I don't understand the calculation of forces acting on a system vs the forces acting on an individual object in that system.
If trolley 1 starts exerting a force so that the whole system accelerates forwards...
- The trolley exerts a forward force on the towbar
- As the towbar pulls the second trolley along, the second trolley exerts a backwards force on the towbar
So the whole system is accelerating forwards, and the towbar has this same acceleration, but at the same time, the forces on the towbar cancel because of the opposite forces exerted by the the two trolleys on it:
How can an object accelerate within a system if the forces acting on that object by itself cancel?