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If a 10kg rock falls of a 10m cliff, what is its speed right before it hits the ground? Show work.



Answer :

AL2006

The rock's potential energy just as it rolls off the edge of the cliff is

   (mass) (gravity) (height) = (10kg) (9.8 m/s²) (10 m) = 980 kg-m²/s²

That's exactly the kinetic energy it will have when it hits the ground.

   Kinetic Energy = (1/2) (mass) (speed)²

   980 kg-m²/s² = (1/2) (10kg) (speed)²

Divide each side by  5kg :

   Speed² = (980 kg-m²/s²) / (5 kg) = (196 m/s)²

   Speed = square root of (196 m²/s²)  =  14 m/s

By the way . . . the mass doesn't matter.  Without air resistance
(which is always ignored in problems like this), It doesn't matter
if it's a rock, a feather, a bowling ball, or a bus.  If it falls from
10 m, then it lands at  14 m/s .  That's how gravity works.