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Homework Help: Help on 3D Motion Prob

  1. Oct 11, 2005 #1
    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.


    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. jcsd
  3. Oct 11, 2005 #2


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    Staff Emeritus
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    Gold Member

    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...
  4. Oct 11, 2005 #3
    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.
  5. Oct 11, 2005 #4
    But someone else got a different answer than me, so I'm probably wrong here......
  6. Oct 11, 2005 #5

    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...?
  7. Oct 11, 2005 #6
    or.. maybe not?

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