The three forces acting on a hot-air balloon that is moving vertically are its weight, the force due to air resistance and the upthrust force. the hot-air balloon descends vertically at constant speed. the force of air resistance on the balloon is f. which weight of material must be released from the balloon so that it ascends vertically at the same constant speed? a f b 2f c 3f d 4f

Explanation :

The forces acting on hot- air balloon are:

Weight, (W)

Force due to air resistance, (F)

Upthrust force, (U)

Its weight W is acting in downward direction. The upthrust force U acts in upward direction. When the balloon is moving upward, the air resistance is in downward and vice versa.

In this case, the hot-air balloon descends vertically at constant speed.

so,

and

so, ....................(1)

when it is ascending let the weight that it is releasing is R, so

..........(2)

solving equation (1) and (2)

2F is the weight of material that must be released from the balloon so that it ascends vertically at the same constant speed.