Answers

2014-08-29T18:26:57+05:30
One third from the celling of the floor = 15*1/3 = 5meter
total mechanical energy of the system is conserved 
loss in potential energy = gain in kinetic energy
gain in kinetic energy (K.E.) = m*g*h
m = 50kg
g = 9.8 m/s²
h = 5m
K.E. = 50*9.8*5 = 2450 joule
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2014-08-30T00:19:01+05:30

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Let us assume that the boy has fallen freely, without any friction (without holding the rope tightly) during 15/3 = 5 meters. He was at rest (speed = 0) at the ceiling of the gymnasium. let his mass be m = 50 kg.

The potential energy corresponding to the drop of 5 meters is converted in to the kinetic energy, due to conservation of (mechanical) energy.

Drop in P.E. = m * g * 5 = gain in kinetic energy = KE - 0
So K.E. = 50 * 9.81 * 5 = 2452.5 Joules    (it is 2,500 J if g = 10 m/sec²)


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