Question

A helicopter, starting from rest, accelerates straight up from the roof of a hospital. The lifting force does work in raising the helicopter. An 790-kg helicopter rises from rest to a speed of 7.0 m/s in a time of 4.0 s. During this time it climbs to a height of 7.3 m. What is the average power generated by the lifting force?

Answer #1

According to the concept of the work energy and power

the avrage power P=(mgh+1/2mv^2)/t

Given that

mass m=790 kg

initial speed u=0

finial speed v=7 m/s

time t=4s

height h=7 m/s

now we find the average power gentrated by lifting force

basing on the concept of the work energy therom

Wnet=1/2mv^2-1/2mu^2

-Wg+Wf=1/2mv^2-1/2mu^2

Wf=12/mv^2-1/2mu^2+Wg

Wf=1/2mv^2-1/2mu^2+mgh

Wf=1/2mv^2+mgh

the average power P=Wf/t=1/2mv^2+mgh/t

=(0.5*790*7^2+790*9.8*7.3)/4

=18967.9 W

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