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Homework Help: Emergency Homework Help: Forces

  1. Apr 4, 2004 #1
    A 4500-kg helicopter accelerates upward at 2m/s^2. What lift force is exerted by the air on the propellers?

    Is the answer just Newtons Second Law of Motion F=ma, so F=(4500kg)*(2)
    , so F= 9000N
  2. jcsd
  3. Apr 4, 2004 #2
    Don't forget that gravity is also acting on the helicopter, and the propellers must overcome the force of gravity as well.

  4. Apr 4, 2004 #3
    so would I have to convert the initial mass into weight. i.e. W=MG


    Is that what I would have to do?
  5. Apr 4, 2004 #4
    The lift must be greater than the weight enough to accelerate the helicopter 2 m/s^2.

    F = aF is an equation you just made up and has no meaning. Try using Newton's 2nd Law.

  6. Apr 4, 2004 #5
    But Newton's Second Law is Force = Mass times Acceleration. Which I used in the first place.
  7. Apr 5, 2004 #6
    Yes, you did a good job of finding the net force on the helicopter. But that's not the lift because you're not taking into consideration gravity.

  8. Apr 5, 2004 #7


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    I think cookie is saying this

    F = (4500)(9.8 + 2)
  9. Apr 5, 2004 #8
    The law is:
    [tex]\Sigma F = ma[/tex]
    But the lift force is not the only one acting on the helicopter:
    [tex]\Sigma F = F_{lift} - mg[/tex]
  10. Apr 5, 2004 #9


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    Although technically correct, your post can cause a great deal of confusion for somebody who doesn't fully understand the concept.

    If I'm understanding correctly, you are saying this.
    ma = F - mg
    F = ma + mg
    F = m(a+g) <----- like my post said
  11. Apr 5, 2004 #10
    Yes, and why would my post cause confusion? Your post confuses me. :wink: I just thought it would be better to show how to get to the solution you posted, so that in future problems he can do the same.
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