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Homework Help: Circular motion car problem

  1. Nov 24, 2009 #1
    1. The problem statement, all variables and given/known data

    A car initially travels north and then turns to the left along a circular curve. This causes a package on the seat of the car to slide toward the right side of the car. Which of the following is true about the net force on the package while it is sliding?

    1. The force is directed north.
    2. There is not enough force directed toward the center of the circle to keep the package
    from sliding.
    3. There is not enough force tangential to the car’s path to keep the package from sliding.
    4. There is not enough force directed north to keep the package from sliding.
    5. The force is directed away from the center of the circle.

    2. Relevant equations

    N/A (I think? Conceptual question as far as I know)

    3. The attempt at a solution

    So from what I understand there is a centrifugal force that is created as the car turns, which wants to push the box outward, along the line perpendicular to the tangent of velocity. So I'll go over what I'm thinking for each answer -

    1. Doesn't seem right at all to me. I believe the direction of the force is changing as the car is turning.
    2. One of my first thoughts. Either friction or centripetal force (I'm thinking the latter) would be smaller than the centrifugal force causing the box to slide to the right.
    3. Don't think this is right, because I'm not even sure of a force tangential to the path, just perpendicular to the tangent.
    4. Again, I didn't think there was a force directed to the north...however two answers involving it make me question myself.
    5. The second of my two best guesses - wouldn't the centrifugal force be directed away from the center?

    I'm sure I'm missing something important - I assume there's a force I'm forgetting...but what is it?
  2. jcsd
  3. Nov 24, 2009 #2
    A. There is no such thing as "centrifugal force".... it does not exist... it is simply inertia's "reaction" to the centripetal force... (this is one of my physics teacher pet peeves...)

    B. Now onto the question
    1) The car turns so....
    2) friction will be directed towards the center ... and what is true about the ability of friction in this case to balance out the packages inertia.....
    3) If there was a tangential force... which way would it accelerate?
    4) see answer 1
    5) what force is acting towards the outside of the circle? or is there one?

    Hint: I like you order
  4. Nov 25, 2009 #3
    Damn you Yahoo Answers! Not sure why I trusted anything from there, but I was fuzzy on the names and needed a refresher, but apparently I looked in the wrong place.

    1. Wouldn't the centripetal force always be directed towards the center, turning 90 degrees when pointing west to pointing south? Other than friction, that's the only force I know of, apart from the reaction to centripetal force (is that named? Is that even considered a force?)
    2. I thought the basis of inertia was that an object in motion wants to stay in motion, an object in rest wants to stay at rest? Unless inertia wanted the box to stay at the start of the turn, wouldn't friction and inertia not balance each other out but be added to each other?
    3. If there was a force tangential to the path, I assume it would accelerate along the tangent, which it is doing because of circular motion and the changing direction of velocity.
    4. I see it, now just to understand it...
    5. Just the reaction to centripetal force, which from what you're saying doesn't sound like a force at all - so I'm assuming it's safe to eliminate this one?

    Also, I don't get the hint, I'm rather braindead right now. You think 1. is the best answer?
  5. Nov 25, 2009 #4
    #2 is correct, i think
  6. Nov 25, 2009 #5
    Yup, #2 was it. Thanks for the help.
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