# Air density and weight?

1. Jul 19, 2010

### Mowgli

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

Air (density= 1.29 kg/m^3) passes over the top surface of an aircraft whoe effective wing surface area is 75.5 m^2 at a speed of 255 m/s. If the air speed beneath the wings is 135 /s, what is the weight of the plane.

2. Relevant equations

W=mg

3. The attempt at a solution

I know how to find the "lift" in newtons due to Bernoulli's effect- but I am thrown off by this question. What is the weight? Please help!

2. Jul 19, 2010

### phyzguy

If you know how to find the lift in newtons pushing upward on the plane, then ask what is the force of gravity in newtons pulling the plane downwards? In stable flight these two are equal.

3. Jul 19, 2010

### Mowgli

I'm sorry- I am not good at this. Are you saying the lift is the same as the weight?

4. Jul 19, 2010

### phyzguy

Yes. Assuming that the airplane is in level flight and neither ascending nor descending, then the lift equals the weight. Just like when you are standing on the floor - the floor is pushing up on you with a force which exactly balances your weight. Otherwise you would be falling, since F=ma.

5. Jul 19, 2010

### Mowgli

Right, that makes sense- thank you so much!