Question:
A man weighing 80 kg is standing on a trolley weighing 320 kg. The trolley is resting on frictionless horizontal rails. If the man starts walking on the trolley with a speed of 1 $ms^{-1}$, then after 4 seconds his displacement relative to the ground will be:
A man weighing 80 kg is standing on a trolley weighing 320 kg. The trolley is resting on frictionless horizontal rails. If the man starts walking on the trolley with a speed of 1 $ms^{-1}$, then after 4 seconds his displacement relative to the ground will be:
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Person moves forward, trolley recoils backward!
Updated On: Jun 6, 2026
- 5 m
- 4.8 m
- 3.2 m
- 3.0 m
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The Correct Option is C
Solution and Explanation
Step 1: Concept
Conservation of linear momentum.
Step 2: Meaning
Recoil effect of a person walking on a platform.
Step 3: Analysis
$M_m v_m + M_t v_t = 0$. $80v_m + 320v_t = 0 \rightarrow v_m = -4v_t$. Relative velocity $v_m - v_t = 1 \rightarrow -4v_t - v_t = 1 \rightarrow v_t = -0.2~ms^{-1}$. Velocity relative to ground $= 1 - 0.2 = 0.8~ms^{-1}$. Displacement $= 0.8 \times 4 = 3.2~m$.
Step 4: Conclusion
The displacement is 3.2 m.
Final Answer: (C)
Conservation of linear momentum.
Step 2: Meaning
Recoil effect of a person walking on a platform.
Step 3: Analysis
$M_m v_m + M_t v_t = 0$. $80v_m + 320v_t = 0 \rightarrow v_m = -4v_t$. Relative velocity $v_m - v_t = 1 \rightarrow -4v_t - v_t = 1 \rightarrow v_t = -0.2~ms^{-1}$. Velocity relative to ground $= 1 - 0.2 = 0.8~ms^{-1}$. Displacement $= 0.8 \times 4 = 3.2~m$.
Step 4: Conclusion
The displacement is 3.2 m.
Final Answer: (C)
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