Bullet Impact Speed Calculation: 2.25x10^3 N/m

[endeavor]

A 0.0125 kg bullet strikes a 0.300 kg block attached to a fixed horizontal spring whose spring constant is 2.25 * 10^3 N/m and sets it into vibration with an amplitude of 12.4 cm. What was the speed of the bullet if the two objects move together after impact?

E = .5 k A² = .5 m v²
Do I use m = mass of bullet + mass of block, or just the mass of the bullet?
Using the former, I get v = 10.5 m/s. Is this right? It just seems kind of slow for a bullet...

 

[christianjb]

This is an energy conservation problem

Initially the only energy is the kinetic energy of the bullet.

Finally, the only energy is the spring energy (potential+kinetic).

Those two energies are the same.