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Potential energy and kinetic energy



Hi, the question is...a body of mass 200kg is raised to a height of 5m before being allowed to fall freely.

Calculate the potential energy and kinetic energy;

a) before it starts to fall

B) when it reaches the ground.

i have calculated for

P.E@5m = PE=m x g x h = 200x10x5 = 10kJ

P.E@0m= m x g x h = 200x10x0= 0J

k.e@5m= ke= (m x v2)/2 = (200x0)/2 = 0J

k.e @0m= (m x v2)/2 = (200x10sq)/2 = 10KJ

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Hi Matt. That looks correct. The total energy is conserved (neglecting friction) in this problem, so at the highest point it is at rest, KE=0, and all its energy is potential energy. As it falls, the potential energy gets converted to kinetic energy, so right before it hits the ground, its potential energy is 0, and its kinetic energy is 10kJ. Once it hits the ground and comes to rest, both potential and kinetic energy are zero (note that all of these are based on the assumption that we're calling the ground level our relative zero point of potential energy).


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