Acceleration is a change in motion . Acceleration is defined as the rate of change of an object’s velocity over time. In physics , the word velocity describes an object’s speed and its direction of motion. Moving objects have velocity, but they are not necessarily accelerating. An object only accelerates when its velocity changes—that is, when the object speeds up, slows down, or changes direction.
Velocity (v) is measured in units of distance (d) divided by time (t).
v = d/t
Acceleration (a) is measured in units of distance divided by time squared—that is, multiplied by itself.
a = d/t 2
Consider a racecar moving down a straight track, picking up speed at a steady rate. Every second, the racecar’s speed increases by 8 meters per second. Thus, the racecar’s acceleration is 8 meters per second per second, or 8 m/s2.
The direction of the acceleration, in this case, is in the same direction as the racecar’s velocity. If the driver hits the brakes and the racecar begins to slow down, the racecar is then decelerating, or accelerating in a direction opposite its velocity.
Acceleration and force.
A force —a push or a pull—causes an object to accelerate. The amount of acceleration depends on the amount of force. The acceleration also depends on the object’s mass (amount of matter). The more massive an object, the more force is required to accelerate it the same amount. For example, accelerating an automobile at a certain rate takes more force than accelerating a shopping cart at the same rate. The Second Law of Motion, an equation first described by the English scientist Isaac Newton in the 1600’s, relates force (F) to mass (m) and acceleration (a):
F = ma
Forces in opposite directions can cancel each other out. Only a net force—the total combination of forces acting on an object—can cause acceleration. The object accelerates in the same direction as the net force.
Acceleration and gravity.
When an object is dropped near the surface of Earth , it is pulled down by the force of gravity . Ignoring air resistance, which has only a small effect on the object’s motion, gravity causes the object’s speed to increase by around 9.8 meters per second each second. The acceleration due to gravity is thus 9.8 m/s2, directed toward Earth’s center.
The moon and other nearby objects in space also are subject to a gravitational force of attraction towards Earth’s center. This force pulls the moon into a nearly circular path called an orbit around Earth. While the moon orbits the planet at a fairly constant speed, its direction of motion is constantly changing, since it moves in a nearly circular ellipse . Thus, the moon is accelerating. This form of acceleration, in which an object moves at a steady speed while changing direction, is called centripetal acceleration.
