Bullet fired and penetrates a block of wood

[talaroue]

1. Homework Statement [/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?

Homework Equations

Dynamics

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]

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

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 don't 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:
https://www.physicsforums.com/showpost.php?p=905663&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