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Homework Help: -newton's laws

  1. Jul 8, 2007 #1
    urgent-newton's laws

    I need help with this question(halliday, resnick and walker, fifth edition, chapter 5 , question 14): PS: I cant draw the figure so I am explaining it as much as possible.

    3 wooden blocks are pushed across a frictionless floor.The blocks have masses of 5kg, 2kg and 10kg. The 3 blocks are joined or pressed against each other in the above order. In other words they are moving together.
    A force of F acts on the first block i e 5 kg.

    The question asks to find the acceleration and force on each block?

    I have considered the 3 blocks as 3 different system. Considering motion only in horizontal direction, I would say I got the following equations:

    F-f(force on 1st block due to second)=5a1(accn of the 1st block)
    f(force on 2nd block due to 1st)-(force on 2nd block due to 3rd)= 2 a2(acc of 2nd block)
    f(force on 3rd due to 2nd)= 10 a3

    adding all the 3 equations and cancelling the action-reaction forces, I got
    F= 5a1+2 a2+10 a3

    The question now asks to rank the blocks accroding to their acceleration and the total mass accelerated by 1) F.

    I have no clue how to proceed from here. Pls can someone help.

    I have seen a similar problem of 2 blocks , and the author assumed common acceleration for both the blocks by considering them as one system
    , and then for each block found the net force from free bodied diagram which should be equal to the mass of the individual block*common acceleration.
    My question: why is this not the case in the above question? If it were a case it why would the ist question ask to rank the blocks in terms of their acclerations?

    I am so confused.
  2. jcsd
  3. Jul 8, 2007 #2


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    You are correct. I think the problem may be asking to rank the blocks by the net force acting on each of them.
  4. Jul 8, 2007 #3
    But dont you think that the second example/question is wrong in the sense why would we assume the 2 blocks to have same acceleration. If the 2 blocks had been joined by a massless inextensible cord/rope it would have a common acceleration but on the other hand lets sayif the 2 blocks had a common acceleration why would in the 3 block case each block have different acceleration?
  5. Jul 8, 2007 #4
    I don't know how the figure actually looks, but from your description of the blocks being joined together I don't see how this situation really differs from a cord/rope situation.
  6. Jul 8, 2007 #5
    in the rope situation, the tension makes sure that the both bodies move with same acceleration in order to prevent the string break. but lets say they have same acceleration, why does the initial question asks them to be rank in order of accelerations?
    i guess the question cant be wrong bcoz it is from FUNDAMENTALS OF PHYSICS by halliday and resknick
  7. Jul 9, 2007 #6


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    a1=a2=a3. How is the problem actually worded? You said something about ranking by mass and acceleration....
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