EXAMPLE 1 (a) How much work is done in lifting a 1.2-kg book off the floor to pu

Question

EXAMPLE 1
(a) How much work is done in lifting a 1.2-kg book off the floor to put it on a desk that is 0.6 m high? Use the fact that the acceleration due to gravity is

g = 9.8 m/s2.

(b) How much work is done in lifting a 25-lb weight 8 ft off the ground?

SOLUTION
(a) The force exerted is equal and opposite to that exerted by gravity, so the force isF = m

= mg = (1.2)(9.8) = N and then the work done isW = Fd =

(0.6) = J.
(b) Here the force is given as

F = 25 lb,

so the work done is

W = Fd = 25 · 8 = ft-lb.

Notice that in part (b), unlike part (a), we did not have to multiply by g because we were given the weight (which is a force) and not the mass of the object.

Explanation / Answer

a) Work = Force * displacement

==> Work =mass * acceleration due to gravity * displacement

==> Work = 1.2*9.8*0.6

==> Work = 7.056 Joules

b) Work = Force * displacement

==> Work = 25 *8 = 200lb-ft

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