Acceleration is a measure of the rate at which the velocity of an object is changing. If you are riding in a car traveling in a straight line at a constant 50 kilometers per hour, you experience no acceleration because the car's velocity (rate of motion) is not changing. If the car begins to speed up, acceleration occurs because the car's velocity increases. If the car slows down, negative acceleration, or deceleration, occurs because the car's velocity decreases.
Our understanding of acceleration is due to the work of two great scientists, Italian physicist Galileo Galilei (1564–1642) and English physicist Isaac Newton (1642–1727). During the late sixteenth and early seventeenth centuries, Galileo first observed the motion of objects rolling down an inclined plane. He wrote mathematical equations that showed how the velocities of these objects increased as they rolled down the planes. These equations first described the idea of accelerated motion.
Some years later, Newton explained the observations made by Galileo. He said that the velocity of an object changes only when a force acts on that object. In the case of a ball rolling down a plane, that force is the force of gravity. Newton's discovery of the relationship between force and acceleration became one of the fundamental concepts in modern physics.
An object moving in a straight line is accelerated only if a force acts on it. For example, imagine a ball rolling across a smooth flat surface with a velocity of 5 meters per second. Then suppose someone hits the ball lightly with a bat. The additional force on the ball provided by the bat will cause the ball to move faster.
Suppose that the ball's new velocity is 10 meters per second and that it takes 2 seconds to accelerate from its original velocity (5 meters per second) to its new velocity (10 meters per second). The acceleration of the ball, then, is the change in velocity of 5 meters per second (10 meters per second minus 5 meters per second) divided by the time it takes to increase in velocity (2 seconds), or 5 meters per second divided by 2 seconds. The acceleration is 2.5 meters per second per second. The unit of measurement used for acceleration—meters per second per second—may sound strange, but it tells by how much the velocity (meters per second) changes in each unit of time (per second).
The acceleration of an object depends on two factors, velocity and direction. An object that moves with constant speed but that changes direction is also accelerating. A car traveling around a curve in the road is accelerating even though its speed remains constant.
Acceleration: The rate at which the velocity and/or direction of an object is changing with respect to time.
Circular acceleration: Acceleration in which the direction of motion is changing.
Force: A push or pull on an object that will accelerate an object.
Gravity: The special acceleration of 9.81 meters per second per second exerted by the attraction of the mass of Earth on nearby objects.
Linear acceleration: Acceleration in which the speed of an object is changing.
Velocity: The rate at which the position of an object changes with time, including both the speed and the direction.
Another example of circular acceleration is the motion of the Moon around Earth. The Moon travels at a constant speed in its orbit. But it also falls constantly towards Earth's surface. The force of Earth's gravity acts on the Moon not to change its speed but to change the direction in which it is traveling. Again, acceleration occurs when a force acts on an object.