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Need help with Incline Plane Problem

  1. Sep 20, 2011 #1
    Need help with Incline Plane Problem!!

    1. The problem statement, all variables and given/known data
    A 10 kg block of ice slides down a ramp 20 m long, inclined at 10 degrees to the horizontal.
    a) If the ramp is frictionless, what is the acceleration of the block of ice? (I was able to do this question the answer is 1.7 m/s²)

    b) If the coefficient of kinetic friction is .10, how long will it take the block to slide down the ramp, if it starts from rest?

    So right now I know I have...
    m= 10 kg
    d= 20 m
    sin10
    cos10
    v1= 0 m/s
    a= 1.7 m/s²


    2. Relevant equations
    I'm really not sure what to do.


    3. The attempt at a solution
    I'm not sure what to do, I need to know where to start!
     
  2. jcsd
  3. Sep 20, 2011 #2
    Re: Need help with Incline Plane Problem!!

    To start with, you should probably draw a FBD of the block on the ramp. :smile:
     
  4. Sep 20, 2011 #3
    Re: Need help with Incline Plane Problem!!

    Ok i did that, I just don't know what to solve for first.
     
  5. Sep 20, 2011 #4
    Re: Need help with Incline Plane Problem!!

    Alright, so you drew your FBD.

    You have a Weight force going down.
    A normal force going perpendicular to the ramp's surface.
    A friction force going the opposite direction of motion.

    Correct?

    Now..... 2 questions for you.

    [itex]\Sigma[/itex]F[itex]_{y}[/itex]= ?
    [itex]\Sigma[/itex]F[itex]_{x}[/itex]= ?
     
  6. Sep 20, 2011 #5
    Re: Need help with Incline Plane Problem!!

    correct,
    fx = sin10mg = sin10 x 10 x 9.8 = 17
    fy = cos10mg = cos10 x 10 x 9.8 = 96
     
  7. Sep 20, 2011 #6
    Re: Need help with Incline Plane Problem!!

    Where is your friction force? I can't tell if you have one or not.
     
  8. Sep 20, 2011 #7
    Re: Need help with Incline Plane Problem!!

    What do you mean? Didn't i just solve for parallel and perpendicular force?
     
  9. Sep 20, 2011 #8
    Re: Need help with Incline Plane Problem!!

    I might be missing something myself; running on very little sleep here. Sorry.
     
  10. Sep 20, 2011 #9
    Re: Need help with Incline Plane Problem!!

    I don't have the force of friction yet.
     
  11. Sep 20, 2011 #10
    Re: Need help with Incline Plane Problem!!

    I was able to get friction because fn = f perpendicular. So i did .10 x 96 and got friction to equal -9.6
     
  12. Sep 20, 2011 #11
    Re: Need help with Incline Plane Problem!!

    Ok, so after looking at this myself....

    I get.

    [itex]\Sigma[/itex]F[itex]_{x}[/itex]=mg*sin[itex]\Theta[/itex] - [itex]\mu[/itex]mg*cos[itex]\Theta[/itex]

    I'm still working on Fy.
     
  13. Sep 20, 2011 #12
    Re: Need help with Incline Plane Problem!!

    Ok, thanks for your help!
     
  14. Sep 20, 2011 #13
    Re: Need help with Incline Plane Problem!!

    Friction = [itex]\mu[/itex]mg*cos[itex]\Theta[/itex]

    The reasoning:

    N = -mg*cos[itex]\Theta[/itex]

    So, friction = -.10*98*cos(10) = -9.65 N

    Correct?


    You're welcome, but I don't know how much help I was. :/ Fy always gets me in these problems.
     
  15. Sep 20, 2011 #14
    Re: Need help with Incline Plane Problem!!

    I got the same answer as you.
     
  16. Sep 20, 2011 #15
    Re: Need help with Incline Plane Problem!!

    Right. And you can find acceleration from there, correct?
     
  17. Sep 20, 2011 #16
    Re: Need help with Incline Plane Problem!!

    Correct, but I'm suppose to find time.
     
  18. Sep 20, 2011 #17
    Re: Need help with Incline Plane Problem!!

    Then you use the acceleration and plug it into a kinematic equation.

    You have delta x, acceleration, and an initial velocity.

    So, you have to find your time using.....

    [itex]\Delta[/itex]x = V[itex]_{i}[/itex]t + 1/2(a)(t[itex]^{2}[/itex])

    Please correct me if I'm wrong.
     
  19. Sep 20, 2011 #18
    Re: Need help with Incline Plane Problem!!

    You are right!!! Thank you so much for helping!!
     
  20. Sep 20, 2011 #19
    Re: Need help with Incline Plane Problem!!

    You're very welcome!
     
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