1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Incline plane problem

  1. Apr 25, 2012 #1
    I've tried a few times on this problem but I can't get the right answer...

    A box with mass m=1,05 kg slides down a ramp with angle=42.39°. Kinetic friction μ=0,1646 between the ramp and the box. It starts from rest. g=9,806m/s². The box hit's a spring with: k=792 N/m after sliding the distance: s= 2,875m, What is the maximum compression of the spring (x)?

    I've tried the following solution based on the law of conservative energy:
    (m*g*(s+x)*sin(42,39)) - (μ*m*g*(s+x)*cos(42.39)) = 0.5*k*x²

    What am I doing wrong?

    Thanks,
     
  2. jcsd
  3. Apr 25, 2012 #2

    Doc Al

    User Avatar

    Staff: Mentor

    Your solution looks OK to me.
     
  4. Apr 25, 2012 #3
    I agree. I came up with the answer: x ≈ 0,203m
    The right answer is: 0,0144
     
  5. Apr 25, 2012 #4

    Doc Al

    User Avatar

    Staff: Mentor

    What textbook is this problem from? (Just in case I have it.)

    (My answer is close to yours.)
     
  6. Apr 25, 2012 #5
    It's from an assignment. I really belive that the answer is wrong, but it has never happend before so...I must have done something wrong.
     
  7. Apr 25, 2012 #6

    Doc Al

    User Avatar

    Staff: Mentor

    I see the problem. Is the spring on the incline? Or on the horizontal section after the incline?

    Assume the latter and you'll get the given answer.
     
  8. Apr 25, 2012 #7

    tms

    User Avatar

    So is mine, if that means anything: 0.211 m.
     
    Last edited: Apr 25, 2012
  9. Apr 25, 2012 #8
    There is no pic. for the problem, but I get your point. Does it really make that big differernce? And We have never had such problems before, so i think the spring is aligned with the plane
     
  10. Apr 25, 2012 #9

    Doc Al

    User Avatar

    Staff: Mentor

    Try it and see for yourself!
     
  11. Apr 25, 2012 #10
    I really don't think it's right. It's going to be the same since Ug2=the height + the compression of the spring.
     
  12. Apr 25, 2012 #11

    Doc Al

    User Avatar

    Staff: Mentor

    Assume the spring is horizontally oriented after the mass slides down the incline. You'll get an answer that matches the one you say is correct.
     
  13. Apr 25, 2012 #12
    Ok! But I don't understand how I should calculate it :/
     
  14. Apr 25, 2012 #13

    Doc Al

    User Avatar

    Staff: Mentor

    Similar to what you did before: you'll use energy conservation. Only this time it's even easier to solve. (Only the incline has friction--assume the flat part after the incline has no friction.)
     
  15. Apr 25, 2012 #14
    I just tried it! It's wierd cause I got the same answer again :/

    k1 = (1/2)mv^2 equals U2= (1/2)kx^2
     
  16. Apr 25, 2012 #15

    Doc Al

    User Avatar

    Staff: Mentor

    Try again. Show me an equation similar to what you had in your first post.
     
  17. Apr 25, 2012 #16
    I calculated the velocity when the box has slided down the plane:
    v= 5,581963937m/s (The speed is just before the box hit's the spring

    Then I put in the velocity in the ecuation I wrote earlier, together with the spring constant and then I solved for x
     
  18. Apr 25, 2012 #17
    I've tried the same formula I used when I started this topic on another assignment of the same type. It shows that the solution in the assignment was wrong!!!!!

    IDIOTS! We were right al along :D Thanks for your help guys!
     
  19. Apr 25, 2012 #18

    Doc Al

    User Avatar

    Staff: Mentor

    It all depends on whether the spring is on the incline or after the incline. You need a diagram.

    Edit: Oops... I think I made an error when I solved the problem the second way (with the spring horizontal). It doesn't give the 'correct' answer. So your suspicion is correct: the answer is wrong no matter how you slice it. Your instructor probably made the same error that I made! That's why I got the 'correct' answer that way. :uhh:
     
    Last edited: Apr 25, 2012
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Incline plane problem
  1. Inclined plane problem (Replies: 2)

Loading...