# Quick question on forces

1. Nov 4, 2008

### qazxsw11111

Hi everyone! Ok, so we have a bird which is flapping its wings. Avg speed of air displaced is v. Area of wings is A. Density of air is p.

Thus mass of air displaced per second is vAp.

This is ok.

However, according to conservation of momentum, (vAp)(v)=mg, where m is mass of bird

Also however, according to Newton's Third Law, Force everted by bird on air=Force exerted on air by bird, (vAp)(g)=mg ???

Is there a mistake in my understanding???

Thanks.

2. Nov 4, 2008

### Staff: Mentor

So what's the problem with that?

3. Nov 4, 2008

### qazxsw11111

I mean like there are two different equations to ma when the bird is hovering in air.

vApg=ma and v(Apv)=ma

Which is correct?

4. Nov 4, 2008

### Staff: Mentor

Check your units. Only one of those equations has units that match. (I assumed it was just a typo before. How did you get that highlighted equation?)

5. Nov 4, 2008

### qazxsw11111

Well, according to Newton's Third Law, Force everted by bird on air=Force exerted on air by bird, (vAp)(g)=mg

I know this is wrong, but why is it?

6. Nov 4, 2008

### Staff: Mentor

That's certainly true.
Where did you get this from? It certainly doesn't follow from Newton's 3rd law. (Note that this equation makes no physical sense, since the units do not match across the equal sign.)