Force of air on parachutist

[JonnyG]

Homework Statement

A 50-kg parachutist descends at a steady 40 km/h. What force does air exert on the parachute?

Homework Equations

The Attempt at a Solution

Okay, I know that F_net = ma, where F_net is the net sum of all the forces acting on the parachutist. The two forces acting on him are gravity (acting downward) and the air resistance (acting upward). These forces are in opposite directions.

I know that if I denote the force caused by gravity (did I word that correctly?) by F_g, then we have F_g = 50(-9.8) = -490. Now I will have F_net = F_g + F_a, where F_a is the force of the air. I imagine that I have to use the fact that he is traveling at 40 km/h downward to figure out the his net force must be, and then solve for F_a. But, the way I see it, if he is traveling at a STEADY speed of 40 km/h, then his velocity is constant. So his acceleration is 0. But then I should get a net force of 0. This doesn't seem correct to me.

 

[collinsmark]

Homework Statement

A 50-kg parachutist descends at a steady 40 km/h. What force does air exert on the parachute?

Homework Equations

The Attempt at a Solution

Okay, I know that F_net = ma, where F_net is the net sum of all the forces acting on the parachutist. The two forces acting on him are gravity (acting downward) and the air resistance (acting upward). These forces are in opposite directions.

I know that if I denote the force caused by gravity (did I word that correctly?) by F_g, then we have F_g = 50(-9.8) = -490. Now I will have F_net = F_g + F_a, where F_a is the force of the air. I imagine that I have to use the fact that he is traveling at 40 km/h downward to figure out the his net force must be, and then solve for F_a. But, the way I see it, if he is traveling at a STEADY speed of 40 km/h, then his velocity is constant. So his acceleration is 0. But then I should get a net force of 0.

Correct!

This doesn't seem correct to me.

Step back and let it sink in.

By the way, now that you have the net force, what is the force exerted by the air?

 

[JonnyG]

Correct!

Step back and let it sink in.

By the way, now that you have the net force, what is the force exerted by the air?

Okay, I think it makes sense now! Since 0 = F_net = F_a + F_g = -490 N, then F_a = 490 N. It makes sense that the magnitude of the air force and the magnitude of the gravity force are equal, for they balance out to make his velocity constant.

 

[NascentOxygen]

A net force of zero accounts for his net acceleration being zero.