# Homework Help: Physics HW kinematics and sig figs

1. Sep 11, 2014

### raho14

1. The problem statement, all variables and given/known data
An indestructible bullet 2.10cm long is fired straight through a board that is 11.7cm thick. The bullet strikes the board with a speed of 416m/s and emerges with a speed of 296m/s.

2. Relevant equations
What is the total time that the bullet is in contact with the board?
What thickness of board (calculated to 0.10cm) would it take to stop the bullet, assuming the bullet's acceleration is the same?

3. The attempt at a solution
i have tried it but its not right.
for the first one:
d=vinitialxt+0.5at^2 solve for t
for second one:
vfinal^2=vintial^2+2ad where v final would be zero.

Last edited: Sep 11, 2014
2. Sep 11, 2014

### SteamKing

Staff Emeritus
Well, how about posting your work? We can't tell what you've done if you won't share.

3. Sep 11, 2014

### DEvens

I'm trying not to give away the entire problem by being too pointy in my questions.

Write down the equations of motion for the first case.

With those equations, can you solve for the acceleration and the time? Do you have to make any assumptions to do it? Hint: Can you work out the average velocity while in contact with the board? And then, so what?

4. Sep 11, 2014

### raho14

i did get the acceleration which was right but i keep getting the time wrong i mean i usually get this but its confusing when u do it ten times so can u just tell me

5. Sep 11, 2014

### Staff: Mentor

raho14,

Welcome to Physics Forums.

It would help if you would show more of your work, including your calculations. This will allow helpers to better understand where you are having difficulties.

Also take note that text-speak such as "u" meaning "you" is not allowed here.