First slide

Relative motion based problems

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 along the rails at speed 1 m/s, then after 4s his displacement relative to the ground will be :

Easy

A

5 m
B

4.8 m
C

3.2 m
D

3.0 m

Solution

Man travels in 4sec, 4 x 1 = 4 m
Trolley moves back word = $\frac{4 \times 80}{80 + 320} = \frac{320}{400} = 0.8 m$
Net displacement w.r.t ground. 4 – 0.8 = 3.2 m

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