Question

example 3
a weight lifter is bench - pressing a barbell whose mass is 110 kg. he raises the barbell a distance of 0.65 m above his chest and then lowers the barbell the same distance. the weight is raised and lowered at a constant velocity. determine the work done on the barbell by the lifter when (a) the barbell is lifted and (b) when it is lowered. (c) what was the net work done?

Explanation:

Step1: Calculate barbell weight

First, find the weight (force due to gravity) of the barbell using $F = mg$. Take $g = 9.8\ \text{m/s}^2$.
$F = 110\ \text{kg} \times 9.8\ \text{m/s}^2 = 1078\ \text{N}$

Step2: Work done lifting barbell

Work is force times distance, with force and displacement in the same direction.
$W_{\text{lift}} = F \times d = 1078\ \text{N} \times 0.65\ \text{m} = 700.7\ \text{J}$

Step3: Work done lowering barbell

Force opposes displacement, so work is negative.
$W_{\text{lower}} = -F \times d = -1078\ \text{N} \times 0.65\ \text{m} = -700.7\ \text{J}$

Step4: Calculate net work

Sum work from lift and lower.
$W_{\text{net}} = W_{\text{lift}} + W_{\text{lower}} = 700.7\ \text{J} + (-700.7\ \text{J}) = 0\ \text{J}$

Answer:

(a) $700.7\ \text{J}$
(b) $-700.7\ \text{J}$
(c) $0\ \text{J}$