Momentum problem with collision

[cristo]

I would say that this collision is to be modeled as a perfectly elastic collision, and so you can use conservation of kinetic energy as well as conservation of momentum.

(This also means you don't need to worry about the coefficient of restitution, which I gather you haven't been taught).

 

[lzh]

if i use conservation of energy why do i need to use conservation of momentum? I know that i should use both but:
before collision: all kinetic energy
after:all kinetic
there's no friction and everything is perfect so if i set the kinetic energy from before equal to after I would just have one unknown(the answer) to solve.
1/2mv^2=1/2mv^2+1/2mv^2
basically the 1/2 and mass all cancel
so:
3.7^2=2.1^2+V^2
V=3.04
this definitely doesn't seem right...

 

[lzh]

yeah i tried what was above and it was right... w/o any momentum...strange

 

[jay ambekar]

hey, check ur gmail inbox