved when the external force on a system is:
- A.maximum
- B.zero
- C.increasing
- D.negative
5.
When you walk, the ground pushes you:
- A.backward
- B.forward
- C.downward
- D.upward only
Section B — Short Answer (2 marks)
4 × 2 = 8 marks
6.
State Newton's Third Law of Motion.
7.
State the law of conservation of momentum.
8.
Why does a boat move backward when a person jumps forward from it?
9.
Give one everyday example of Newton's Third Law.
Section C — Short Answer (3 marks)
3 × 3 = 9 marks
10.
A gun of mass $5\ \text{kg}$ fires a bullet of mass $0.025\ \text{kg}$ at $300\ \text{m/s}$. Find the recoil velocity of the gun.
11.
Explain why action and reaction forces do not cancel each other.
12.
A $3\ \text{kg}$ trolley at $4\ \text{m/s}$ sticks to a stationary $1\ \text{kg}$ trolley. Find their common velocity.
Section D — Long Answer (5 marks)
1 × 5 = 5 marks
13.
State the law of conservation of momentum. A $60\ \text{kg}$ skater moving at $5\ \text{m/s}$ catches a $4\ \text{kg}$ ball thrown toward her at $5\ \text{m/s}$ in the opposite direction. Find their common velocity after she catches it.
Answer Key
Section A — Multiple Choice Questions
- (B) two different bodies
- (B) momentum
- (B) downward
- (B) zero
- (B) forward
Section B — Short Answer (2 marks)
- To every action there is an equal and opposite reaction, acting on two different bodies.
- In the absence of an external force, the total momentum of a system remains constant.
- The person's forward momentum is balanced by the boat's equal backward momentum (conservation of momentum).
- While walking, the foot pushes the ground back and the ground pushes the person forward.
Section C — Short Answer (3 marks)
- $v_g = -m_b v_b/m_g = -(0.025 \times 300)/5 = -1.5\ \text{m/s}$, i.e. $1.5\ \text{m/s}$ backward.
- They are equal and opposite but act on two different bodies, so they cannot cancel.
- $3 \times 4 = (3+1)v \Rightarrow 12 = 4v \Rightarrow v = 3\ \text{m/s}$.
Section D — Long Answer (5 marks)
- Conservation: total momentum stays constant with no external force. Taking skater's direction positive: $(60 \times 5) + (4 \times -5) = (60+4)v \Rightarrow 300 - 20 = 64v \Rightarrow v = 280/64 = 4.375\ \text{m/s}$ in the skater's direction.
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