# Homework Help: Bullet penetrating a block problem

1. Nov 5, 2008

### mjolnir80

1. The problem statement, all variables and given/known data
a rifle with a barell length of 60 cm fires a 10g bullet with a horizontal speed of 400 m/s. the bullet strikes a block of wood and penetrates to a depth of 12 cm. how long does it take the bullet to come to rest( from when it penetrates the block to when it stops)?

2. Relevant equations
kinematic equations

3. The attempt at a solution
we can figure out the acceleration of the bullet while its in the block with
Vf2= Vi2 + 2aX which gives -6.7*105

but now when i try to use X= Vit + 1/2 at2 to figure out time it give me a function with no real roots
what am i doing wrong here?

2. Nov 5, 2008

### Kurdt

Staff Emeritus
Any other kinematic equations you may have overlooked?

3. Nov 5, 2008

### mjolnir80

dont worry i havent forgotten about the other equations :)
i was just wondering why this one didnt work

4. Nov 5, 2008

### gabbagabbahey

I get a real, double root when I solve it using your method, so you must be making an error somewhere in your work...If you actually post your work, I can point it out to you.

5. Nov 5, 2008

### mjolnir80

oops guess i better double check that
thanks for the help