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Q & A: Exchanging potential energy and kinetic energy

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Most recent answer: 10/07/2012
Is it possible to calculate the kinetic energy of a falling object from just its mass, the distance it falls and acceleration due to gravity? i.e. without measuring velocity or time
- Joe (age 31)
Yes, the law of conservation of energy allows you to do this.   The potential energy of an object raised to a height h above the ground is E = mgh. When you drop the body it loses potential energy and gains kinetic energy.  Since the kinetic energy is T = mv2/2 you can easily solve for v  by equating T and V to give v2 =  2gh. This assumes that the air resistance is negligible.  Notice that the mass drops out of the equation and the velocity is same for any body.   Galileo showed this with his famous experiment of dropping a heavy mass and a light mass from the tower of Pisa and noting that they both reached the ground at the same time.  Actually he was trying to show that the mass of a body associated with inertia was the same as the mass associated with gravity.


p.s We're not sure he actually performed that experiment, but he certainly did related ones. /mw

(published on 10/07/2012)

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