Several special types of motion are easily described. First, velocity may be constant. With constant velocity, the average velocity is equal to the velocity at any particular time. In the second special type of motion, acceleration is constant. Because the velocity is changing, instantaneous velocity, or the velocity at a given instant, must be defined.
Circular motion is another simple type of motion.
If an object has constant speed but an acceleration always at right angles to its velocity, it will travel in a circle. Because of gravitation, the ball undergoes a constant downward acceleration that first slows its original upward speed and then increases its downward speed as it falls back to Earth. The vertical and horizontal components of the motion are independent, and they can be analysed separately. The resulting path of the ball is in the shape of a parabola. To understand why and how objects accelerate, force and mass must be defined.