How fast would a descending pencil have to be to crack a rock in half? And the rock's density is about 2.7?

S = 1/2at^2 + Vo(t) + So (s = displacement) You should be able to plug in the back car as having displacement 0 and the front car as having a positive displacement. You could also think of So being the initial amount that the front car is ahead of the back car. The acceleration terms will cancel when you set them equal to each other Vo1 = initial vel of the back car Vo2 = initial vel of the front car 1/2at^2 + Vo1(t) + 0 = 1/2at^2 + Vo2(t) + So Vo1(t) = Vo2(t) + So Vo1(t) - Vo2(t) = So t(Vo1 - Vo2) = So t = So/(Vo1 - Vo2).

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