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Homework Help: Newtons laws again

  1. Sep 20, 2006 #1
    At the local grocery store, you push a 15.2 kg shopping cart. You stop for a moment to add a bag of dog food to your cart. With a force of 12.0 N, you accelerate the cart from rest through a distance of 2.24 m in 3.00 s. What was the mass of the dog food?
    F=ma m=f/a
    I rearranged x(f)=x(i)+v(i)+1/2at^2 to be a=(1/2t^2)/x
    so,
    1/2(3)^2/2.24= 2.01 m/s^2.

    m=f/a
    m=12.0/2.01=5.97kg.
    This defenately looks wrong. How can the cart's mass of decreased? I'm confused what did I do wrong?
    I tried 15.2-5.97 and that was also wrong.
     
  2. jcsd
  3. Sep 20, 2006 #2
    The only thing i regognise there is 1/2at^2.
    This wudv worked:

     
  4. Sep 20, 2006 #3
    awsome, got it. 8.80kg. I don't know how i mess these up. What are the steps you use to convert x = vt +1/2at^2 to a= 2x/t^2? thanks for your help.
     
  5. Sep 20, 2006 #4

    BobG

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    Science Advisor
    Homework Helper

    Initial velocity was 0.

    Multiply both sides by 2 to get rid of the 1/2.

    Divide both sides by t^2 to isolate your a.
     
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