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Homework Help: Finding critical Angles

  1. Mar 15, 2010 #1
    1. The problem statement, all variables and given/known data
    No specific question.
    How would one find the critical angle of a ramp given the mass of the block sliding down it and the mu between ramp and block?
    It's not a single question but something my teacher explained that i didn't understand.

    2. Relevant equations
    F=(mu)mg
    F=ma
    I'm not sure of any more

    3. The attempt at a solution
    I just don't get how to calculate the critical angle.
    Is there a formula or a way to derive some equations?
    What i know is i have to get the gravity vector that points down the ramp to exceed friction.
    But how would i set up that equation?
     
  2. jcsd
  3. Mar 15, 2010 #2

    rl.bhat

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    The critical angle is that angle at which component of g along the inclined plane is equal to the acceleration due to frictional force.
    Can you write the expression for frictional force?
     
  4. Mar 15, 2010 #3
    Wouldn't the vector of gravity along the ramp have to exceed the friction for it to move? If it were equal, then the block would be stationary, right?
    Frictionforce=(mu)mg
    Frictionforce=(mu)normalforce
    Also thanks for the quick reply.
     
  5. Mar 15, 2010 #4
    Do you know what a Free Body Diagram is? Split up the components of the objects weight and set them equal to the opposite forces. Remember that cosine is horizontal while sine is vertical.
     
  6. Mar 15, 2010 #5

    rl.bhat

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    The block will move with uniform velocity.
    Now what is the expression for the normal force?
    The component of g along the inclined plane is g*sin(θ) where θ is the critical angle of the inclined plane with the horizontal.
     
  7. Mar 15, 2010 #6
    Now i'm confused.
    The normal force is pointing perpendicular to the ramp, right?
    Why would i need that?
    And for the g vector along the ramp, i need to find the critical angle, so how could i use it in an equation, unless i can cancel it somehow?
     
  8. Mar 15, 2010 #7

    rl.bhat

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    If you resolve g in to two components, g*sinθ is along the inclined plane and g*cosθ is perpendicular to the ramp, which is thew normal force.
    At critical angle g*sinθ = μ*g*cosθ.
     
  9. Mar 15, 2010 #8
    Hmm, ok.
    However, for now i only have 2 of those 3 variables, g, and mu.
    If i wanted to get theta equals something, what i got from deriving is
    sin(theta)/cos(theta)=mu.
    So now what do i do?
    I have sin and cos on one side, so would i take cos^-1(mu)/sin^-1 ?
    Or what.....
    Also, how would the mass come into play?
     
  10. Mar 15, 2010 #9
    Sorry, i researched a little and found that sin/cos =tan.
    I got it now, thanks so much rl.bhat!
    again.
     
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