# Homework Help: Help on 3D Motion Prob

1. Oct 11, 2005

### Anisotropic Galaxy

A jet fighter takes off at an angle of 27.0 degrees w/ the horizontal, accelerating at 2.62 m/s*s. The plane weighs 79,300 N. I need to find the thrust T of the engine on the plane and the lift force L exerted by the air perpendicular to the wings.

Thanks

My reasoning so far...

79,3000 N / g = 8084 kg.

F = ma = 2.62 m/s^2 * 8092 kg -> 21179 N.

So I'm thinking that be the thrust.

As for the lift force L though, I don't know. Could I use sin or cosine?

2. Oct 11, 2005

### vanesch

Staff Emeritus
This is the mass of the airplane. You'll need it.

Here's a list of all the forces that act upon the plane:
- gravity
- lift of the wings
- thrust of the engines

That's 3 forces (try to find in what direction they are applied ; make a drawing of the forces). Find the total force.

Now, the airplane is not in equilibrium. It is accelerating. So what should you do with the 3 forces ? What equation should you use ? (hint: Newton...)

That should be the total force, yes....

No, it is not the thrust. It is the total force (of which thrust is only a part)...

Leave you here...

3. Oct 11, 2005

### Anisotropic Galaxy

Is the thrust simply cos(27.0)*21179 N then?

So...the force of the lift is 89,000 N, right? a_y = 2.62 m/s^2*sin27.0 = (Lift - mg)/m so then by substituting things, Lift = 89,000 N.

4. Oct 11, 2005

### Anisotropic Galaxy

But someone else got a different answer than me, so I'm probably wrong here......

5. Oct 11, 2005

### Anisotropic Galaxy

Does

Should i switch the axis around, and then the Force of the thrust = (2.62cos27-gsin27)m and L equal to (2.62sin27+gcos27)m?

sound right...?

6. Oct 11, 2005

### Anisotropic Galaxy

or.. maybe not?

L = mgcos27 and T = m(a_x) + mgsin27?