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Find force parallel to ramp given mass and theta?

  1. Mar 10, 2010 #1
    1. The problem statement, all variables and given/known data
    There's a block on a ramp. The block weighs 48.9 kg and the ramp's theta (the angle the ramp makes to the ground) is 45. Assumng gravity is 9.8 m/s^2, how would i find the force of the block parallell to the ramp?


    2. Relevant equations
    Um..maybe
    F=(mu)(m)(g)
    F=(mu)(normal force)
    or F=ma
    and all 4 kinematics equations:
    D=((vi+vf)/2)*t
    vf=vi+at
    D=vi*t+((at^2)/2)
    vf^2=vi^2+2(a)(d)

    3. The attempt at a solution
    So first i drew it out. the block is accelerating down the ramp, and for the FBD i found the force of gravity acting straight down (Which can be broken down into two vectors, one perpendicular to the ramp and one parallel to it), the normal force perpendicular to the slope, the acceleration downwards, and the friction backwards.
    Then i'm totally baffled. Am i supposed to use the kinematics equations to find acceleration to find the block's force or soh cah toa to find his friction?
    And sorry if i'm doing this wrong, this is my first post and i'm one confused little boy.
     
  2. jcsd
  3. Mar 10, 2010 #2

    rl.bhat

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    Hi cellfoneguy, welcome to PF.
    In the problem they have not mentioned the friction. So you need not take into account the frictional force while calculating the force on the block. The component of the weight of the block parallel to the ramp is the force on the block.
     
  4. Mar 10, 2010 #3
    So if i draw a right triangle using down (Gravity=9.8) as its hypotenuse, and the adjacent side perpendicular to the block, and the opposite side parallel to the block, maybe i can find the adjacent side? But i don't know the theta of above mentioned triangle.
     
  5. Mar 10, 2010 #4

    rl.bhat

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    the ramp's theta (the angle the ramp makes to the ground) is 45
    What is this? Draw the FBD and identify the angles.
     
  6. Mar 10, 2010 #5
    here's both the given info and the FBD i drew
     

    Attached Files:

  7. Mar 10, 2010 #6

    rl.bhat

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    From the diagram can you find the angle between the gravity and the perpendicular to the ramp? Once you do that, find the parallels and the perpendicular components of the weight.
     
  8. Mar 10, 2010 #7
    uh......i'm a little confused.
    so i know that if i get the angle between gravity and the ramp's perpendicular, i can find both the parallel and the perpendicular to the ramp, the two vectors of the gravity force. So i know that to find the force of the block ill need net force. To find net force, that's just the vector of the gravity force that is parallel to the ramp, right?.......
    If that is right, then how do i find the theta for the gravity vector triangle?
     
  9. Mar 10, 2010 #8

    rl.bhat

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    Right.
     
  10. Mar 10, 2010 #9
    hmm. So then the gravity force triangle's theta is..........45?
     
  11. Mar 10, 2010 #10
    AH HA I HAVE IT NOW
    thanks so much rl.bhat!
     
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