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Solving for Acceleration when weight is in Newtons instead of Kg

  1. Jan 21, 2013 #1
    1. The problem statement, all variables and given/known data

    On the earth, an astronaut and the equipment weigh 1,960 N. Weightless in space, the motionless astronaut and equipment are accelerated by rocket pack with
    100 N thrust, that fires for 2.0s. What is the resulting final velocity? (Hint: First calculate the acceleration. Also take the initial velocity as zero)

    2. Relevant equations

    I am not sure how to solve for the Acceleration when mass is in Newtons and not Kg. I can not use the formula which I think i use which is f=ma. then the final velocity = acceleration * time - initial velocity

    3. The attempt at a solution

    I divided 1960 N by 9.8 m/s^2= 200kg <<If that is right please explain because I don't understand.

    200kg * a = 100N which equals a=0.5m/s^2

    Vf = 0.5m/s^2 * 2sec - 0 = 1m/sec for the final velocity.
  2. jcsd
  3. Jan 21, 2013 #2


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    Weight is a force, measured in newtons. Mass is, well, mass, measured in kg. You must get the distinction clear.
  4. Jan 21, 2013 #3
    i know but the problem has given me a weight of the astronaut and his equipment in newtons not in kg so how can i solve for the acceleration
  5. Jan 21, 2013 #4


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    tms is pointing out the misuse here: "when mass is in Newtons and not Kg".
    You are told the weight at Earth's surface in N, not the mass. From that you correctly deduced the mass.
  6. Jan 21, 2013 #5
    I think i got it now. But one more question, so is that always how you would change the newtons into a mass in kg by dividing it by the acceleration due to gravity or is there a formula to manipulate.
  7. Jan 21, 2013 #6


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    Yes. Weight is only meaningful in a specified context (i.e. the local gravitational field) and is given by mass * field strength. So given the weight you can divide by the field strength to get the mass.
  8. Jan 21, 2013 #7
    okay well thanks guys the rest should be right then.
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