Hence we have in the direction of the net force. In our examination of Newton's second law of motion, we find that the first law is expressed in the second. If the net force on an object is zero, then its acceleration is zero. That is, there is no change in velocity, and the object must be moving with a constant velocity or be at rest. This is essentially what the first law states, in sort of a reverse fashion.
Cars get better gas mileage on the highway than in the city.
Newton's second law provides an explanation. When a car is driven in city traffic, it must be accelerated again to normal speed after it slows down or stops. All this acceleration burns a lot of gasoline. It does not take as much fuel to keep a car going at a constant speed down a level highway.