A gang of robbers is planning to hold up a train. Their plan is simple and elega

Question

A gang of robbers is planning to hold up a train. Their plan is simple and elegant: one of them (a former physics major) will perch atop a 100 m vertical cliff immediately adjacent to the tracks. As the train approaches, she will drop a stick of dynamite from the top of the cliff onto the tracks to blow off its front wheels, bringing it to a stop. The rest of the gang can then rob the train at their leisure. A potential minor hitch to this plan is that the sticks of dynamite the gang can get their hands on are all pretty small, so the dynamite dropper must be very accurate, and drop the stick so that it is just at ground level when the front of the train reaches it. (Being a clever physics major, she has carefully trimmed the length of the dynamite

Explanation / Answer

time taken by the dynamite to reach ground = t1 = sqrt(2*h/g) = sqrt(2*100/9.8) = 4.51 s

using s = u*t2+0.5*a*t2^2

500=50*t2+0.5*3*t2^2==========> t2 = 8.054 seconds

robber must release her dynamite,after time t = t2-t1 = 3.544seconds

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