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Homework Help: Net Force

  1. Oct 4, 2007 #1
    1. The problem statement, all variables and given/known data
    A 5400 kg helicopter accelerates upward at 2.17 m/s2. The acceleration due to gravity is 9.8 m/s2. What life force is exerted by the air on the propellers? Answer in N.

    2. Relevant equations

    3. The attempt at a solution

    I am making the acceleration positive (2.17m/s2) and the gravitational acceleration negative (-9.8 m/s2) since the helicopter is travelling upwards.

    F(lift) - F(gravity) = ma
    F(lift) - (5400kgx(-9.8m/s2)) = (5400kg)(2.17 m/s2)
    F(lift) = 11718-52920 N
    F(lift) = -41202 N

    Shouldnt the lift force acting upon the propellers by positive?
  2. jcsd
  3. Oct 4, 2007 #2


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    Net force means the SUM of all forces, positive and negative.
    Thus, you have a minus too much here:

    F(Lift)=ma-F(grav), remember that F(grav) is here a negative quantity.
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