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Man pulling himself up at 0.2m/s^2

  1. Oct 25, 2009 #1
    1. The problem statement, all variables and given/known data
    A man with an 70kg mass, sits on chair of 10kg, and pulls himself upwards at a acceleration of 0.2m/s^2. What is the force needed to acheive this acceleration?


    2. Relevant equations

    F=ma

    3. The attempt at a solution
    F=(80)*(9.81+0.2)
    F=(80)*(9.81-0.2)
    Neither are giving a correct answer, please help!
     
  2. jcsd
  3. Oct 25, 2009 #2
    Well, he's not going to take the chair with him when he pulls himself up, I take it? So the mass is 70kg only.

    Remember that the force of gravity is acting in the opposite direction to the direction the man is accelerating in. Taking the direction of acceleration due to gravity to be positive, and the direction of acceleration of the man to be negative (since it is against gravity) then the overall acceleration is 9.81 + (-0.2) = 9.61.
     
  4. Oct 25, 2009 #3
    no, he is pulling himself and the chair upwards, he is on a pully system. sorry for not being clear. appreciate the quick response though!
     
  5. Oct 25, 2009 #4

    Redbelly98

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    One of these last 2 equations is correct (do you know which?) There might be a problem with the number of significant figures you gave in your answer, or you could have done the arithmetic wrong. Since you didn't say what answer you got, it's hard to tell.
     
  6. Oct 25, 2009 #5

    Redbelly98

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    Just read this more carefully, I didn't realize before there is a pulley system involved.

    Have you drawn a free body diagram for the man+chair? In how many different places is a rope attached to them, including where the man is holding the rope to pull himself up?
     
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