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Homework Help: Force Colliding

  1. Nov 28, 2005 #1
    Hi, I just need some help with this homework question. It says that two blocks in contact with each other move to the right across a frictionless horizontal surface by two forces shown.

    Forces shown:
    58N force to the right on the left block with a weight of 2.7kg.
    18N force to the left on the right block with a weight of 8.4kg.

    Determine the magnitude of the force exerted on the block with mass 2.7kg by the block with mass 8.4 kg. Answer in units of N.

    Well, from what I've got from submitting some answer online is that it isn't 18N, -18N, -76N, 76N, 40N, or -40N. I have one answer left and I don't know how to figure out what the force is on the left block. I've tried all sorts of subtraction, addition, and just plain stating the forces, but I don't understand how those aren't the answers. Isn't Newton's 3rd law every action has an equal and opposite reaction? That means that if you push on the right block with a force of 58N, then it pushes back with 58N, plus the force that it's pushing on the left block, which is 18N. So I added them up, typed in the answer, and got nothing. What am I doing wrong?

    Edit: I realized that forces cannot be negative, so avoid telling me that I'm wrong in that area, I just now realized my mistake. I still cannot figure out how to solve this one. Do I need to take the acceleration of the two blocks, then take one minus the other, then convert the acceleration back into newtons?
    Last edited: Nov 28, 2005
  2. jcsd
  3. Nov 28, 2005 #2
    Oh, and the formula for Force is:
    F being force, m being mass, a being acceleration
  4. Nov 28, 2005 #3
    Is it the answer is 48.28 N ? If my answer correct , please follow the steps below .

    First you must calculate the acceleretion of the both two block .
    40=(8.4 + 2.7 )a
    Then make the 2.7 kg object is being acted by two forces , that is one is 58 N and another one is F which is acted by 8.4 kg object . Then you will get formula below :
    We use 58-F because the acceleration is in the direction of 58 N and the a is the acceleration we calculated before .
    Substitute the a and you should able to calculate the answer =48N.

    If my answer wrong , I am so sorry because I still a High Scholl student and not the proffessional . :!!)
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