What happens to the force of an air balloon if the total weight if the air balloon is higher than the lift force?

Question

A hot air balloon resting on the ground has a 770 N lift force. The weight of the basket is 150 N, and the balloon plus the gas in it weigh 80 N. If a rider weighing 560 N gets in, which statement is correct?

A. The net force on the balloon is 20 N upward.
B. The net force on the balloon is 790 N upward.
C. The total weight of the balloon is 790 N upward.
D. The normal force of the ground on the balloon is 20 N upward.

Answer 1

The net force on the balloon due to lift, and weight of the basket + balloon + gas + rider is

 

F = [770 - (150 + 80 + 560)] N = -20 N (downward).

 

Since the balloon is not accelerating downward, there must be an equal and opposite normal force applied by the ground on the balloon, which would be 20 N (upward).  The correct answer is (D).