Acceleration is often understood simply as “speeding up,” but the physics definition is far broader and encompasses three distinct scenarios. Any change to an object’s state of motion is defined as acceleration. Because motion is described by a quantity that includes both a rate and a path, acceleration is possible even when an object is moving at a steady pace.
What Acceleration Means
The concept of acceleration relies on understanding velocity, which is more than just speed. Speed is a scalar quantity, meaning it only has a magnitude (e.g., 50 kilometers per hour). Velocity is a vector quantity, possessing both a magnitude (speed) and a specific direction (e.g., 50 kilometers per hour north).
Acceleration is formally defined as the rate at which an object’s velocity changes over time. Because velocity is a vector, any alteration to its magnitude (speed), its direction, or both simultaneously, counts as an acceleration. This distinction explains why an object can accelerate in three different ways, even if its speed remains constant.
Acceleration By Changing Speed Only
The most intuitive type of acceleration involves changing an object’s speed while maintaining a straight path. This occurs when a net force is applied parallel to the direction of motion, causing the velocity vector to grow or shrink. For example, if a car travels in a straight line and the driver presses the accelerator, the car speeds up, and the acceleration vector points in the same direction as the velocity.
Slowing down, often called deceleration, is also a form of acceleration. When braking, the acceleration vector points opposite to the car’s movement, reducing the magnitude of the velocity vector. In this scenario, the direction of travel remains constant, with only the numerical value of the speed changing.
Acceleration By Changing Direction Only
An object can experience continuous acceleration even if its speed remains constant, provided it follows a curved path. Although the object is not speeding up or slowing down, its velocity vector is constantly being reoriented. Examples include a car making a turn or a satellite orbiting the Earth at a steady pace.
This change in direction requires a force that pulls the object inward, perpendicular to the direction of its motion, resulting in centripetal acceleration. For circular motion, this acceleration is always directed toward the center, continuously bending the object’s path. This force alters the object’s direction without changing the magnitude of its velocity.
Acceleration By Changing Both Speed and Direction
The third way an object accelerates is by changing both its speed and its direction simultaneously. This motion combines the two previous cases, involving a net force that is neither perfectly parallel nor perfectly perpendicular to the object’s velocity. The resulting acceleration vector has components that act both to change the object’s speed and to alter its path.
A jet plane executing a climbing turn is a clear example of this combined acceleration. As the pilot increases thrust and banks the wings, the aircraft gains speed and alters its direction of travel. Similarly, a skier navigating a winding downhill slope while picking up speed experiences this complex form of acceleration.