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Homework Help: Helicopter Lifting off(Newtons Laws). Help please?

  1. Jan 19, 2009 #1
    1. The problem statement, all variables and given/known data
    A medical helicopter weighs 7000 lbs. How much upward force must the propeller supply for it to lift off at 10ft/sec(squared)?

    2. Relevant equations

    T-w=ma, w=mg

    3. The attempt at a solution
    Here is what I tried:
    T-7000= 217.39(-10ft/s)

    What am I doing wrong here? Shouldnt the acceleration be a negative value since it is going upward? and what should the actual final answer be?
  2. jcsd
  3. Jan 19, 2009 #2
    F = m * a

    now, to go up, it must counter both gravitational acceleration, plus add that additional 10 ft/sec acceleration

    so "a" = 10 ft/sec + .....
  4. Jan 19, 2009 #3
    I see, thank you.
    So technically, the only thing I did wrong was the negative sign.
  5. Jan 19, 2009 #4
    Yeah, because if you're doing T-w then you're saying that 'g' is acting in the negative direction since you're subtracting it from upward force. Since the 10 ft/s are in the opposite direction from g, it can't be negative TOO
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