The diagram represents a motorised vehicle designed to transport one person. The vehicle has two wheels mounted on the same axle. The passenger stands on a platform positioned between the wheels. The weight of the machine is $600\,\text{N}$. Its centre of mass is $200\,\text{mm}$ in front of the axle. The wheel radius is $400\,\text{mm}$. When the vehicle is stationary, a passenger with weight $600\,\text{N}$ stands with his centre of mass $200\,\text{mm}$ behind the axle so that the machine is balanced. The motor is now turned on so that a horizontal force of $90\,\text{N}$ acts at the ground and drives the vehicle forwards. How far, and in what direction, must the passenger shift his centre of mass to keep the machine balanced?
- A60 mm backwards
- B60 mm forwards
- C140 mm backwards
- D140 mm forwards