If the net force is zero, why keep running the engine?

Answer by  HarshVardhan (73)

Running engine provides the energy against which frictional force acts to provide momentum to wheels i. e. governed by each action has equal and opposite reaction. If the engine is not running there will be no force to which frictional force will counteract to displace the wheels.

Answer by  MechE (78)

For non-hybrid engines, it's actually more reliable in the long run to keep the engine running versus stopping the engine and using the starter (motor) to restart the engine. Starters themselves take a lot of energy to turn the engine to the correct RPM to self run.

Answer by  rogera (788)

Running the engine provides the energyagainst which friction force acts to provide momentum to wheels, Akey point is that the net force is zero. It is not the same as having zero forceacting on it. The engine generatesa force through the wheels that balance out thus force, keeping you at a constant velocity.