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Problem Solving: Forces

  1. Oct 10, 2005 #1
    Hi there,

    I'm having some trouble with this problem right now. We are learning about Newton's First/Second/Third Laws.

    With a friction coefficient of 0.15, what is the force that the crates exert on each other if the crate 1 is 75kg and crate 2 is 110kg with a push of 730N?

    Can anyone give me pointers or starters?
  2. jcsd
  3. Oct 10, 2005 #2
    Treat the system as a whole to get acceleration.
  4. Oct 10, 2005 #3
    Errrr... I was looking for the force that each box exerts on each other, not the acceleration...

    But thanks anyways.
  5. Oct 10, 2005 #4
    I didn't give you the complete story. You asked for pointers/starters.

    Let me elaborate: Treat the system as a whole to find the acceleration of the entire system (use F = ma.) The system moves as a whole, thus each box will have the acceleration that you found. Continue by drawing free-body diagrams for each box and keep in mind that [itex]F_{12} = -F_{21}[/itex] ([itex]F_{12}[/itex] - read "force acted on box two by box one.")
    Last edited: Oct 10, 2005
  6. Oct 10, 2005 #5
    -_-, still confused, sorry... acceleration is 2.48m/s^2-- what do i do with that number?
  7. Oct 10, 2005 #6
    I don't know if you read my edited post but use [itex]\Sigma F = ma[/itex] on each box. You know both m and a.

    [tex]\Sigma F_1 = m_1a[/tex]

    You now have the net force on box 1. If you drew your free-body diagrams you would also know that

    [tex]\Sigma F_1 = F + F_{21}[/tex]

    Our last two expressions are both give us the net for on box 1 so we can equate them

    F + F_{21} &= m_1a \\
    F_{21} &= m_1a - F \\

    Tada! You have the force acting on box 1 by box 2. If you read what I said in my last post you should be able to get [itex]F_12[/itex].
    Last edited: Oct 10, 2005
  8. Oct 10, 2005 #7
    is 435 correct then?
    Last edited: Oct 11, 2005
  9. Oct 11, 2005 #8
    Looking back, I see your problem includes friction. It shouldn't be too hard for you to include that in with what I've given you. Recheck your calculation of acceleration too.
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