A 6.5-kg box is being lifted by means of a light rope that is threaded through a

Question

A 6.5-kg box is being lifted by means of a light rope that is threaded through a single, light, frictionless pulley that is attached to the ceiling.

(a) If the box is being lifted at a constant speed of 2.0 m/s, what is the power delivered by the person pulling on the rope?
0.128 kW

(b) If the box is lifted, at constant acceleration, from rest on the floor to a height of 1.8 m above the floor in 0.42 s, what average power is delivered by the person pulling on the rope?
________ kw

I solved for part a but doesn't know how to solve for part B.
I've used the equations (mdg)/t and (2m*d2)/(t3) But none works.

Explanation / Answer

Work done by man -mgh = K ;

s = ut + 1/2 at^2 ;

1.8 = 0 * t + 1/2 * a * 0.42^2 ;

or a = 20.408 m /s^2 ;

v= u + at ;

v = 0 + 20.408 * 0.42 = 8.57 m /s

Work done by man -mgh = K ;

Work done by man -mgh = 1/2 m v^2 ;

W = mgh + 1/2 mv^2 ;

W = m ( gh + 0.5 v^2 ) ;

P = m ( gh + 0.5 v^2 ) / t ;

P = 6.5 ( 9.8 * 1.8 + 0.5 * 8.57^2 ) / 0.42 ;

P = 841.32 W <-------ans

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