How Do You Calculate the Angle in a 2D Elastic Collision?

[LTZach]

Homework Statement

Consider an elastic collision (ignoring friction and rotational motion). A queue ball initially moving at 2.9 m/s strikes a stationary eight ball of the same size and mass. After the collision, the queue ball’s final speed is 1 m/s. Find the queue ball’s angle with respect to its original line of motion.
Answer in units of ◦.

Homework Equations

I can't seem to figure out how you're supposed to get an angle for this, if none is given.

The Attempt at a Solution

in 1-d the equation would be something like m1v1+m2v2=(m1+m2)*V. In 2-d, I assume you need a trig function, but how is that useful with no given angle?

 

[tiny-tim]

Welcome to PF!

Hi LTZach ! Welcome to PF!

Consider an elastic collision (ignoring friction and rotational motion). A queue ball initially moving at 2.9 m/s strikes a stationary eight ball of the same size and mass. After the collision, the queue ball’s final speed is 1 m/s. Find the queue ball’s angle with respect to its original line of motion.
Answer in units of ◦.
…
how is that useful with no given angle?

(btw, it's a cue ball! )

Call the angle θ, and find three equations, for momentum in the x and y directions, and for energy.

What do you get? ​