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Homework Help: Few Questions on Newton's Laws

  1. Oct 17, 2009 #1
    1. The problem statement, all variables and given/known data

    An 81 kg man lowers himself to the ground from a height of 10.6 m by holding onto a rope that runs over a frictionless pulley to a 60 kg sandbag.
    (a) With what speed does the man hit the ground if he started from rest?

    (b) What can he do to make him hit the ground slower?

    2. Relevant equations

    3. The attempt at a solution

    attempt for part a: no idea... i drew diagram, labeled forces, but cant seem to go beyond that.
    attempt for part b: hold the string attached to the sandbag and pull down as he is going downward.
  2. jcsd
  3. Oct 17, 2009 #2


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    If you drew your free body diagram and labeled all the forces, the next step should be to write down newton's laws. Ultimately, you want to deduce the man's acceleration.

    Hint: What must be the relationship between the man's acceleration and the sandbag's acceleration?
  4. Oct 17, 2009 #3
    both accelerations should be the same shouldnt it? so this is what i have so far...

    Sum of forces for m1: Ft - m1g

    Sum of forces for m2: m2g - Ft

    m1a = Ft - m1g

    m2a = m2g - Ft

    Ft = m2g - m2a

    m1a = ( m2g - m2a ) - m1g

    m1a + m2a = m2g - m1g

    80a + 60a = 60*9.8 - 80*9.8

    140a = -196

    a = -1.4 m/s^2? but since the man is going down, and my reference frame for going down for a is positive, should it be 1.4? and if a = 1.4 or -1.4, how do i find the impact velocity? V^2 = Vo^2 + 2ax?...
  5. Oct 17, 2009 #4


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    Your original statement of the problem says the man's mass is 81kg, yet you use 80 in your work. Typo? Either way, your work is correct. That equation will do nicely to find velocity. Remember, having acceleration (which is constant in this problem), it's just a kinematics question!
  6. Oct 17, 2009 #5
    Oh sorry about that. Yes, it forgot it was supposed to be 81. But I redid it and got the right answer! Thank You very much!
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