1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Finding Speed and Coefficient of Friction

  1. Oct 2, 2004 #1
    Finding Speed with Coefficient of Friction

    You testify as an "expert witness" in a case involving an accident in which car A slid into the rear of car B, which was stopped at a red light along a road headed down a hill (Fig. 6-27). You find that the slope of the hill is = 12.0°, that the cars were separated by distance d = 24.0 m when the driver of car A put the car into a slide (it lacked any automatic anti-brake-lock system), and that the speed of car A at the onset of braking was v0 = 18.5 m/s.

    With what speed did car A hit car B if the coefficient of kinetic friction was 0.60 (dry road surface)?

    I've figured out this equation since there is constant acceleration:
    V^2=Vo^2+2a(x-xo) where a=-(Mk)g. I haven't taken into account the incline and I'm not sure how. How do I incorporate the incline into this equation, if my equation is even right.
     

    Attached Files:

    Last edited: Oct 2, 2004
  2. jcsd
  3. Oct 2, 2004 #2

    Pyrrhus

    User Avatar
    Homework Helper

    Let me point out change of Mechanical Energy will be equal to the work done by friction.

    [tex] \Delta E = W_{f} [/tex]
     
    Last edited: Oct 2, 2004
  4. Oct 2, 2004 #3
    I don't see the relation....
     
  5. Oct 2, 2004 #4

    Pyrrhus

    User Avatar
    Homework Helper

    [tex] E - E_{o} = W_{f} [/tex]

    What's not to see?

    Tell me exactly what you don't understand.
     
  6. Oct 2, 2004 #5
    Well in my class we haven't dealt with that equation yet, and this problem should have nothing to do with Mechanical Energy, just Newton's Laws and Acceleration and Friction.
     
  7. Oct 2, 2004 #6

    Pyrrhus

    User Avatar
    Homework Helper

    Oh, should had said so :smile: , then use newton's 2nd law (to find acceleration) and kinematics for final speed.
     
  8. Oct 2, 2004 #7
    I can't do that lol I wrote out my equation in my original post I just can't incorporate the 12 degree angle.
     
  9. Oct 2, 2004 #8

    Pyrrhus

    User Avatar
    Homework Helper

    Put your X-axis along the 12 degree straight line, so the normal force will have only a non-zero component, and the weight of the car will have 2 non-zero components one pointing left, and the other pointing down., and the friction force will be on the right, against movement with 1 non-zero component.
     
    Last edited: Oct 2, 2004
  10. Oct 2, 2004 #9
    hmm not understanding
     
  11. Oct 2, 2004 #10

    Pyrrhus

    User Avatar
    Homework Helper

    Draw a Free body diagram.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?