Question

A man weighing 80 kg is standing in 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 m/s, then after 4 sec his displacement relative to the ground will be .....​

Answer 1

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In 1s, man has walked, 4m/s×1s = 4m, with respect to the trolley.

As position of the CoM stays same, let the distance moved by trolley be x in direction opposite to that of man.

So net diplacement of man is (4−x)m

Thus, M man × (4−x) = M trolley x

⟹ 80(4−x) = 320x

⟹ x = 320/400

= 0.8m

So net displacement of man in 1s is, 4 − 0.8 = 3.2m.(Answer).

Explanation:

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Answer 2

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In 1s, man has walked, 4m/s×1s = 4m, with respect to the trolley.

As position of the CoM stays same, let the distance moved by trolley be x in direction opposite to that of man.

So net diplacement of man is (4−x)m

Thus, M man × (4−x) = M trolley x

⟹ 80(4−x) = 320x

⟹ x = 320/400

= 0.8m

So net displacement of man in 1s is, 4 − 0.8 = 3.2m.