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**1. Homework Statement**

A cart with mass 310 g moving on a frictionless linear air track at an initial speed of 1.4 m/s undergoes an elastic collision with an initially stationary cart of unknown mass. After the collision, the first cart continues in its original direction at 0.64 m/s.

**2. Homework Equations**

(1/2)m1vo^2 +(1/2)m2vo^2 = (1/2)m1vf^2 + (1/2)m2vf^2

**3. The Attempt at a Solution**

(310)(1.4)^2 + 0 = (310)(.64)^2 + m2vf^2

(310)(1.4)^2 - (310)(.64)^2 = m2vf^2

but I feel like I need vf to find m2 so how am I supposed to go from here?