1. Tardigrade
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  4. A man weighing 60 kg is standing on a trolley weighing 240 kg. The trolley is resting on frictionless horizontal rails. If the man starts walking on the trolley with a velocity of 1 m s-1, then after 4 s, his displacement relative to the ground is

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Q. A man weighing 60 kg is standing on a trolley weighing 240 kg. The trolley is resting on frictionless horizontal rails. If the man starts walking on the trolley with a velocity of $1\, m\, s^{-1}$, then after $4\, s$, his displacement relative to the ground is

Work, Energy and Power

Solution:

The trolley shall move backwards to conserve momentum. The backward momentum would be shared by both the trolley and man.
Applying conservation of momentum
$60 \times 1 = (240 + 60)v$ or $60 = 300v$
or $\upsilon=\frac{60}{300}=\frac{1}{5}\,m\,s^{-1}=0.2\,m\,s^{-1}$
Speed of man w.r.t. ground $=\left(1-0.2\right)m\,s^{-1}$
$=0.8\,m\,s^{-1}$
Displacement of man $= 0.8 × 4 = 3.2\, m$