(A) What is the retardation? Write its formula. Give an example of where it acts on the body.
(B) A train starting from rest attained a velocity of ${30m}/{\sec }\;$ in $30$ minutes. What is its acceleration?
(C) What is the distance travelled during this time?

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(A) Hint: Give the definition of retardation and explain the difference between acceleration and retardation.

Complete Step by Step Solution
When final velocity of the body is less than initial velocity, then the body is said to be under retardation.
Retardation can also be named as negative acceleration or deceleration.
We know that
$\text{v = u + at}$
On measuring, we get
$\text{a = }\dfrac{\text{v}-\text{u}}{\text{t}}$
$\text{v =}$Final velocity
$\text{u =}$Initial velocity
As, $\text{v u}$
$\therefore \text{a = negative =}$Retardation
Example: When we apply brakes to the vehicle, the velocity of the vehicle decreases (Final velocity becomes less than initial velocity). This means that retardation has taken place.

Note: The student should clearly know the difference between retardation and acceleration. Retardation means reduction of velocity of body (Final velocity less than initial velocity). Acceleration means increase in velocity of body (Final velocity greater than initial velocity.