1. Chapter 11: Motion and Speed
Section 3: Acceleration

2. Acceleration
As we have discussed so far, speed describes the distance something travels in a certain amount of time.
But speed can change.
The rate at which the velocity of an object changes is called acceleration.
Acceleration doesn’t always mean than an object increases in speed. It can also mean a decrease in speed and/or a change in direction.

3. Positive and Negative Acceleration
When an object speeds up over a given time period, this is known as a positive acceleration. A car that starts at rest and increases in speed to 60 mph over a short amount of time is an example of a positive acceleration.
When an object slows down over a period of time, it is known as a negative acceleration. If a car slows down to a stop at a traffic signal, it has undergone a negative acceleration as the speed decreases each second.

4. Direction change - acceleration
Since acceleration deals with velocity, a direction change will cause a change in acceleration.
So, even if you don’t speed up or slow down, you can still accelerate by turning as velocity is changed.

5. Some terms with acceleration
Constant acceleration – This is a term that often confuses students. As constant means unchanging, a constant acceleration is where the acceleration continues to increase or decrease. A rocket or a plane at take off undergo a constant acceleration until they reach a normal cruising speed.
No acceleration – Acceleration is unchanged. This is where a steady speed is maintained. Cruise control on a car attempts to keep your speed the same (no acceleration).

6. Calculating acceleration
Any type of straight line motion can be calculated using the following formula
a = vf - vi t or
acceleration = (final velocity – initial velocity) / time

7. Units for acceleration
Since acceleration is the change in velocity (m/s) divided by time (s), the SI unit for acceleration is m/s2 or m/s/s. The seconds squared shows that velocity changes per second.
Acceleration, like velocity can also be written in other units too. You can measure your acceleration on a car’s speedometer in miles/hour/second for example.

8. Speed-time graphs
Speed-time graphs show acceleration unlike the distance-time graphs which show only speed.