Oh man, I don't understand anything about forces Dx

[lolsherry]

I just started taking physics this year (junior year in high school) and I can honestly say that I have no idea what's going on. Alright, so I know about free fall (well...the falling part of free fall, not when the ball bounces up and back down again) and acceleration, but pretty much anything other than stuff that's clearly cut I have no idea where to even START. One problem is:
As a large jet flies at a constant altitude, its engines produce a forward thrust of 8.4 x 10 ^5 N. The mass of the plane is 2.6 x 1^5 kg.
a. What is the forward acceleration of the plane, ignoring air resistance?
b. How much upward force must the air exert on the plane when it is flying horizontally?

And is there a site or something that I can learn forces from? I don't know which forces are opposites (like one is positive and one is negative) and which forces are completely the same and balance each other out.

 

[fss]

As a large jet flies at a constant altitude, its engines produce a forward thrust of 8.4 x 10 ^5 N. The mass of the plane is 2.6 x 1^5 kg.

a. What is the forward acceleration of the plane, ignoring air resistance?

You are given a force (F) and a mass (m). Do you know any equations involving F and m you could use to get acceleration (a) ?

b. How much upward force must the air exert on the plane when it is flying horizontally?

What's a force that pulls objects back to earth? In order for a plane to remain in level flight at altitude, what must be true about the sum of forces in the "vertical" direction?

 

[lolsherry]

What's a force that pulls objects back to earth? In order for a plane to remain in level flight at altitude, what must be true about the sum of forces in the "vertical" direction?

Oooo, I think I drew the diagram wrong. xP Wait, so is the acceleration 3.23 m/s^2? And for the second part...hmmm...the force has to be greater in the "vertical" direction because the net force and acceleration goes in the direction of the greater force, right? Huh. But I don't know how to answer the second part. :/ I just can't see how the upward force and the plane flying horizontally is related.

 

[fizzynoob]

>>the force has to be greater in the "vertical" direction because the net force and acceleration goes in the direction of the greater force, right?

No, you have 1 force in the Y direction initially. Its gravity, which is pulling the plane down towards the earth. They want to know the vertical force required to counter act gravity.

Heres a force diagram
[PLAIN]http://img842.imageshack.us/img842/1926/gradiant.png

Question 1 :

The Forces in the X direction are:

F(thrust) = m*a
a = F(thrust)/m

Question 2:

You must find F(air) that counteracts F(gravity)

Sum of the forces in the Y direction

F(air) - F(gravity) = 0 //it must equal zero to be in equilibrium

now solve for F(air), remember that F(gravity) is m*g