conservation of momentum, elastic collision, find other mass? help!! Two titanium spheres approach each other head-on with the same speed and collide elastically After the collision, one of the spheres, whose mass is 300 g, remains at rest. What is the mass of the other sphere? What i did: m1v1 + m2v2 = m1u1 + m2u2 v1 = 0 b/c at rest m2v2 = m1u1 + m2u2 m2v2 - m2u2 = m1u1 m2(v2 - u2) = m1u1 ????? This is where I got stuck! please help!