I'm stuck on this problem.. My futile attempt: the wood block's initial momentum is zero so it's [tex]m_b v_b = (m_b + m_w) v_f[/tex] Where [tex]m_w[/tex] is the mass of the wooden block. In order to get the bullet's velocity (which is [tex]v_b[/tex]) I need [tex]v_f[/tex] which is the final velocity of both the bullet and the block in this inelastic collision. So I tried to use kinematics to get that final velocity which is equal to initial velocity from the start of motion (bullet + block) until it comes to rest after moving .05 meters. [tex]v_f^2 = v_i^2 + 2 x a[/tex] [tex]0 = v_i^2 + 2 (.05) a[/tex] As you can see, I don't know the acceleration... so maybe I should do some force analysis? The problem didn't give the magnitude of force the bullet exerted on the block nor the coefficients for friction so I don't know... Thanks for any help!