What is dynamics? What is mechanics?

$A$ particle is moving with a constant speed along a straight line path. $A$ force is not required to

$A$ trolley of mass $300 \; kg$ carrying a sandbag of $25 \; kg$ is moving uniformly with a speed of $27 \; km/h$ on a frictionless track. After a while,sand starts leaking out of a hole on the floor of the trolley at the rate of $0.05 \; kg \cdot s^{-1}$. What is the speed (in $km/h$) of the trolley after the entire sandbag is empty?

"The uniform motion is possible when no frictional forces oppose" is the concept of

$Assertion$ : The driver in a vehicle moving with a constant speed on a straight road is an inertial frame of reference.
$Reason$ : $A$ reference frame in which Newton's laws of motion are applicable is non-inertial.

$A$ girl jumps down from a moving bus,along the direction of motion of the bus,tilting slightly forward. She falls on $(a)$ a sheet of ice $(b)$ a patch of glue. Which of the following statements is correct regarding her fall?