# Homework Help: Force of air resistance?

1. Oct 2, 2011

### axlg38

1. The problem statement, all variables and given/known data

an object with a mass of 1.21g is falling at a constant velocity of 1.1 m/s. What is the force of air resistance exerted on the object?

2. Relevant equations

f=ma

3. The attempt at a solution

i imagined that if the object is falling at a constant velocity, then acceleration is 0. so it seems that f=0, but thats obviously not true. i have no idea how to do this

EDIT: according to the book, the answer is about .0119N, im not sure how to arrive at that answer

Last edited: Oct 2, 2011
2. Oct 2, 2011

### lewando

The acceleration is 0. The velocity is constant. If by "f=0", you mean Ʃf = 0, that is correct.

[Edit: the book is correct]

Last edited: Oct 2, 2011
3. Oct 2, 2011

### lewando

What are the forces acting on the falling body?

4. Oct 2, 2011

### axlg38

gravity and air resistance?

5. Oct 2, 2011

### lewando

Yes! In which direction is gravity working? In which direction is air resistance working?

6. Oct 2, 2011

### axlg38

gravity is working downward, and air resistance is working upward

7. Oct 2, 2011

### lewando

Right, so if acceleration is zero and Ʃf (sum of the forces) = ma = 0, what can you conclude about the magnitudes of the 2 forces?

8. Oct 2, 2011

### axlg38

They cancel each other out? i thought if that happened, the object wouldnt move at all

9. Oct 2, 2011

### lewando

Ah, I see where you are hung up. You should review Newton's 1st law. It is okay for an object to have a constant velocity as long as there is no net force acting on it (think of an object travelling through space, away from significant sources of gravity). In this case, you correctly assert that "they cancel each other out" -- there is no net force acting on the body.

Would you say that the 2 forces are equal and opposite?

10. Oct 2, 2011

### axlg38

yes?

11. Oct 2, 2011

### lewando

Yes! Then we are almost home. What is the force of gravity on the body. Please report in units of newtons.

12. Oct 2, 2011

### axlg38

11.858 n

13. Oct 2, 2011

### lewando

Almost. [Edit: I'm using "almost" in the broadest sense, meaning you could be off by 3 orders of magnitutde ] Newtons are (kg*m)/s2

Last edited: Oct 2, 2011
14. Oct 2, 2011

### axlg38

alright, thanks for the help haha this is going to save me on the test scheduled for tuesday