What is the speed of the bullet in a ballistic pendulum collision?

[nns91]

Homework Statement

A 16g bullet is fired into the bob of a ballistic pendulum of mass 1.5kg When the bob is at its maximum height, the strings make an angle of 60 degree with the vertical. The length of the pendulum is 2.3 m. Find the speed of the bullet.

Homework Equations

p_i=p_f

The Attempt at a Solution

So I think it is an inelastic collision so v_f = (m_bullet*v_bullet) / (m_bullet+m_block)

After collision:

(1/2)*(m_bullet+m_block)v_f² = (m_bullet+m_block)*g*h

h=2.3-2.3*cos60=1.15m

then I solved for v_f²=2*9.981*1.15=22.563

the I substitute into the first formula ( I actually squared both sides to use vf^2) to find v_bullet= 346.2 m/s. However it is wrong. Where did I do wrong ??

 

[Doc Al]

Your method looks perfect. Check your arithmetic in your last step.

 

[nns91]

Cool. I found out that I got the wrong mass for the last computation, I put in 1.15 instead of 1.5