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# Question about motion along a straight line. If the displacement, s=t^2-4t. Find the minimum displacement.?

The minimum displacement should be 0 or -4? We should take the shortest distance between fixed point and particle or use ds/dt to find minimum displacement.

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- Steve4PhysicsLv 72 months agoFavourite answer
Given that ‘s’ is the displacement, the minimum (smallest) value of displacement is -4 (which occurs when t=2).

If the question had been the minimum magnitude of displacement (minimum distance), the answer would be 0 (which occurs when t=0 or t=4s).

But I agree the question could be better-worded.

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- KnrLv 42 months ago
since, given s = t’2 - 4 t, for the displacement to be minimum or maximum, the time derivative of displacement should be zero. Hence, ds/dt = 2 t - 4 = 0 , which gives t= 2 sec.

thus the displacement will be minimum when t = 2 sec and this gives s = 4 - 8 = - 4 m.

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