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Homework Help: Net force

  1. Oct 28, 2007 #1
    1. The problem statement, all variables and given/known data

    Hey everyone, this problem has been bugging me for days now, and no matter how many times i've attempted it, I feel like its going wrong.


    The buillder has a mass of 75kg and the cradle has a mass of 35kg. The builder pulls on the rope with a force of 650N.

    The rope will exert a force of 650N upwards on the man and 650N upwards on the cradle.

    Explain why the net upward force on the man is:

    Force exerted by floor of lift - (weight of man - 650N)

    2. Relevant equations

    3. The attempt at a solution

    Force exerted by floor of lift = 650N
    Weight of man = 735N

    Now what I don't get is where the other 650N comes from? If the force exerted by the floor of the lift is 650, and the weight of man is 735N, then surely his net upward force is minus 85 Newtons? The only way the equation makes sense is if the the force exerted by the floor of the lift is equal to the upward force, minus the weight? Which again gives me negative 85 newtons?

    If anyone could help me and tell me where im going wrong i'd really appreciate it.
  2. jcsd
  3. Oct 28, 2007 #2
    1) Is he trying to pull the whole thing up? As in, is the rope he is pulling connected to the box he's in?

    2) Check what cancels out, meaning, what is equal and opposite? Him pulling on the rope, makes the rope pull on him, right? His weight on the floor, means the floor pushes back. So what's left?
  4. Oct 28, 2007 #3
    Thanks for replying! :D

    Well he's accelerating (doesnt say how much), so he's definetely pulling the whole box up including himself.

    I just took into account that if he's pulling the cradle up then the floor is lifting him 650N, and the fact he's pulling the rope means a further 650 must be lifting him. Then I took away his weight, which must be exerted back onto the floor and got a net force of 1300N - (his weight) 735 N = 565N, but again I think thats wrong.
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