Final answer:
There is no acceleration when a car moves at a constant velocity because net force is zero, so it neither speeds up nor changes direction. A rocket launching into space and a car turning at constant speed experience acceleration due to changing velocity and direction, respectively, resulting in a nonzero net force.
Step-by-step explanation:
In situations where there is no acceleration, the object does not change its velocity, which implies that both its speed and direction remain constant. When a car is moving at a constant velocity, it does not accelerate because there is no change in speed or direction, and according to Newton's first law, this indicates that the net force acting on the car is zero. On the contrary, a rocket being launched into space is under considerable acceleration as its speed increases rapidly when it ascends. Lastly, a car turning at a constant speed is indeed accelerating, contrary to common misconceptions; even though the speed remains constant, the change in direction means the velocity is changing, thus creating acceleration.
Would we measure a net force in these situations? For the car moving at a constant velocity and a car parked with no movement (as a stationary reference), the net external force would be zero as forces are balanced and the car is in equilibrium. However, for the rocket and the turning car, there would be a net force since these motions involve changing velocity which cannot occur without a resultant force acting upon them, as stated by Newton's second law, F = ma; a nonzero acceleration implies a nonzero net force.