A tractor with mass $1000\,\text{kg}$ is attached to a trailer of mass $1000\,\text{kg}$ by a tow-bar. The overall resistance to motion is constant and equal to $4000\,\text{N}$. A quarter of this resistance acts on the trailer. If the tractor and trailer are travelling over horizontal ground at a steady speed of $6\,\text{m s}^{-1}$, what force does the tow-bar exert on the tractor?
- A$0\,\text{N}$
- B$1000\,\text{N}$
- C$3000\,\text{N}$
- D$4000\,\text{N}$