# Homework Help: Air Resistance Question

1. Dec 22, 2008

### missmerisha

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

A table tennis ball of mass 10 g is falling towards the ground with a constant speed of 8.2 m/s.

Calculate the magnitude and direction of the air resistance force acting on the ball.

2. Relevant equations
Not quite sure

3. The attempt at a solution

m= 10 g which equals .01 kg
v = 8.2 m/s

.01* 8.2
= 8.2

Textbook Solution .098N

2. Dec 22, 2008

### Hootenanny

Staff Emeritus
Welcome to Physics Forums,

Let's start from the top. Since the tennis ball is moving with a constant velocity, what can you say about the forces acting on it?

3. Dec 22, 2008

### missmerisha

There's an unbalanced force because the object is travelling. Air resistance is opposing gravity in this case.

4. Dec 22, 2008

### Hootenanny

Staff Emeritus
True.
Not true. I suggest you think about Newton's Second and First Laws.

5. Dec 22, 2008

### missmerisha

Newton's First Law states an object will remain at rest or travel at a constant velocity unless there's an unbalanced force.

Newton's Second Law
F= ma

Thinking time:

Gravity

g/a = 9.8 ms ^-2
m = .01 kg
F= .98 N

So gravity has a force of .98 N

6. Dec 22, 2008

### Hootenanny

Staff Emeritus
You might want to check you arithmetic, but you're on the right lines.

Now know that that the ball is at constant velocity, what can you say about the sum of all the forces acting on it?

7. Dec 22, 2008

### missmerisha

I'm still really confused.
The sum of all forces acting on it is equal.

8. Dec 22, 2008

### Hootenanny

Staff Emeritus
... equal to zero: Correct! So if the weight of the ball is mg, what must be the magnitude of the drag?

9. Dec 22, 2008

### missmerisha

wait, I think I get it now
The sum of all forces acting on it is equal because it's travelling at a constant velocity.
hence, gravity and air resistance must have the same magnitude.

using mg
m = .01 kg
g(gravity) = 9.8 m/s^2
= .098 N

Is that correct?

10. Dec 22, 2008

### Hootenanny

Staff Emeritus
Spot on

11. Dec 22, 2008

### missmerisha

^THANK YOU! I am no longer confused and I get it now!
=D

12. Dec 22, 2008

### Hootenanny

Staff Emeritus
It was a pleasure