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Homework Help: Homework Mass Question

  1. Nov 29, 2006 #1
    1. The problem statement, all variables and given/known data
    On a planet far, far away, an astronaut picks up a rock. The rock has a mass of 4.40 kg, and on this particular planet its weight is 40.0 N. If the astronaut exerts an upward force of 49.5 N on the rock, what is its acceleration?

    Magnitude _____m/s2

    2. Relevant equations
    Acceleration = Velocity/Time

    3. The attempt at a solution

    I know that the rock is moving in the upward direction, because the force applied to the rock is more than its mass. One thing that I do not understand however, is how to find the acceleration. I am given the velocity, 49.5 but I do not get the time. How can I figure out the time knowing that the velocity is 49.5 and the rock weighs 40 N?
  2. jcsd
  3. Nov 29, 2006 #2
    anyone got any ideas on this?
  4. Nov 29, 2006 #3
    Think about it this way.
    The key law that you're supposed to use is that the net force on an object = its mass times its acceleration, or F = ma...
    Now, you have 2 forces, one acting up and one acting down...
    Read between the lines. :)
  5. Nov 29, 2006 #4
    Well if F = ma then

    49.5 = 40a

    49.5/40= a

    a = 1.2375 but the answer is not correct, what did I do wrong!?
  6. Nov 29, 2006 #5
    How much force would the astronaut have to exert to keep it in equilibrium?
  7. Nov 29, 2006 #6


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    No, it's F_net =ma. The 49.5N force acts up, and the gravity force (weight)acts down. So what is F-net??
  8. Nov 30, 2006 #7
    So 49.5-40! = Fnet Fnet = 9.5

    9.5/4.40 = a?
  9. Nov 30, 2006 #8
    Ah I see thanks I got it right!
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