A bullet is fired through a wooden board with a thickness of 12.0 cm. The bullet

Question

A bullet is fired through a wooden board with a thickness of 12.0 cm. The bullet hits the board perpendicular to it, and with a velocity of +390 m/s. The bullet then emerges on the other side of the board with a velocity of +351 m/s. Assuming constant acceleration (rather, deceleration!) of the bullet while inside the wooden board, calculate the acceleration. (Note that the positive velocities define the positive direction to be in the bullet's direction or travel.)
^ For this problem, I got 120412.5 m/s2 but is telling me that it is wrong? ^

Calculate also the total time the bullet is in contact with the board (in sec).

Explanation / Answer

thickness, t = 0.12 m
Vi = 390 m/s
vf = 351 m/s

Using Netwon equation of motion,
vf^2 = vi^2 + 2*a*s
351^2 = 390^2 + 2*a*0.12
a = - 120413 m/s^2

Let the total time be, t
v = u + a*t
351 = 390 - 120413*t
t = 3.24 * 10^-4 s

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