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Friction acting on box not slipping in truck

  1. Sep 30, 2011 #1
    Sorry for not using the proper post format. I'm typing on my phone and the post template doesn't show up.

    Question: a box is sitting in the back of a truck which accelerates forward when a traffic light turns green. Which of the following statements is correct if the boxdoes not slip?

    The answer is: the force of static friction acts on the box in the forward direction.

    I don't understand why it's in the forward direction. I thought friction was always in the direction opposite motion, and motion, in this case, is forward. Therefore, shouldn't the friction be acting backward?
     
  2. jcsd
  3. Sep 30, 2011 #2
    The truck goes forward but the wheels of the truck aren't attached to the box. The box wants to stay still and let the truck slip underneath it. Friction however decides to push the box and force it to stay on the truck.
    Drawing a picture helps too.
     
  4. Sep 30, 2011 #3
    The direction of motion of the car is forward, but the direction of motion of the box relative to the car would be backwards. Thus, friction must be in the forwards direction to keep the box from moving.
     
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