Newton's First Law of Motion: When ΣF = 0
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.
Newton's First Law of Motion: When ΣF = 0
ΣF = 0 means no change in velocity, not no motion. When the drive and the resistance cancel, a = ΣF/m = 0, so v stays what it was: 0 for a parked car, 15 m/s for one already cruising. The v–t line goes flat at that height, over a ΣF line on zero. That is Newton's first law: the car's inertia keeps its velocity until a net force changes it.
Cruising still takes a drive force. At a steady speed the drive has not switched off; it balances the resistance. Lower Fdrive below Fresist and ΣF points backward: the car still moves forward, but slows down. Moving needs no net force; changing velocity does.
Resistance only pushes back. Fresist stands for road friction and air resistance together. It opposes the motion, so it can stop the car but never drive it backward: at rest it matches the drive, up to Fresist, and ΣF stays 0. With both forces at 0, a moving car coasts at constant velocity.
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.