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2014-07-22T19:04:19+05:30

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There is a force on the lift exerted by some motors etc. due to which it accelerates and decelerates.  Let us say its acceleration is 'a' upwards. let time interval be t1.
 a = (6 m/s - 4 m /s )/ t1  = 2/t1
distance traveled in t1 = 4 t1 + 1/2 a t1 * t1 = 5 t1
there is change in PE :  400 * g * 5 t1    positive if lift moved upwards.
change in Kinetic energy =  1/2 * 400 (6*6 - 4 * 4) = 1000 Joules  positive
total change in energy = 2000 g t1 + 1000 
force on the lift = m a = 800 / t1
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