How much kinetic energy is released

[gijungkim]

Homework Statement

A 6.0-kg object, initially at rest in free space, “explodes” into three segments of equal mass. Two of these segments are observed to be moving with equal speeds of 20 m/s with an angle of 60° between their directions of motion. How much kinetic energy is released in this explosion?

a.2.4 kJ

b.2.9 kJ

c.2.0 kJ

d.3.4 kJ

e.1.2 kJ

Homework Equations

m1v1 + m2v2 + m3v3 = m1v1f + m2v2f + m3v3f
K = 0.5*m*v^2

The Attempt at a Solution

The answer is c, 2000J, but I got 1500J and I don't know where I did wrong. Can anyone help me?
My work is in the picture

 

[gijungkim]

Nevermind I got it. Just made an algebric mistake..

 

[BvU]

Even so, herer's a tip: place the x-axis halfway between m1 and m2. That way p_y cancels. Less chance of error. But you do need cos(60) which at first you had 0 !

 

[gijungkim]

Even so, herer's a tip: place the x-axis halfway between m1 and m2. That way p_y cancels. Less chance of error. But you do need cos(60) which at first you had 0 !

Thanks for the tip!