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Homework Help: Finding Acceleration of a cart. NEW

  1. Mar 29, 2007 #1
    1. The problem statement, all variables and given/known data
    A grocery cart is being pushed with a force of 450 N at an angle of 30 degrees to the horizontal. If the mass of the cart and the groceries is 42 kg:
    a) Find force of friction if coefficient of friction is 0.6
    b) Find acceleration of cart.


    2. Relevant equations
    Ff=uFnormal
    F=ma
    Fn=mg



    3. The attempt at a solution

    I figured out part A and I got 3.8 X 10^2 N as force of friction.

    For part B, I tried to find force applied.
    Fa = 450 N X cos30 degrees
    = 389.7 N

    and then find net force:

    Fnet = 389.7 - (3.8 X 10^2)
    = 9.7

    and then find acceleration:

    a = 9.7 / 42
    = 0.23

    But the answer is wrong and I checked from my book. Can someone tell me if I am right on part B of this question. Thanks.
     
  2. jcsd
  3. Mar 29, 2007 #2
    it is so much easier if you post the math that got you to the answer,
    assuming the force is downward on the cart,
    N=mg+f*sin 30 so N=.5*450+mg=9.8*42+225=636.6
    can you do it from here?
     
  4. Mar 29, 2007 #3
    But I don't understand what to do with 636.6. I actually did that before but then I thought that's wrong since the number is too big.

    I am guessing that probably I subtract 450 from 636 to get net force. But I am too confused. Can you please help more? Thanks.
     
  5. Mar 30, 2007 #4
    sure. but lets make certain you understand the first part:

    friction resists motion, but it only dependent on the force pushing the objects into contact. that force is the Normal or perpendicular force. So if we have a weight of 10okg*g, the normal force directed straight up is 980N. When we multiply this by the coefficient of friction, it is now a horizontal resistance of magnitude say .6*980.
     
    Last edited: Mar 30, 2007
  6. Oct 24, 2010 #5
    hi. i'm currently doing this question too.
    but for some reason i am not getting part A. could you explain what you did to get 3.8 x 10^2 N?
    what i did was:
    Fn=mg
    Fn=(42kg) (9.81m/s^2)
    Fn=412.02N
    Ff=0.6(412.02N)
    Ff=247.212N

    my answer is no near to the book's answer. what am i doing wrong?!!
    please help me
     
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