9. (Problem Solving)

# Collision Problems

- Start with momentum conservation: $\vec p_i=\vec p_f$. If there are two objects, then $$m_1v_{1i} + m_2v_{2i} = m_1v_{1f} + m_2v_{2f}$$
- Fill in what is given, and solve for what's left.
- Notice that the masses $m_1$ and $m_2$ appear on both sides of the equation.
- Remember that the velocities have direction and therefore
*sign*: + or -. - If the two objects "stick together" after the collision, then they have the same velocity: $v_{1f}=v_{2f}$
- If the problem involves something that comes apart (like a bomb), then their
*initial*velocities are the same: $v_{1i}=v_{2i}$