Kinematic Equation: Accel of Bullet Through 8.1cm Thick Board

[krazykaci]

another one!

An indestructible bullet 1.86cm long is fired straight through a board that is 8.1cm thick. the bullet strikes the board with a speed of 452 m/s and emerges with a speed of 298 m/s. what is the average acceleration of the bullet through the board? answer in units of m/s^2

what would the length of the bullet and the width of the wall be used for in a kinematic equation? I know 452=v. and 298=v ... or is that wrong?

 

[tony873004]

Use the same formula for acceleration as your tennis ball question.

 

[EP]

Think. You have to find the acceleration. List the things you know. Then look at your equations again. List all things that are relavant to kinematics.

 

[krazykaci]

what would the time variable be then?

 

[EP]

If you don't know it label it as just t=?. But look at your equations, they all don't have something in it. Each one doesn't have one of these a,v,t,d so which oe should you use.

 

[krazykaci]

v=v.+a(t)

i know v=452m/s
v.=298 m/s
a= unknown
but i don't know what t should be, and it is known isn't it?

 

[EP]

Do you need time?
Your missing a variable, distance.
There are other equations you can use.

 

[krazykaci]

oh, so I am using v^2=v.^2+2a(x-x.)? right?

 

[EP]

Yes, List out everything you know before you even start a problem. This will help with some headaches. Look for what you don't know also and choose your equation with this information.

 

[krazykaci]

so do I use 8.1 and 1.86 to find displacement?

 

[EP]

The bullet is only deaccelerated over 8.1cm. The width of the board.

 

[krazykaci]

can you explain it in a different way. I don't understand how that fits into the equation.

 

[EP]

Your initial x=0 because the bullet has not been acted upon yet so your final x=8.1cm

 

[krazykaci]

ok so the equation is 298^2=452^2+2a(8.1-0) right?

 

[EP]

looks good to me!

 

[krazykaci]

the answer I am getting (-7129.63) is apparently incorrect. can you work it out and see if I am just doing my math wrong?

 

[EP]

whats the right answer?

 

[krazykaci]

i don't know! i am doing the equation and I am getting that answer, but the computer is saying it is the wrong answer!

 

[EP]

well I'm getting a different answer than you also. Check your units. Stay consistent.