Newton's first law states that in an inertial reference frame a body's velocity cannot change unless a nonzero net external force acts on it. The relationship runs both ways: if ΣF = 0 the velocity is constant, and if the velocity is constant then ΣF = 0. Constant velocity includes the special case v = 0, so an object sitting still and an object cruising at steady speed are governed by exactly the same condition — a first law situation, not a second law one.

Adjust the forces below and watch what happens the moment ΣF stops being zero.

Watch the three readouts move together. Net force ΣF sets the acceleration through a = ΣF/m, and the acceleration is what changes the velocity. While the forward and backward forces balance, ΣF = 0, a = 0, and the velocity holds flat — you can see it as a horizontal line on the v–t graph. Nudge either force out of balance and the line immediately tilts.

The car's mass never affects whether the velocity is constant; it only affects how quickly the velocity changes once ΣF is nonzero. Doubling the mass halves the acceleration for the same net force, but a balanced car at 500 kg and a balanced car at 2000 kg both hold their speed equally well.

This is also why an engine never appears on a car's free body diagram. The engine is internal to the car, and internal forces cannot change the motion of the system as a whole. What actually pushes the car forward is static friction from the road acting on the tyres — the tyre pushes backward on the road, and the road pushes forward on the tyre. Remove the road, as on sheet ice, and the engine can spin the wheels as fast as it likes without producing any forward acceleration.