The velocity of a body depends on time according to the equation $v = 20 + 0.1t^2$. The body is undergoing

Define acceleration,average acceleration,and instantaneous acceleration.

The distance-time graph of a particle at time $t$ makes an angle of $45^{\circ}$ with the time axis. After one second,it makes an angle of $60^{\circ}$ with the time axis. What is the average acceleration of the particle?

$A$ body in one-dimensional motion has zero speed at an instant. At that instant,it must have:

$Assertion$ : Retardation is directly opposite to the velocity.
$Reason$ : Retardation is equal to the time rate of decrease of speed.

Suggest a suitable physical situation for the following $a-t$ graph.