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Homework Help: Help . Need help with these problems.

  1. Oct 17, 2007 #1
    Help plz. Need help with these problems.

    Hey. I'm really not good at physics and my professor only tells stories all day so I need help with these problems. Please be as descriptive as you can so I could understand the solution.
    1) A woman pushes two crates across a frictionless horizontal floor. The crates mass 4kg and 20kg respectively. If the woman exerts a force of 36 N on the smaller crate, then approximately what force does the smaller crate exert on the larger crate. (The crates are next to and touching each other. I tried to draw a free body diagram but I'm confused because the answer is 30 N and i thought it was 36N)

    2) There is a picture for this question that I need so I'll just describe it. Two cables, B and C, are at 60 degrees from the ceiling and are holding a 72 N weight attached to another cable A directed straight down that is holding a 150 N weight. I have to find the tension in cables A, B, anc C. I added a diagram that I made from paint but i don't know if it worked. Thanks a lot.

    Attached Files:

  2. jcsd
  3. Oct 17, 2007 #2
    hey could someone please reply. I'm not getting anywhere with these problems
  4. Oct 17, 2007 #3
    From what I know, please correct me if I am wrong.

    F=ma where m = 4kg +20kg and F=36N

    we solve for a which is 1.5m/s/s

    now see what is the force on each block individually.
    F=ma where m1 is 4 and a is 1.5 = 6N
    F=ma where m2 is 20 and a is 1.5 = 30N

    So the woman exerts a force of 36N in total which gets divided...so 6N on the first block by the woman and 30N exerted on the second block by the first block.

    please correct me if i am wrong.
  5. Oct 17, 2007 #4
    ya that sounds right. I never thought to add the masses. I kept trying to solve for different parts in f=ma seperately for each block. Thanks a lot. Now im trying to solve the second problem. I trying to figure out if the Y component of the tension is the sum of the two hanging weights or if it is the sum multiplied by cos 60.
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