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Asked in JEE Main 29th Jan 2nd Shift 2023 · Applying the second law and free-body diagrams
Idea: once the force stops the body coasts at constant speed, so the second stretch hands you the speed it had reached.
Constant-velocity phase: v=(50)/(10)=5 m/s.
That is the speed at the end of the accelerated phase, which started from rest and lasted 20 s:
a=(v-0)/t=5/(20)=0.25 m/s².
F=ma=20×0.25=5 N.
The 50 m is deliberately placed after the force ceases; used as an accelerated distance it gives a quite different answer.
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