### Theory:

Consider that you are stopped at the signal due to traffic, and once you see the green signal, suddenly you will increase the speed of your bike to move quick. This phenomena is called acceleration.

Acceleration ($$a$$):
An acceleration is the rate of change in velocity. In other words, if a body changes its speed or direction, then it is said to be accelerated.
$\mathit{Acceleration}\phantom{\rule{0.147em}{0ex}}\left(a\right)=\frac{\mathit{Change}\phantom{\rule{0.147em}{0ex}}\mathit{in}\phantom{\rule{0.147em}{0ex}}\mathit{velocity}}{\mathit{Time}\phantom{\rule{0.147em}{0ex}}\left(t\right)}=\frac{v-u}{t}$
The change in velocity can be written as $\mathit{Final}\phantom{\rule{0.147em}{0ex}}\mathit{velocity}\phantom{\rule{0.147em}{0ex}}\left(v\right)\phantom{\rule{0.147em}{0ex}}-\mathit{Initial}\phantom{\rule{0.147em}{0ex}}\mathit{velocity}\phantom{\rule{0.147em}{0ex}}\left(u\right)$.

Where $$v$$ is the final velocity of an object and $$u$$ is the initial velocity of an object.

The SI unit of acceleration is $m/{s}^{2}$.

Types of acceleration:
1. Positive acceleration
2. Negative acceleration
Positive acceleration:
If the velocity of an object increases with respect to time, then the object is said to be in positive acceleration or just acceleration.
Example:
A bike moving with increasing speed has a positive acceleration
A vehicle moves fastly following the green signal.
Negative acceleration:
If the velocity of an object decreases with respect to time, then the object is said to be in negative acceleration or deceleration or retardation.
Example:
A running car slowly stops after applying the brake
A rolling ball that slowly stops