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Homework Help: Need help with newtons third law and friction.

  1. Oct 14, 2009 #1
    1. The problem statement, all variables and given/known data

    Hi guys i am trying to do question number 4 but i have not done a Question liek this in a very long time so i am forgetting where to start.

    http://img405.imageshack.us/img405/6976/101420092845.jpg" [Broken]

    Image is to big for the forum so its easier that way.

    2. Relevant equations

    3. The attempt at a solution

    Ok so first thing i did was make some FBDs and got my equations, idk if this is the first thing i should do....

    for Garfield

    Fnet = T+Fg
    ma = mg-T

    and for the mug

    Fnet = T (since FN and FG cancel out correct)

    Like i said its been about a year so i am rusty, could someone help me get the correct answer for 4?

    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. Oct 14, 2009 #2


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    Homework Helper

    Unless you mean for Fnet, T, and Fg to be vectors, you have to write down Newton's second law for both the x and y directions. (You can't add forces unless they're acting in the same direction or opposite directions.) Keep in mind that the force pulling Garfield up is not T; it's the horizontal component of T.
    Actually, Fnet has to be 0 because the mug isn't acceleration. Again, write Newton's second law in both the x and y directions, and note that the tension in the diagonal rope isn't the same as the tension in the horizontal rope.
  4. Oct 14, 2009 #3


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    Homework Helper

    Resolve tension in the rope attached to the wall in to components. At equilibrium, vertical component balances the weight of Garfield, horizontal component balances the friction force between mug and the table. Mass of the mug is given. Assume m as the mass of the coffee. from that find the total normal reaction and hence frictional force.
  5. Oct 14, 2009 #4
    Ok thansk guys, i understand it better now, i am am makign progress. I dont want to make another topic so i will just ask here.

    In Question number 2a, I would start by making an FBD then an equation for each mass correct? and then using those equations to find T1 and T2? But i am confused because they are two different tensions and my examples have not dealt with a Question like this.
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