Two spheres move towards one another along the same straight line. Before they collide, their speeds are $u_1$ and $u_2$. Once the collision is over, the spheres move apart with speeds $v_1$ and $v_2$ in the directions shown below. The collision is perfectly elastic. Which equation must be correct?
- A$u_1 - u_2 = v_2 + v_1$
- B$u_1 - u_2 = v_2 - v_1$
- C$u_1 + u_2 = v_2 + v_1$
- D$u_1 + u_2 = v_2 - v_1$