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A girl on a swing varies in height from 60 cm abov the ground to 180 cm.The girl's greatest velocity

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at highest point, velocity = 0, height (potential energy) is max
at lowest point, velocity(Kinetic energy) is max, height (PE) is reduced

total energy at 2 points is equal --- conservation of energy. in the other words, potential energy transform to kinetic energy when decending.

PE reduce = KE increase
PE@highest - PE@lowest = KE@lowest - KE@highest
mg(180) - mg(60) = 0.5mv^2 - 0
g(180-60) = 0.5v^2
work from here
... remark ... unit of v is cm/s
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