A weight lifter lifts a mass of 250 kg with a force 5000 N to the height of 5 m.
(a) What is the work done by the weight lifter!
(b) What is the work done by the gravity?
(c) What is the network done on the object?

Explanation

(a) When the weight lifter lifts the mass, force and displacement are in the same direction which means that the angle between them `theta= 0^(@)`. Therefore, the work done by weight lifter `w_("weight lifter")=F_(u)hcostheta=F_(w)h(coso^(@))`
`=5000xx5xx(1)=25,000"joule"=25KJ`
(b) When the weight lifler lifts the mass, the gravity acts downwards which means that force and displacement are in opposite direction. Therefore, the angle between them `theta=180^(@)`
`w_("gravity")=F_(g)hcostheta=mgh(cos180^(@))`
The net work done (or total work done ) on the object
`w_("net")=w_("weight lifter")+w_("gravity")`
`=25KJ-12.5KJ=+12.5KJ`