Very simple high school phisics question

[God64bit]

A stationary object explodes into two fragments. A 4.0 kg fragment moves westward at 3.0 m/s. What are the speed and kinetic energy of the remaining 2.0 kg fragment?
why can't i assume the kinect enegery of each object is the same?


I loked at it and assumed 1/2mv^2 for object 1 should equal object 2 subsituded numbers got 18J=1/2 2 v^2

but the answer says its 36J's

[dx]

A better question to ask is "why did I assume kinetic energy is divided equally?". Try to think about the explosion. The explosion forces the pieces away. You may assume that the force on both pieces is the same. How long does the force act on each particle, and how far does it act?

[God64bit]

thats simple enough the t part for how long the force was acting.... :P
ill just solve it with momentum.

[Pengwuino]

If you solve this with conservation of momentum, you can figure out the problem. I've come to realize that solving things with conservation of momentum is a better way to start in these sort of macro-classical mechanic problems.