An object P with mass $80\,\text{g}$ collides with a second object Q of mass $40\,\text{g}$. Following the collision, P and Q join together and move off as one. Prior to the collision, P has speed $6.0\,\text{m s}^{-1}$ and Q is stationary. What is the speed of P and Q after the collision?
- A$2.0\,\text{m s}^{-1}$
- B$3.0\,\text{m s}^{-1}$
- C$4.0\,\text{m s}^{-1}$
- D$6.0\,\text{m s}^{-1}$