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Homework Help: Collision of Two Swinging Bars, Maximum Angle

  1. Aug 7, 2014 #1
    1. The problem statement, all variables and given/known data

    Two uniform bars have masses and lengths of [itex]m[/itex], [itex]l[/itex] and [itex]2m[/itex], [itex]2l[/itex], respectively. The bars are pivoted at the common frictionless hinge as shown. The bars are released from the horizontal position in such a way that they collide and stick together at the bottom position. After that, the bars swing upward together. Find the maximum angle θ the bars would make with the vertical after the collision.

    2. Relevant equations

    I chose to solve this in 3 steps:
    1) Use conservation of energy of C.O.M. of each bar to determine ω of each bar just prior to collision.
    2) Use conservation of angular momentum to determine ω of the two bar system immediately after collision.
    3) Use conservation of energy to determine final height of the C.O.M. of bar 2, and use that to find θ.

    3. The attempt at a solution
    Step 1:
    Bar 1:
    [itex]U_{gi} + E_{ki} = U_{gf} + E_{kf} [/itex]
    [itex]mgl + 0 = \frac {mgl}{2} + \frac {I_{pivot}ω_1^2}{2} [/itex]

    [itex]I_{pivot} = \frac {m L^2}{3} [/itex]
    [itex]m = m, L = l [/itex]

    [itex]mgl = \frac {mgl}{2} + \frac {ml^2ω_1^2}{6} [/itex]
    [itex]\frac {g}{2} = \frac {l}{6}ω_1^2 [/itex]
    [itex]ω_1 = \sqrt{\frac {3g}{l}}[/itex]

    Bar 2:
    [itex]U_{gi} + E_{ki} = U_{gf} + E_{kf} [/itex]
    [itex]2mgl + 0 = 0 + \frac {I_{pivot}ω_1^2}{2} [/itex]

    [itex]I_{pivot} = \frac {8m l^2}{3}[/itex] [itex](m = 2m, L = 2l) [/itex]
    [itex]2mgl = \frac {8ml^2ω_2^2}{6} [/itex]

    [itex] 2g = \frac {8l}{6}ω_2^2 [/itex]
    [itex]ω_2 = \sqrt{\frac {3g}{2l}} = \frac{1}{2}\sqrt{\frac {6g}{l}}[/itex]
    Last edited: Aug 7, 2014
  2. jcsd
  3. Aug 7, 2014 #2
    Step 2: Apply conservation of angular momentum to determine the angular velocity of both bars immediately after collision.
    [itex]L_{before} = L_{after}[/itex]
    [itex]I_1ω_1 - I_2ω_2 = -(I_1 + I_2)ω_{both}[/itex]

    [itex]\frac {ml^2}{3}\sqrt{\frac{3g}{l}} - \frac {8ml^2}{3}(\frac{1}{2}\sqrt{\frac{6g}{l}}) = -(\frac {ml^2}{3} + \frac {8ml^2}{3})ω_{both}[/itex]

    [itex]\frac {1}{3}\sqrt{\frac{3g}{l}} - \frac {4}{3}\sqrt{\frac{6g}{l}} = -(\frac {1}{3} + \frac {8}{3})ω_{both}[/itex]

    [itex]\frac {4\sqrt{6g} - \sqrt{3g}}{3\sqrt{l}} = 3ω_{both}[/itex]

    [itex]ω_{both} = \frac {\sqrt{g}}{9\sqrt{l}}(4\sqrt{6} - \sqrt{3})[/itex]
    Last edited: Aug 7, 2014
  4. Aug 7, 2014 #3
    Step 3: Apply conservation of energy to the swinging 2 bar system to determine [itex]h_2[/itex].
    This picture shows the change in position of the center of mass of each bar from the collision to the top of the swing. I truncated the 2nd half of each bar to try to simplify the picture. I have drawn both bars separately.


    Starting with the 2nd bar:
    [itex]l cos θ = l - h_2[/itex]
    [itex]h_2 = l - l cos θ[/itex]

    Now the first bar:
    [itex]\frac{l}{2} cos θ = \frac{l}{2} - h_1[/itex]
    [itex]h_1 = \frac{l}{2} - \frac{l}{2} cos θ = \frac{1}{2}h_2[/itex]
    Since the C.O.M. of the shorter bar starts at a position of [itex]\frac{l}{2}[/itex] above the reference point,
    [itex]h_{1 final} = \frac{1}{2}h_2 + \frac{l}{2}[/itex]

    Now apply conservation of energy:

    [itex]U_{gi} + E_{ki} = U_{gf} + 0 [/itex]

    [itex]mg(\frac{l}{2}) + 2mg(0) + \frac{1}{2}(I_{pivot1} + I_{pivot2})ω_{both}^2 = mgh_{1 final} + 2mgh_2 [/itex]

    [itex]mg(\frac{l}{2}) + \frac{1}{2}(\frac {ml^2}{3} + \frac {8ml^2}{3})ω_{both}^2 = mg(\frac{h_2}{2} + \frac{l}{2}) + 2mgh_2 [/itex]

    [itex]\frac{1}{2}(\frac {l^2}{3} + \frac {8l^2}{3})ω_{both}^2 = g\frac{h_2}{2} + 2gh_2 [/itex]

    [itex]3l^2ω_{both}^2 = gh_2 + 4gh_2 [/itex] **Equation 1


    Take a detour to simplify [itex]ω_{both}^2[/itex]:

    [itex]ω_{both} = \frac {\sqrt{g}}{9\sqrt{l}}(4\sqrt{6} - \sqrt{3})[/itex]

    [itex]ω_{both}^2 = \frac {g}{81l}(16*6 - 2*4\sqrt{3*6} + 3)[/itex]

    [itex]ω_{both}^2 = \frac {g}{81l}(99 - 8\sqrt{2*9})[/itex]

    [itex]ω_{both}^2 = \frac {g}{81l}(99 - 24\sqrt{2})[/itex]

    [itex]ω_{both}^2 = \frac {g}{27l}(33 - 8\sqrt{2})[/itex]

    Insert our expression for [itex]ω_{both}^2[/itex] into Equation 1 above:
    [itex]3l^2(\frac {g}{27l}(33 - 8\sqrt{2})) = 5gh_2 [/itex]

    [itex]\frac {l}{9}(33 - 8\sqrt{2}) = 5h_2 [/itex]

    [itex]h_2 = \frac {l}{45}(33 - 8\sqrt{2}) [/itex]

    Insert our expression for [itex]h_2[/itex] into [itex]l cos θ = l - h_2[/itex] from the very top:

    [itex]l cos θ = l - \frac {l}{45}(33 - 8\sqrt{2})[/itex]

    [itex]cos θ = \frac {12 + 8\sqrt{2}}{45}[/itex]

    θ = 58.8 degrees Note: This is the correct answer. I discovered my mistake above.
    Last edited: Aug 7, 2014
  5. Aug 7, 2014 #4
    So, as noted, I found the mistake I made and did get the correct answer. This was an arduous problem.
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