Question

A weight lifter picks up a barbell and 1. lifts it chest high 2. holds it for 30 seconds 3. puts it down slowly (but does not drop it). rank the work w that the weight lifter does during each of these three operations. label the quantities as w1, w2, and w3. (hint: think about how work is defined in terms of who is applying forces and who is doing work.) w3 = w2 = w1 w3 = w1 > w2 w2 > w1 > w3 none of the above w1 > w2 > w3 w3 > w2 > w1 w2 > w3 > w1 justify your ranking order.

Answer 1

1. lifts it chest high

The force opposing to this action is the force due to gravity. Therefore the work done is:

W1 = m g d

where m is mass of the barbell, g is gravity and d is displacement

 

2. holds it for 30 seconds

Work is a product of force and displacement, since there is no displacement, therefore work done is zero.

W2 = 0

 

3. puts it down slowly

If the barbell was dropped, then it would simply be a free fall. But since it was not, so the work done here is also equal to the weight of the barbell times displacement:

W3 = m g d

 

We can see that W1 = W3, and since W2 = 0, therefore the answer is:

w3 = w1 > w2