I understand this is a homework problem. I don't expect anyone to do my homework for me, but help me understand what I am doing wrong.

The problem goes: Two shuffleboard disks of equal mass, one orange and the other green, are involved in a perfectly elastic glancing collision. The green disk is initially at rest and is struck by the orange disk moving initially to the right at oi = 5.30 m/s as in Figure (a) shown below. After the collision, the orange disk moves in a direction that makes an angle of θ = 35.0° with the horizontal axis while the green disk makes an angle of = 55.0° with this axis as in figure (b). Determine the speed of green disk if the final velocity of the orange disk is 4.34m/s

First I draw the collision out. with the angles and everything. This helps me with visualizing what is happening.

Now i separate the Elastic Collision into x & ycomponents:

since the masses are all the same does that mean in the elastic collision equation the masses cancel out?

if so this is the equations

My x equation

5.30m/s + 0 = 4.34m/s*cos(35) + Vgf

Vgf(x) = -1.3513 m/s

y equation

0 + 0 = 5.30*sin(35)+Vgf

Vgf(y) = 2.6412

Then I do sqrt( (Vgf(x))^2 + (Vgf(y))^2) = 2.64 m/s (which is the incorrect velocity of the green disk.)

Though this is incorrect.

My only idea to why this is incorrect is probably because of the mass part of the Elastic Collision equation. I don't know how to use the collision equations without the masses given. What do I do?

  • $\begingroup$ Hint: Part (b) can be solved without understanding the geometry of collision at all, just that a disk moving with known velocity strikes a stationary one and one final velocity is given. $\endgroup$ – dmckee --- ex-moderator kitten Dec 14 '11 at 22:18
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    $\begingroup$ Note to potential answerers: Take care to address the concepts at play here and not the particular exercise in keeping with the FAQs requirement to not do homeowrk problems. $\endgroup$ – dmckee --- ex-moderator kitten Dec 14 '11 at 22:19
  • $\begingroup$ @dmckee, are questions like this one allowed to be asked? With given the information i have provided. $\endgroup$ – John Riselvato Dec 14 '11 at 22:45
  • $\begingroup$ This question is borderline, and could benefit from be re-focused a little. See the section in the FAQ and How do I ask homework questions on Physics Stack Exchange?. The basic 2D elastic collision has a lot to teach in terms of independence of vector components and conservation rules, but you should focus on those and not on this particular instance. $\endgroup$ – dmckee --- ex-moderator kitten Dec 14 '11 at 22:59

First of all, check your arithmetic on the X equation. You should get Vg(x) = +1.74 m/s.

Second, in the Y equation you should use the final orange speed 4.34 instead of 5.3 on the right hand side. You should get Vg(y) = -2.48 or so.

  • $\begingroup$ Ugh.. simple mistake. thanks for pointing that out. $\endgroup$ – John Riselvato Dec 14 '11 at 22:43

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