Final answer:
An airplane traveling with a constant velocity of 210 m/s is not accelerating as its speed and direction stay the same; acceleration occurs with changes in speed or direction.
Step-by-step explanation:
The concept of acceleration in physics is often misunderstood. It is important to recognize that an object is accelerating not only when it is speeding up but also when it is slowing down or changing direction. For example, if an airplane travels with a constant velocity of 210 m/s, it is not accelerating because its speed and direction remain constant. However, if that airplane changes its speed or direction at any point, it would then be considered to be accelerating.
An airplane that lands with an initial velocity and slows to a final velocity is a clear illustration of deceleration, which is a type of acceleration where the change in velocity is negative (the object slows down). Similarly, if an airplane is traveling in a straight line at a constant speed and then starts turning, it is also accelerating because its direction is changing.