Bullet & 2 Blocks Momentum Question

[lando45]

Hey,

I have been set this question and thought I'd solved it correctly but it turns out I got the answers wrong...

A 5.50 g bullet is fired horizontally at two blocks at rest on a frictionless tabletop. The bullet passes through the first block, with mass 1.20 kg, and embeds itself in the second, with mass 1.80 kg. Speeds of 0.630 m/s and 1.40 m/s, respectively, are thereby given to the blocks. Neglect the mass removed from the first block by the bullet. Find the speed of the bullet immediately after it emerges from the first block, and find it's original speed.

For the first question (finding the speed after it emerges from the first block) I found 1/2mv2 for the second block then subtracted the 1/2mv2 of the first block and then calculated v and got a value of 20m/s but this is wrong...can anyone shed some light?

 

[Fermat]

Use conservation of momentum rather than conservation of energy.