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Physics Question: Friction

  1. Oct 26, 2005 #1
    4. Before your friend releases the brakes, you start pushing the motorcycle up the ramp with a 20 pound force. The motorcycle is still stationary half-way down the ramp. At this point the frictional force is now:
    (A) Kinetic friction, and it is equal to 50 pounds up the ramp.
    (B) Static friction, and it is equal to 50 pounds up the ramp.
    (C) Static friction, and it is equal to 30 pounds up the ramp.
    (D) Static friction, and it is equal to 20 pounds up the ramp.
    I know that it is static friction, so it's not A, and I know that static friction would act in the upward direction (oppostie to the motion of the bike), but i am not certain on how my pushing of 20 pounds up on the bike affects the way in which static friction reacts.




    6. The motorcycle is skidding uncontrollably down the ramp! You need to stop the sliding, so you apply 10 pound force up the ramp and it slows down to a stop. During the time the motorcycle slows down, the frictional force is:
    (A) Static friction and equal to 50 pounds up the ramp.
    (B) Static friction and equal to 50 pounds down the ramp.
    (C) Kinetic friction and equal to 50 pounds up the ramp.
    (D) Kinetic friction and equal to 40 pounds up the ramp.

    Again, i know that since it is in motion that this implies kinetic friction, and I know that the direction of the kinetic friction would be upward, but again, i am uncertain at how my pushing of 10 pounds affects kinetci energy.

    Thanks for the help.
     
  2. jcsd
  3. Oct 26, 2005 #2
    for the first question, I should add this info:


    ***Extra Information: You Find that in order to keep the motorcycle stationary halfway down the plank, you have to supply a force up the ramp of exactly 50 pounds***

    What I think I know: I know that since the bike is stationary, it implies that static friction (friction of motionless objects) is at work.

    What I am unsure of: I am not sure if the "me" in the problem pushing on the bike with a force of 20 pounds would cause static friction to do that much less work and hence static Friction would only have to exert a force of 30 pounds now.
     
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