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Homework Help: Work and kinetic energy (dynamics)

  1. Feb 5, 2012 #1
    1. The problem statement, all variables and given/known data

    See attachment.

    2. Relevant equations

    (1) F=ma
    (2.13) W=ΔT
    (2.20) (weight) x (decrease in altitude of mass center of body)

    3. The attempt at a solution

    I don't get where |v(t1)| and |v(t2)|^2 comes from. The book says |v(t1)| = 0, which I suppose is because initially there was no movement, so kinetic energy is 0? Does |v(t1)|=0 have anything to do with T=mgsinθ+mlθ'^2?

    Then it says |v(t2)|^2 = l^2θ'^2, and this totally confused me. First, why is v(t2) squared and not v(t1)? Second, obviously v(t2) is somehow related to T=mgsinθ+mlθ'^2, but I don't see how you get v(t2) from that. Also originally θ'^2 was already squared, so wouldn't |v(t2)|^2 = l^2θ'^4??
    Very confused on this.

    Any help would be appreciated thanks.

    Attached Files:

  2. jcsd
  3. Feb 5, 2012 #2


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    |v(t1)| = 0 as no movement at t=t1.

    |v(t2)|^2 = l^2θ'^2 because v(t2)= radius x angular velocity. So it is just squared. (I am ignoring the modulus signs now).

    For the second part, the change in PE = change in KE

    so the height went from lsin(30) to lsin(θ) so the change in PE is mg[lsinθ-lsin30]

    similarly change in KE is 1/2mv22-1/2mv12 = 1/2mv22 = 1/2m(lω)2 = ½ml2ω2

    Do you understand how they the first part for the tension T at least?
  4. Feb 5, 2012 #3
    I understand where T=mgsinθ+mlθ'^2 came from. But for |v(t2)|^2, why is l squared but theta dot not to the 4th power? I totally don't get this. And PE = ΔKE is the same as W=ΔKE right? Thanks.
  5. Feb 5, 2012 #4


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    Right, for the velocity, they multiply the radius (l) by the angular velocity (theta dot or ω)

    so v(t2)=lω so that v(t2)2=l2ω2

    Yep, same idea.
  6. Feb 5, 2012 #5
    so |v(t2)|^2 has absolutely nothing to do with T=mgsinθ+mlθ'^2? Meaning it wasn't derived from there or anything? It's just something you should intuitively know?
  7. Feb 6, 2012 #6


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    Well the kind of did it backwards in a way.

    Normally you'd start with ΔPE=ΔKE

    to get mg[lsinθ-lsin30]= 1/2mv22

    and it would be better to have everything in terms of θ so putting [itex]v_2= l \dot{\theta}[/itex] is easier.

    so you'd get

    [tex]mg[lsin\theta - lsin30]=\frac{1}{2}ml^2 \theta ^2 \Rightarrow mg[sin\theta -sin 30]=\frac{1}{2}ml \dot{\theta}^2[/tex]

    and if we look at the the [itex]ml \dot{\theta}^2[/itex] you'd see that is mass*radius*angular velocity squared i.e. centripetal force.

    The only way that relates to tension T is from the first line

    [tex]T+mgsin\theta = ml \dot{\theta}^2[/tex]
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