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How to calculate the gravity on a hill?

  1. Jan 25, 2017 #1
    • Member advised to use the homework template for posts in the homework sections of PF.
    The question is about a box with no movement standing on a hill. The hill has an angle of 25 degrees. The box has a mass of 40 kg.
    1. Calculte the gravity
    This I still get: F= M x A = 40 x 9,81 = 3,9 x 10^2
    The next question tough:
    2. Calculate the component Fgravity,x off the gravity alongside the hill.
    The component I think they ment is marked on the picture included.

    What I did was 390 / sin(25) because the opposite is known (the gravity) and you want to calculate the hypotenuse. The answer indicates this as the right answer tough: Fgravity,x = Fgravity sin(a) = 165,8

    Why do they multiply when you should divide?
    Am I missing something?
    Thanks in advance! Knipsel.PNG
     
  2. jcsd
  3. Jan 25, 2017 #2

    Doc Al

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    Staff: Mentor

    You want the component of the weight acting down the incline. When finding the component of a vector using a right triangle, the full vector is always the hypotenuse. (The components are always smaller than the full vector.)

    You're using the wrong triangle: Draw the weight vector acting straight down. That's the hypotenuse of the correct triangle.
     
  4. Jan 25, 2017 #3

    Doc Al

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    Staff: Mentor

    Check this out: Inclined Planes
     
  5. Jan 25, 2017 #4

    Aaah I get it, thank you!
     
  6. Jan 25, 2017 #5

    CWatters

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    Science Advisor
    Homework Helper

    If you find yourself in an exam and can't remember if it's sin or cos... consider what happens if the angle of the slope is reduced towards zero degrees (no slope). The force you calculate down the slope should approach zero. Your calculator will tell you sin(0)=0 but cos(0)=1.
     
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