# Bullet, block, spring (again)

• Spraypaint

## Homework Statement

A rifle bullet of mass 7.2 grams strikes and embeds itself in a block with mass 0.819 kg that rests on a frictionless horizontal surface and is attached to a coil spring. The impact compresses the spring 17 centimeters. Calibration of the spring shows that a force of 0.68 Newtons is required to compress the spring 0.31 cm. Find the magnitude of the block’s velocity just after impact. Give your answer in m/s to the first decimal place.

## The Attempt at a Solution

I have looked at at least 4 other posts on this forum about this question, but I still cannot quite get it.

m1v1 = (m1+m2)v2

Using some dimensional analysis, I get that force is equal to some 37 N. Multiply this with the 17 cm, and you get the work done on the spring. This is equal to .5 * m * v1^2. Solve this for V1, plug back into the momentum equation to get V2. Is this not correct because my answer differs from the solution.

Ok, so I finally found the solution, and it's something pretty stupid that I never thought of. k the spring constant is given, then everything else is simple.