Physics (forces)

Question

Suppose the rocket had a mass of 850 grams at the time the parachute opened. What must the force of air resistance have been once terminal velocity was reached?

Answer 1

Consider the free-body diagram:

Once "terminal velocity" has been reached, the rocket/parachute system will fall at constant velocity. That means there will be no acceleration. By Newton's 2nd law (F = ma), if there is no acceleration then there is no net force. If there is no net force then, from the free body diagram, F_{AIR RESISTANCE} - F_GRAVITY = 0 or F_{AIR RESISTANCE} = F_GRAVITY

We know that the F_GRAVITY equals mass times the acceleration due to gravity:

F_GRAVITY = mg

F_GRAVITY = (0.850)(9.8) = 8.3 N

So F_{AIR RESISTANCE} = 8.3 N