Now suppose the crate is lifted so rapidly that air resistance was significant during the raising. How much work was done by the lifting force as the box was raised 1.5 m?

A

Explanation:

- The complete question is:

" A crate is lifted vertically 1.5 m and then held at rest. The crate has weight 100 N (i. e., it is supported by an upward force of 100 N). Now suppose the crate is lifted so rapidly that air resistance was significant during the raising. How much work was done by the lifting force as the box was raised 1.5 m?"

Options:

1. More than 150 J

2. A bit less than 150 J because the air partially supported the crate.

3. No work was done.

4. Still 150 J

5. None of these

Solution:

- As the box is raised the work is done against gravity and air resistance opposes the motion of box. Hence, both the force of gravity (Weight and air resistance act downward).

- The amount of work done is the sum of both work done against gravity and against air resistance.

- W = W_g + W_r

W > W_g

W > 100*1.5

W > 150 J

- Hence, the work done is greater than 150 J i. e work is also done against ai r resistance.