2D Collisions

A collision at an angle: conserve momentum along x, and separately along y.

$$\textstyle\sum p_x \text{ before} = \sum p_x \text{ after}, \quad \sum p_y \text{ before} = \sum p_y \text{ after}$$

After the hit, A goes up and to the right, B down and to the right.

No y momentum before: the y momenta after are equal and opposite. Tip to tail, the momenta after add up to the momentum before.

The two momenta after the collision add up to the momentum before.