Bullet fired and penetrates a block of wood

talaroue

1. The problem statement, all variables and given/known data[/b]

A rifle with a barrel length of 61.54 cm fires a 8.17 g bullet with a horizontal speed of 400 m/s. The bullet strikes a block of wood and penetrates to a depth of 8.9 cm.
What resistive force (assumed to be constant) does the wood exert on the bullet?


2. Relevant equations
Dynamics


3. The attempt at a solution

talaroue

am i even in the right direction? If not where did I go wrong?

Doc Al

Did you give the correct attachment?

talaroue

haha oops good catch give me a second

talaroue

now its the correct image. Thank you for that.

Doc Al

Hint: Use energy considerations.

talaroue

Is that the only way that we are able to solve this because we haven't gone over engeries in class yet?

Doc Al

No, you can also use dynamics/kinematics. Start by finding the average acceleration.

talaroue

Am i suppose to assume that the distance the bullet traveled is 70.44 cm? Which is the barrel length+ the depth of the bullet in the wood?

talaroue

I got the anwser it was 7343.822 N.....only problem is I dont know how I got it I found someone who did this problem online and just plugged my numbers in his equation but it doesn't show how he derived the equation!

LowlyPion

The distance it decelerates is .089 m.

The initial velocity is 400 m/s. The final velocity of course is 0.

Does anything occur to you as to how to find the magnitude of the deceleration?

See maybe:
http://www.physicsforums.com/showpos...63&postcount=2

talaroue

I think working on physics so long actually made me worse at it. It was right in front of me I just needed you to remind me of Kinematics! just us xf=xi+(vf^2-vi^2)/2a....then multiply the mass of the bullet by acceleration and that gives me fs.....wow FML