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Homework Help: Basic Rectilinear Motion Question

  1. Jan 6, 2008 #1
    I am confuse with the following Kinematics problem

    A helicopter accelerates uniformly upward at 1m/s^2 to a height of 300 m. Determine the time required for this motion.


    s = 300 m
    v0 = 0
    a = 1 m/s^2
    v = v2

    s = (v0) (t) + (1/2) (a) (t^2)
    300 m = 0 + (1/2) (1m/s^2) (t^2)
    t = 24.5 s

    As the Helicopter move upward, there should exist any Gravitational Acceleration downward.

    But as you can see on the Solution the only available acceleration is the helicopter acceleration.

    Can anyone here tell me what’s wrong with the Gravitational Acceleration?

    Please advance

  2. jcsd
  3. Jan 6, 2008 #2

    Doc Al

    User Avatar

    Staff: Mentor

    The acceleration due to gravity--9.8 m/s^2 downward--would apply if gravity were the only force acting on the helicopter. But that's not the case here--the air is exerting an upward force that more than balances the weight of the helicopter.
  4. Jan 9, 2008 #3
    I don’t know that…

    The book doesn’t say anything about that…

    We are student forced to understand but we aren’t understood.
  5. Jan 9, 2008 #4
    I'm sure the book tells you how to solve the problem of finding how long it takes for something under constant acceleration to travel a certain distance, which is what is asked here, nothing more

    An object in freefall will accelerate downward at 9.8 m/s^2

    Helicopter rides would be less popular if the helicopter were in freefall :)

    What Doc Al was pointing out is an extension of that, if you've studied forces yet(if not just ignore this)then what you can say is that an object will accelerate at 9.8m/s^2 if gravity is the only force acting on it. If the helicopter were hovering there'd be a force acting upwards to cancel it, and if the helicopter were accelerating upwards(as it is here)there's a force acting upwards greater in magnitude than the gravitational force
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