# Block Bullet moment problem

#### nakatheking

Hello,
I got v = 5.48 m/s for part a) and f = 24 Hz for part b). Is this right? If not, what is the right approach to this problem?

Thank you so much!

#### nakatheking

Scratch that - I got 5.03 m/s and 7.92 Hz

#### Cervesa

A bullet moving at about 5 m/s ... does that even make sense? I agree with the digits, but not the order of magnitude. Check your units.

#### nakatheking

I mean, if the spring is only moving by .1 m, then the bullet can't be going that fast, right? I checked my units and everything appears correct...

#### nakatheking

I think this may be the problem... I had calculated v final = sqrt(kA^2/(m1+m2)), should I just be dividing my the one mass?

#### Cervesa

$mv_0 = (m+M)v_f \implies v_f = \dfrac{mv_0}{m+M}$

$KE = U_e \implies \dfrac{1}{2}(m+M)v_f^2 = \dfrac{1}{2}kA^2$

$\dfrac{1}{2}(m+M)\left(\dfrac{mv_0}{m+M}\right)^2 = \dfrac{1}{2}kA^2$

$\dfrac{m^2v_0^2}{m+M} = kA^2 \implies v_0^2 = \dfrac{(m+M)kA^2}{m^2} \implies v_0 = \sqrt{\dfrac{(m+M)kA^2}{m^2}}$

plugging the numbers into a calculator