Third Law and Conservation of Momentum

Third Law and Conservation of Momentum MDCAT MCQs with answers

10 past paper MCQs on Third Law and Conservation of Momentum, from the Force and Motion unit of MDCAT Physics. The year each question appeared is shown where known; tap “Show answer” for the correct option and a short solution.

Read the notes first Practise the unit as a quiz

1.Which of the following is incorrect? (MDCAT 2022)

  1. the reaction force on a body is always balanced by the action force on the same body
  2. reaction and action forces are always equal in magnitude
  3. action and reaction forces never act on the same body
  4. Newton's third law is always valid
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Correct answer: (a) the reaction force on a body is always balanced by the action force on the same body

Action and reaction act on two different bodies, so they can never balance each other on one body.

2.A canon is placed on a smooth surface. When it fires a shell, the canon moves backward, this recoil occurs due to: (UHS MDCAT 2025)

  1. law of conservation of energy
  2. Backward thrust of the gases
  3. Newton's third law of motion
  4. Newton's first law of motion
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Correct answer: (c) Newton's third law of motion

The shell is pushed forward and the cannon takes an equal and opposite push backwards.

3.A 0.02kg bullet moving at 300m/s embeds itself in a 2 kg block at rest on a smooth surface. What is the velocity of the block-bullet system just after impact? (KMU MDCAT 2025)

  1. 2m/s
  2. 3m/s
  3. 4m/s
  4. 5m/s
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Correct answer: (b) 3m/s

Momentum is conserved, so $0.02\times300$ is shared by $2.02\,\mathrm{kg}$, giving about $3\,\mathrm{m\,s^{-1}}$.

4.Two bodies in contact experience forces in: (PMC MDCAT 2022)

  1. Same direction
  2. Opposite directions
  3. Perpendicular directions
  4. five different direction
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Correct answer: (b) Opposite directions

Action and reaction are equal and opposite.

5.Conservation of momentum is always applicable in a: (PMC MDCAT 2022)

  1. Open system
  2. Closed system
  3. Isolated system
  4. Any system
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Correct answer: (c) Isolated system

Momentum is conserved when no external force acts, that is in an isolated system.

6.While moving a boat forward we have to: (PMC MDCAT 2022)

  1. Push water forward
  2. Push water backward
  3. Keeps the water steady
  4. Keeps the water undisturbed
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Correct answer: (b) Push water backward

The oar pushes the water back and the water pushes the boat forward.

7.The horse pulling the wagon is able to move forward because of: (PMC MDCAT 2022)

  1. The force exerted by the horse on the ground
  2. The force exerted by the ground on the horse
  3. The force exerted by the wagon on the horse
  4. The force exerted by the horse on the wagon
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Correct answer: (b) The force exerted by the ground on the horse

The horse pushes back on the ground and the ground pushes it forward.

8.If a bird catches its prey from a height which was initially at rest then momentum will be conserved for: (PMC MDCAT 2022)

  1. Bird
  2. Prey
  3. Either bird and prey
  4. Both bird and prey
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Correct answer: (d) Both bird and prey

No outside force acts on the pair, so the momentum of the bird and prey together is conserved.

9.The total momentum of a flock of identical birds could be zero only if the birds are: (PMC MDCAT 2022)

  1. Taking off from the ground
  2. Flying in the same direction
  3. Flying in different dlrections
  4. Very tired and coming down to rest
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Correct answer: (c) Flying in different dlrections

Momentum is a vector, so equal and opposite momenta can cancel only if the birds fly different ways.

10.Q.79 Which of the following is NOT TRUE? (NUMS MDCAT 2022)

  1. Action and reaction have same nature
  2. Action and reaction have same line of action
  3. Action and reaction never act on same body
  4. Action and reaction can cancel each other
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Correct answer: (d) Action and reaction can cancel each other

They act on different bodies, so they can never cancel one another.

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