The rate of change in velocity over time is acceleration. An accelerating object is either speeding up, slowing down, or changing direction. Acceleration is a vector quanity that is both a magnitude and a direction.Acceleration describes both the magnitude of an object’s change in velocity, and the direction in which it is accelerating. Acceleration can thus involve changes of speed, changes of direction, or both. As acceleration is a rate of change of velocity over time and velocity is measured in meters per second (m/s), the units of measurement of acceleration are meters per second per second (m/s2).Objects do not speed up, slow down, or change direction unless they are pushed in some way. Newton's second law sums up this idea, stating that the acceleration of an object results from the application of a force. The acceleration (a) of an object with mass (m) produced by a given force (F) may be calculated using the equation F = ma. A larger force produces a greater acceleration; a larger mass results in a smaller acceleration given the same force.

Change in Speed

A car that starts at a standstill and then increases its speed along a straight road is subject to an acceleration. That acceleration is due to the application of a force originating in its engine. A car that reduces its speed, by application of a force generated by its brakes for example, is also subject to an acceleration. In such situations, where acceleration is in a direction opposite to velocity, the acceleration is often called deceleration. A constant acceleration (a) over a given time interval (?t), results in a change in velocity (?v) that can be calculated using the equation ?v = a?t m/s (the ? symbol is often used in physics equations to indicate a change in the quantity that follows it.)The force of gravity near Earth’s surface results in a very familiar form of straight-line acceleration. The strength of Earth’s gravitational field near the surface (g) is an acceleration equal to 9.8 m/s2. So every second that an object falls, its speed increases by 9.8 m/s. A ball dropped from a rooftop, for example, would start with 0 velocity. It would have a velocity of 9.8 m/s one second after it was dropped. After two seconds, it would be moving 2(9.8) = 19.6 m/s.

Change in Direction

Acceleration can also involve a change in the direction an object is moving. A ball on the end of a string being whirled overhead at a constant speed is an example of this type of acceleration. Since velocity is a vector quantity like acceleration, velocity has a speed component (magnitude) and a direction component. At every instant in its motion overhead, the ball’s velocity is changing because the velocity’s direction is different at every point on the circular path. Changing velocity is acceleration. The acceleration of the object is directed toward the center of the circle, and is of constant magnitude a=v2/r, where r is the radius of the circle and v is the speed of the object (with mass m). This type of acceleration is called radial or centripetal acceleration. Radial acceleration results from the action of the force generated by the string that pulls the ball toward the center of the circle. In the case of a satellite in orbit, the force causing the radial acceleration is Earth’s gravity pulling the satellite toward the center of the planet.