Question

4. Define acceleration and give its SI unit. When is acceleration of a body negative? Give two examples of

situations in which acceleration of the body is negative.

Answer 1

In physics, acceleration is the rate of change of velocity of an object with respect to time. 

m/s^2 (meter per sec square )

An example of "negative acceleration" would be a car braking, or decelerating. If the car is moving in a "positive" direction, and the brakes are applied, the acceleration is pointed in the opposite, or "negative" direction. However, you won't often see a physicist writing acceleration as a negative value (-2m/s^2).
In that case friction acts on the hockey puck in the direction opposite its motion, and so causes a 'negative' acceleration. Acceleration is the change of velocity in a unit time. It is denoted by 'a'. Acceleration can be positive (+) , negative (-) or can be zero (0).

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Answer 2

Acceleration is defined as the rate of increase or decrease in velocity in unit time. SI unit of acceleration is m/s^2.

Negative acceleration is called retardation or declaration. Situations when acceleration is negative are when the velocity decreases.Example, When a car comes to rest and during free fall.