Air resistance force on a falling table tennis ball at constant speed

[missmerisha]

Homework Statement

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.

Homework Equations

Not quite sure

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

 

[Hootenanny]

Homework Statement

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.

Homework Equations

Not quite sure

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

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?

 

[missmerisha]

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

 

[Hootenanny]

Air resistance is opposing gravity in this case.

True.

There's an unbalanced force because the object is travelling.

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

 

[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

 

[Hootenanny]

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

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?

 

[missmerisha]

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

 

[Hootenanny]

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

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

 

[missmerisha]

wait, I think I get it now
The sum of all forces acting on it is equal because it's traveling 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?

 

[Hootenanny]

wait, I think I get it now
The sum of all forces acting on it is equal because it's traveling 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?

Spot on

 

[missmerisha]

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

 

[Hootenanny]

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

It was a pleasure