Answer the following question given this paragraph: 

External forces are forces that act on the system from outside. In our previous example, external forces include the force of gravity acting on both cars (because the other part of the force pair, the pull of gravity the Earth experiences coming from the cars, is not included in the system) and the forces of friction between the tires and the road.


Q: What do we call forces that act on the system from outside?


A:
external