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: $v_{1i}=v_{2i}$