Acceleration: speeding up

Acceleration and Deceleration

Acceleration is another way to describe movement. It is the way that an object's velocity changes over time. Like velocity and displacement, acceleration is concerned with both the direction and time the travel takes.

When an object goes from being at rest to being in motion, it accelerates to a certain speed. If the object stays at constant speed, it is not accelerating. If the object changes direction, it is also accelerating, only in a different direction. If an object slows down or stops, it is accelerating. This type of acceleration is referred to as negative acceleration or deceleration. It is called negative acceleration because it is only negative compared to the direction of the original motion. If acceleration in the same direction were applied to a stationary object, the object would start moving in the other direction.

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Acceleration is determined using the following formula:

Acceleration is generally measured in metres per second per second or m/s².

Like speed and velocity, there can be instantaneous and average acceleration. An object could speed up and slow down many times over a period of time, but its average acceleration would not show that. Let's say we have a person travelling to school. Over that time, she has to stop at road crossings and has to run to avoid being late. Her average acceleration for the first half of her trip would be an average of all those accelerations, negative and positive.

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Also like speed, if average velocity is measured over smaller units of time, a magnitude closer to the instantaneous acceleration can be determined. If we were to only measure our school traveller's acceleration over one block, for example, or half a block, we would get a number much closer to her instantaneous acceleration.

Velocity-time and speed-time graphs

A velocity-time graph is a graph that shows how the velocity of an object changes over time. It can be useful in calculating the acceleration of an object.

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In this graph, an object positively accelerates, then stays at a constant speed. Acceleration can be found by determining the slope of the line. The distance travelled can be determined by finding the area of the space beneath the line.