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A boy and the skateboard on which he is standing have a combined mass of 60 kilograms He throws a 1-kilogram rock forward at 20 miles per second At what speed will the boy roll backward?
Wiki User
∙ 14y ago
He uses muscular energy to impart forward momentum to the rock,
in the amount of MRVR = 20 kilogram-miles per second.
Since the total momentum of the boy-skateboard-rock system is conserved,
the boy and skateboard must acquire the same amount of momentum in the
opposite direction.
M(B+S)V(B+S) = 20
V(B+S) = 20 / M(B+S)
= 20 / 60 = 1/3 mile per second.
(The questioner's typo ... writing "miles per second" where he meant "meters per second" ...
makes this quite a story ! The boy's pitch is no ordinary fling, endowing the rock with over
32,000 joules of kinetic energy. Thereafter, the rock ... or whatever is left of it after its
incendiary trip through the atmosphere at 20 miles per second ... is well on its way to the
moon, while the boy and his skateboard recoil backwards at a mere 1,200 miles per hour.
But the physics and the arithmetic are sound, even if they have no resemblance to reality.)
Wiki User
∙ 14y ago
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