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Magnitude of acceleration of block

  1. Feb 13, 2009 #1
    How do I find the magnitude of the acceleration of a block along a inclined plane?

    I am given the mass of the block = 4.15kg on a no-friction inclined plane of an angle 30 degrees.
     
  2. jcsd
  3. Feb 13, 2009 #2

    Doc Al

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    Staff: Mentor

    What forces act on the block? Find the net force, then apply Newton's 2nd law.
     
  4. Feb 13, 2009 #3
    What forces act on the block?

    Would that be normal force which is the weight? (40.67? Am I right?)

    Newton's 2nd law? F = ma

    Can you explain that? How do I find F? m is given as 4.15. Am I right?
     
  5. Feb 13, 2009 #4

    Doc Al

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    The normal force is one of the forces acting on the block, but it does not equal the weight.

    The normal force acts perpendicular to the plane; the weight acts down. Draw a diagram and find components of the forces parallel and perpendicular to the plane.
     
  6. Feb 13, 2009 #5
    My diagram has the normal force that keeps it up. The weight force pulling it down which opposite of normal force. And tension moving it forward. Am I right?
     
  7. Feb 13, 2009 #6

    Doc Al

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    No. If the weight were equal and opposite to the normal force, the block would just sit there.
    Tension from what?

    The weight force has components parallel and perpendicular to the incline. (What are those components?) The perpendicular component of the weight is equal and opposite to the normal force--so they cancel out. But that leaves the other component of the weight acting down the incline.

    Read this: http://www.physicsclassroom.com/Class/vectors/u3l3e.cfm" [Broken]
     
    Last edited by a moderator: May 4, 2017
  8. Feb 13, 2009 #7
    Okay...so I used the m*g*sin30

    m = 4.15
    g = 9.8

    and I still got the wrong answer.

    then i divided by mass again and got wrong answer again.
     
  9. Feb 13, 2009 #8

    Delphi51

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    We need more information to understand what has gone wrong. Show your calcs for normal force, friction force and force along the ramp. Finally, your F=ma calc to find the acceleration.
     
  10. Feb 14, 2009 #9

    Doc Al

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    OK, that's the net force acting on the block.

    Wrong answer for what?

    If you want to find the acceleration, divide the net force by the mass. (F = ma → a = F/m.)

    That's really all there is to it, assuming you've described the full problem exactly as it was given to you. If you are still not getting the right answer, please post the original problem word-for-word, including any diagrams given.
     
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