### Theory:

Consider that you're stooped at the signal due to traffic, and once you see the green signal, suddenly you're increasing the speed of your bike/car to move fastly, and this act of increasing the speed is called, acceleration.

Acceleration (a):
The acceleration can be defined as the rate of change in velocity.

When the object undergoes acceleration, its speed and/or direction change(s).

If an object 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 bus moving with increasing speed has a positive acceleration.
After the green signal, a vehicle moves fastly.
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.
A rolling ball that slowly stops.
Reference:
Image credits:
Car image: https://www.pxfuel.com/en/free-photo-xstno