Solving for v: "Calculating the Speed of Puck After Collision

[BadSkittles]

A 0.440kg ice puck, moving east with a speed of 3.16m/s, has a head-on collision with a 0.940kg puck initially at rest. Assuming a perfectly elastic collision, what is the speed of the lighter puck? Use east as the positive axis.

I got -2.09.

I made two equations and used substitution to find the speed of the lighter puck. But its wrong. Would appreciate all help.


initial final
1/2mv^2= 1/2 mv^2 + 1/2 mv^2

m(v) = m(v) + m(v)

 

[rude man]

How about distinguishing symbolically between the lighter and the heavier puck? Like, call the lighter m1 and the heavier m2, and same for the velocities.

Rewrite your equations that way and try again. You seem to have the right basic idea.

Hint: there are two solutions to (v1,v2). The second set is physically impossible even though not violating either momentum or energy conservation.