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Force that's acting on the truck

  1. Feb 6, 2009 #1
    1. The problem statement, all variables and given/known data

    A large box containing your new computer sits on the bed of your pickup truck. You are stopped at a red light. The light turns green and you stomp on the gas and the truck accelerates. To your horror, the box starts to slide toward the back of the truck. (Assume that the truck is accelerating to the right.)

    Draw clearly labeled free-body diagram for the truck. (The bed of the truck is not frictionless.)
    Draw the force vectors with their tails at the dot. The orientation of your vectors will be graded. The exact length of your vectors will not be graded but the relative length of one to the other will be graded.

    2. Relevant equations

    Nground: normal force
    Nb: reaction to the normal force on the box from the truck
    Froad: Reaction force to tires' backward push
    Fk: box friction force on the truck bed
    Wt: weight of truck

    3. The attempt at a solution

    1st attempt:

    Nground and Nb pointing upwards, length of Nground longer than Nb.
    Fk pointing right.
    Froad pointing left.
    Wt pointing down.
    *length of my Nground and Wt arrows are equal

    2nd attempt:

    Nground and Nb pointing upwards, length of Nground longer than Nb.
    Fk and Froad pointing right, length of Froad longer than Fk.
    Wt pointing down.
    *length of my Nground and Wt arrows are equal


    However i can't seem to get the correct free body diagram for this. Can anyone help?
    Should i change the Nb of my 2nd attempt to acting downwards instead of acting upwards?
     
    Last edited: Feb 6, 2009
  2. jcsd
  3. Feb 6, 2009 #2

    PhanthomJay

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    Yes, that will help. The direction of Nb can be determined from Newton 3, by first examining the forces on the crate. Check that friction direction also in the same manner....is it correct?
     
  4. Feb 6, 2009 #3
    How long should the length of my Nb be?
     
  5. Feb 6, 2009 #4

    PhanthomJay

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    Just the way you said it, Ng longer than Nb. Do you understand why? And don't forget to rexamine the direction of the friction force and why it is in that direction. Just guessing won't do you any good. You have another error as well.
     
  6. Feb 3, 2010 #5
    I just figured this one out! Don't forget that the forces in the y direction have to add to zero...might need to change the length of a vector to make that true...
     
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