External forces are the which act from outside the system. (T or F)

Fill in the blanks: When no external force acts on a system the total _____ of the system remains constant.

When brakes are applied in a car,the brake-shoes apply frictional force on the wheels.these forces are internal for the system(car).For retardation,external forces ust act on the system.The force of friction at the road surface remains the same before and after teh brakes are applied (f = mu N) .How does the car stop?

Underline the correct alternative :- The rate of change of total momentum of a many-particle system is proportional to the external force // sum of the internal forces on the system.