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Projectile problem: with coefficient of kinetic friction?

  1. Oct 26, 2011 #1
    1. The problem statement, all variables and given/known data
    Julia is delivering newspapers. Suppose she is driving at 15 m/s along a straight road and wants to drop a paper out the window from a height of 1.00m so it slides along the shoulder and comes to rest in the customer's driveway. At what horizontal distance before the driveway should she drop the paper? The coefficient of kinetic friction between the newspaper and the ground is 0.40. Ignore air resistance and assume no bouncing and rolling.


    2. Relevant equations

    delta y = Viy* delta t + 1/2 ay* delta t^2
    delta x= Vix * detal t
    3. The attempt at a solution
    delta y = Viy* delta t + 1/2 ay* delta t^2
    -1m=0 + 1/2(-9.8m/s^2)*delta t^2
    ==> delta t = sq. root of (-2* -1m/-9.8m/s^2) = 0.45s
    delta x= Vix * detal t=15m/s * 0.45s = 6.78m

    I am not sure what to do with the coefficient of kinetic friction between the newspaper and the ground???? Please help!!! Or is it a trick to throw me off... the answer is just simply 6.78 m????
     
  2. jcsd
  3. Oct 26, 2011 #2

    ehild

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    There are two stages of the motion. First the newspaper is a projectile, with horizontal initial velocity. In the second stage, it slides on the ground. Find the distance travelled in the horizontal direction as a projectile and then the distance travelled to full stop.

    ehild
     
  4. Oct 26, 2011 #3
    Thank you so much.. NOw it makes sense why they thro that coefficient of K in there. soFriction the only force working against the newspaper at that phase?!!!

    net force=ma = - Friction force = ma
    - (coefficient k * m* g) = ma
    a = -3.92
    vf^2 = vi^2 + 2a delta x
    x= 28.7m???

    the distance from second stage is kinda high, it took 28.7m to reach full stop????
    that's kinda crazy!!! Did I do it right? Thanks in advance!
     
  5. Oct 26, 2011 #4

    ehild

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    That 28.7 m is correct.
    I think that the newspaper will not slide that long in reality, but that coefficient of friction is rather low for a rough surface as the road, and air resistance also acts on the newspaper.


    ehild
     
  6. Oct 26, 2011 #5
    Thanks so much! You're so helpful! Have a wonderful day :)
     
  7. Oct 26, 2011 #6

    ehild

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    Thank you:smile:

    ehild
     
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