Question

What do you mean by retardation? What is its SI unit?

Answer 1

Retardation is merely a negative acceleration. The velocity of the body will either rise or decrease. The change in velocity is referred to as acceleration. If the velocity of the body rises, acceleration is considered to be positive. Similarly, if the speed declines, the acceleration is said to be negative.
The train slows down as it approaches the station; thus, we can assume that the train is slowing down in this circumstance. The negative acceleration value shows that the velocity of a body is decreasing. Here we are going to learn about retardation and its SI unit briefly.

Complete step by step solution:
Retardation:
A moving body's rate of change in velocity with time might be either positive or negative. If the rate of change of velocity with time is positive, it is referred to as acceleration; if it is negative, it is referred to as retardation.
When the acceleration acts in the opposite direction of the velocity, this is referred to as retardation. It is an abbreviation for negative acceleration. Because acceleration is a vector quantity, it is determined by the coordinate system or reference frame used to describe motion. If it is positive, it is referred to as acceleration; if it is negative, it is referred to as retardation.
For example, when you apply a brake to your cycle while riding to school/college, the effect on tires is known as retardation.
That is why negative acceleration is referred to as retardation.
Retardation is defined as the amount by which a body's velocity varies in one second. The unit and dimension of retardation are identical to those of acceleration. If a body's velocity decreases, its acceleration decreases as well. Negative acceleration is negative, yet there is no decrease or increase in speed; in the event of free fall, the acceleration is constant. Negative acceleration is sometimes referred to as retardation or slowdown.
SI unit of retardation:
Because it is negative acceleration, the SI unit of retardation is the same as the unit of acceleration. Meter/$second^2$ (m/$s^2$) is the SI unit of retardation.

Note:
When a train arrives at a station, it is said to be retarded since its velocity reduces. Acceleration with a negative sign is referred to as retardation. A negative acceleration value indicates that a body's velocity is decreasing.
A body's velocity can raise or decrease. When the velocity of a body changes, it accelerates. So, when a body's velocity increases, the acceleration is said to be positive, and when a body's velocity decreases, the acceleration is said to be negative (Retardation). If a body's velocity decreases, it is said to be retarded.