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Math Help - Physics friction

  1. #1
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    Physics friction

    A crate is sitting on the back of a truck at 20 m/s with a μ of 0.4. With what acceleration can the truck stop without having the crate slide?

    I'm not sure if I am supposed to use this equation:

    a = μ(9.8)

    Any help is appreciated.
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  2. #2
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    first draw a picture. Realize that these forces are acting on it:
    Normal, friction, and weight.

    Now break up those forces into componets (x and y)
    when you do this, you should get friction = ma(in x dir.)
    Acceleration in y dir. is zero.

    Normal Force = mg

    w=mg
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  3. #3
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    Thanks for the help, but how would I use those equations if I do not know the mass?
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  4. #4
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    when I did it, mass canceled. So in theory, it should be negligable.
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  5. #5
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    let me just do the whole equation for you.

    in words.

    the sum of the forces in the x dir. = f = ma

    now just figure out what friction is, the (f).
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