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Length along ramp from end of spring, picture included

  1. Oct 11, 2010 #1
    1. The problem statement, all variables and given/known data
    A(n) 2300 g block is pushed by an external
    force against a spring (with a 14 N/cm spring
    constant) until the spring is compressed by
    19 cm from its uncompressed length. The
    compressed spring and block rests at the bot-
    tom of an incline of 33◦ .
    The acceleration of gravity is 9.8 m/s2 .
    Note: The spring lies along the surface of
    the ramp (see figure).
    Assume: The ramp is frictionless.
    Now, the external force is rapidly removed
    so that the compressed spring can push up
    the mass.
    [PLAIN]http://img64.imageshack.us/img64/5812/problem6l.jpg [Broken]
    (a)How far up the ramp (i.e., ℓ the length
    along the ramp) will the block move (measured
    from the end of the spring when the
    spring is uncompressed) before reversing di-
    rection and sliding back?
    Remember, the block is not attached to the
    Answer in units of cm.

    (b)Repeat the first part of this question, but
    now assume that the ramp has a coefficient of
    friction of μ = 0.308 .
    Keep all other assumptions the same.
    Answer in units of cm.

    2. Relevant equations
    [tex]\Delta[/tex]K= Kf-Ki=1/2mvf2-1/2mvi2

    3. The attempt at a solution
    I don't know where the angle comes in...
    I know that N=mgcos[tex]\theta[/tex]
    Last edited by a moderator: May 5, 2017
  2. jcsd
  3. Oct 12, 2010 #2


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    Note that the block starts off with no speed, and it has no speed when it comes to a stop at the top of the incline, so delta K =0 . Note also that your energy equation is wrong. W_nc = (delta K + delta U_spring + delta U_gravity), where W_nc = work done by non conservative forces, like friction.
  4. Oct 12, 2010 #3

    Wnc= 1/2kx2+mgh

    is k the 19cm (.19m)?

    and do I figure out the height from the angle? and if so how?
    Last edited: Oct 12, 2010
  5. Oct 12, 2010 #4
    okay k is the spring constant which is given above 14 N/cm or 1400N/m if you convert it like I did.

    so you know that initial potential energy is the potential energy in the spring plus the gravitational potential energy. If you set you reference point there you can say that gravitational potential energy there is 0 because in mgh, h=0.

    and we also know that when we get to the top of the slope K=0 and U=mgh
    so use the information up to there to solve for the height.

    if you've got the max height you can set up a triangle (sin = opp/hyp, and you know the angle and the opposite side) and solve for the distance up the ramp, but the distance you get from that is going to be the distance from the top of the "compressed" spring so compensate for that in your answer. (i.e. subtract the distance that the spring was compressed)
    Last edited: Oct 12, 2010
  6. Oct 12, 2010 #5

    would it be

    well i tried and did 1400(.19)2/(2*2.3*9.8)

    then sin33=o/h

    2.05846-.19 m=1.8684m

    which is 186.84 cm which is wrong

    well I reread the problem and it says when it is uncompressed so .57 m
    I misused 2.05846-.57=1.48846

    which is 148.46 cm which is wrong
    Last edited: Oct 12, 2010
  7. Oct 12, 2010 #6
    We have the same homework service im pretty sure... mine was due today so here's how I did it.

    Since there is no friction on the first part of the problem, mechanical energy is conserved and you can use Ui+Ki=Uf+Kf

    Ui= (1/2kx2 + mghi) where hi = 0
    I put the reference point a the top of the compressed spring.
    x is 19cm or 0.19m the distance the spring was compressed

    Ki = 1/2mvi2
    but the block is at rest so v=0 so the whole quantity is 0
    so Ki=0

    Uf = (mgh)
    (only mgh because there's no spring potential energy like there was before)

    Kf = 1/2 mvf2
    but again at the top of the motion the velocity is 0 so
    Kf = 0

    (1/2kx2+0) + 0 = mghf + 0

    1/2kx2 = mghf
    solve for hf

    (kx2/2) x (1/mg) = hf

    convert everything to meters btw, I dunno why I do it .. i think it has to do with g being
    in units of m/s2, and i converted the mass to kilos

    k=1400 N/m (14 N/cm)
    x=.19m (19cm)
    m=2.3Kg (2300g)


    hf=1.1211 Meters (112.1118 cm)

    [tex]\theta[/tex] = 33 degrees
    sin(33) = (hf)/d
    let d be the distance from the top of the compressed spring to the top of the motion

    (d)sin(33) = hf
    d = hf/sin(33)

    d=112.1118/sin(33) = 205.8460cm

    L = d-x (this is the x from earlier, or the distance the spring was compressed)

    L = 205.8460 cm - 19cm = 186.846 cm

    L = 186.8460 cm

    the part with friction is similar but you have to account for energy lost to internal energy
    Last edited: Oct 12, 2010
  8. Oct 12, 2010 #7
    did you actually do sin(30) ? maybe you just messed up the angle it looks like you did it right for the most part..
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