Assertion : A particle having negative acceleration will slow down.

Reason : Direction of the acceleration is dependent upon direction of the velocity.

Assertion : Magnitude of average velocity is equal to average speed.

Reason : Magnitude of instantaneous velocity is not equal to instantaneous speed.

Assertion : When velocity of a particle is zero then acceleration of particle must be zero at that instant.

Reason : Acceleration is equal to $a=\left(\frac{dv}{dx}\right),$ where $v$ is the velocity at that instant.

Assertion : A particle moves in a straight line with constant acceleration. The average velocity of this particle cannot be zero in any time interval.

Reason : For a particle moving in straight line with variable acceleration, the average velocity in a time interval is $\frac{u+v}{2},$ where $u$ and $v$ are initial and final velocity of the particle in the given time interval.

Assertion : Retardation is directly opposite to the velocity.

Reason : Retardation is equal to the time rate of decrease of speed.

Assertion : If separation between two particles does not change then their relative velocity, will be zero.

Reason : Relative velocity is the rate of change of velocity of one particle w.r.t. another.

Assertion : If speed of two particles is increasing at the same rate then their relative velocity will be constant.

Reason : If the acceleration vector of the two particles is same then their relative acceleration will be non-zero.