Two equal masses move towards one another on a frictionless air track with speeds of $60\ \text{cm s}^{-1}$ and $40\ \text{cm s}^{-1}$. On collision, they remain stuck together. What speed do the masses have after the collision?
- A$10\ \text{cm s}^{-1}$
- B$20\ \text{cm s}^{-1}$
- C$40\ \text{cm s}^{-1}$
- D$50\ \text{cm s}^{-1}$