# Homework Help: Bullet hits mass attached to spring with pic

1. Dec 16, 2008

### hodgepodge

1. The problem statement, all variables and given/known data
A 20 gram bullet strikes a 15 kg mass attached to a spring. The mass aslides and compresses the spring by 12 cm. Calculate the velocity of the bullet initially.

Bullet............................Mass..........Spring
...D.............................I--------I
..................................I...........I~~~~~~~~|
.........................._____I_______I___________|
2. Relevant equations

3. The attempt at a solution

i was thinking to use (1/2) x (K) x (x squared) = (1/2) x (m) x (v squared)
then i was told to solve for p (momentum) (??) after using the above formula

2. Dec 16, 2008

Your attempt is good. From that equation, you can find v (the speed of the mass just before it compresses the spring).
But what happens to the bullet? Does it get embedded in the mass? That information is important.

3. Dec 16, 2008

### glueball8

Hi, is the spring constant given? Yes, its different if the bullet stick with the mass or bounces off, but in this problem it should stick.

4. Dec 16, 2008

### hodgepodge

spring constant is 200 n/m sorry forgot to add it

5. Dec 16, 2008

### glueball8

In the cases that it sticks energy is NOT conserved but momentum always is.

$$m_{1}v_{1}+m{2}v_{2}=m{1}v_{3}+m{2}v_{3}$$

they stick soo mass and the bullet has the same velocity.

Then again use energy.

6. Dec 16, 2008

### hodgepodge

thanks once again bright wang