dralion87 said:
i did (1/2)(.02)(800)^2 / (.56) =11428
All right, the conservation of energy equation you are using is
KE_init + W_non-conservative = KE_final ,
with the non-conservative work being given by
W_non-con = -F · (delta_x) .
Since the final KE is zero, you have
W_non-con = -KE_init and
-F · (delta_x) = -KE_init gives F = KE_init / delta_x .
You did this and found F = 11,430 Newtons.
That is the part you can do by conservation of energy principles. To find the power involved in stopping the bullet, you need to use the definition of power
P = W / delta_t , where delta_t is the time over which the force you found has acted.
You know the initial speed (800 m/sec) and final speed (0 m/sec) of the bullet. If you can find the acceleration that acted on the bullet while the trunk of the tree was stopping it, you can find the time it took to bring the bullet to rest. You also now know the force that acted on the bullet. Can you find the bullet's acceleration?
(Actually, you're getting close: the number you gave in post #17 is off by a factor of about 2...)