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Homework Help: Block in an incline connected to a spring

  1. Sep 26, 2009 #1
    1. The problem statement, all variables and given/known data

    Supposed the incline is frictionless for the system. The angle of inclination is 28 degrees, the spring constant is 692 N / m and the mass of the block is 4.95 kg. The block is released from rest with the spring initially unstretched. The acceleration of gravity is 9.8 m/s2.

    How far x does it move down the incline before coming to rest?

    Choosing down the incline as the positive direction, what is its acceleration at its lowest point?

    2. Relevant equations

    For part A - mgh = 1/2kx2

    h = xsin28

    3. The attempt at a solution

    I plugged h into the equation and got:

    mg(xsin28) = 1/2kx2

    I cannot seem to solve for x.

    I do not know if this equation is correct.

    Thanks for any help.
  2. jcsd
  3. Sep 26, 2009 #2
    You have to be careful, when doing inclines, to keep your directions clear and separate. I recommend using x and y for the untilted world and x' and y' for the incline.
  4. Sep 27, 2009 #3
    I don't entirely understand what you are saying but when I drew out the diagram I set the positive x direction pointing down with the block moving down the incline and I don't even think a y direction has to be taken into account.
  5. Sep 27, 2009 #4
    Perhaps I misunderstood your setup. You have x being positive down the incline. Where is h? You might also say something about the level of the course this is for since the answer is more mathematically involved than the question seems to suggest.
  6. Sep 27, 2009 #5
    Here is a link to the exact question I am doing but I wanted to try and solve it for myself before looking at this thread:


    I did glance over it and I got the same equation for the first part but he seemed to be able to solve for x and I could not. I think it's an algebra problem.
  7. Sep 27, 2009 #6

    Doc Al

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    Staff: Mentor

    The equation is fine. Try dividing both sides by x.
  8. Sep 27, 2009 #7
    mg(xsin28) = 1/2kx2
    mg(sin28) = 1/2kx
    mg(sin28) / 1/2k = x

    Does that look okay?
  9. Sep 27, 2009 #8

    Doc Al

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    Staff: Mentor

    Looks good.
  10. Sep 28, 2009 #9
    I am going to try out this answer in a few minutes, I will report back.
  11. Sep 28, 2009 #10
    Okay, my x value (distance moving down the incline) turned out to be .066 m. I managed to solve b by using a Fnet equation and an acceleration equation given by my professor. My Fnet equation looked like this:

    Fnet = mgsin22 + kx

    After solving for F, I plugged that into F / m (mass) = a

    However, my answer should be negative because it is moving up the slope at its lowest point.

    Thanks for the help on part a.
  12. Sep 28, 2009 #11

    Doc Al

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    Staff: Mentor

    That "acceleration equation" is just Newton's 2nd law.
    Careful with the direction (and thus the sign) of those forces. Which way does gravity act? Which way is the spring pulling?

    (Also, is 22 degrees a typo? Before it was 28.)
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