One concept about motion that is often hard for students to grasp is that an acceleration can occur even when the speed or magnitude of the velocity of an object is constant. This means that the object may not speed up or slow down, but there can still be a change in its velocity. The key here is that the direction of the velocity is changing, even though the magnitude of the velocity is not. Thus any object following a curved path, even when it is traveling at a constant speed, will be experiencing an acceleration. Any object that is experiencing a curving motion is traveling along the circumference of a circle.
Tight turns have a very short radius, whereas gradual turns have a longer radius.
A model airplane fastened to a long string held tightly by a child may follow such a path, as will a bicycle rider going around a turn, or the Moon as it orbits Earth. Centripetal motion is covered in more detail in the textbook under the discussion of centripetal force in Chapter 3. Because of the continuous pull of gravity on all objects located near Earth's surface, any object moving horizontally through the air will be pulled downward in a curved path. This results in a two-dimensional type of motion that is referred to as projectile motion . The path followed by objects experiencing this type of motion has been found to be a parabola .