# Another one

1. Sep 4, 2006

### 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?

2. Sep 4, 2006

### tony873004

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

3. Sep 4, 2006

### 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.

4. Sep 4, 2006

### krazykaci

what would the time variable be then?

5. Sep 4, 2006

### EP

If you dont know it label it as just t=?. But look at your equations, they all dont have something in it. Each one doesnt have one of these a,v,t,d so which oe should you use.

6. Sep 4, 2006

### krazykaci

v=v.+a(t)

i know v=452m/s
v.=298 m/s
a= unknown
but i dont know what t should be, and it is known isnt it?

7. Sep 4, 2006

### EP

Do you need time?
There are other equations you can use.

8. Sep 4, 2006

### krazykaci

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

9. Sep 4, 2006

### EP

Yes, List out everything you know before you even start a problem. This will help with some headaches. Look for what you dont know also and choose your equation with this information.

10. Sep 4, 2006

### krazykaci

so do I use 8.1 and 1.86 to find displacement?

11. Sep 4, 2006

### EP

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

12. Sep 4, 2006

### krazykaci

can you explain it in a different way. I dont understand how that fits into the equation.

13. Sep 4, 2006

### EP

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

14. Sep 4, 2006

### krazykaci

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

15. Sep 4, 2006

### EP

looks good to me!

16. Sep 4, 2006

### krazykaci

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

17. Sep 4, 2006

### EP

18. Sep 4, 2006

### krazykaci

i dont know! i am doing the equation and im getting that answer, but the computer is saying it is the wrong answer!

19. Sep 4, 2006

### EP

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