Newton's First Law: Mass and Inertia Questions in English

(C) An inertial frame of reference is a frame in which Newton's first law of motion holds true. This means an object in this frame remains at rest or moves with a constant velocity if no net external force acts on it.
$1$. $A$ pilot in an aeroplane taking off is accelerating,so it is a non-inertial frame.
$2$. $A$ child on a merry-go-round is undergoing circular motion,which involves centripetal acceleration,making it a non-inertial frame.
$3$. $A$ driver in a bus moving with constant velocity has zero acceleration,making it an inertial frame.
$4$. $A$ man in a train slowing down is undergoing deceleration (negative acceleration),making it a non-inertial frame.
Therefore,the correct option is $C$.

(D) According to Galileo's law of inertia,an object in motion will continue to move with a constant velocity in a straight line unless acted upon by an external force. He concluded that uniform motion is possible when no frictional forces oppose the motion of the object.

(D) According to Newton's $1^{st}$ Law of Motion,an object in motion will continue to move with a constant velocity in a straight line unless acted upon by an external unbalanced force.
Since the object is already moving with a constant speed along a straight-line path,its velocity is uniform.
Therefore,no external force is required to maintain this state of uniform motion.
Options $A$,$B$,and $C$ all involve changing the state of motion (acceleration),which requires an external force.

(B) When a bus takes a sudden turn,the passengers experience a force that pushes them outwards. This phenomenon occurs due to the property of inertia,specifically the inertia of direction. According to Newton's First Law of Motion,an object continues to move in its state of uniform motion in a straight line unless acted upon by an external force. When the bus turns,the lower part of the passenger's body moves with the bus,but the upper part tends to maintain its original direction of motion due to inertia,causing the passenger to lean or be thrown outwards relative to the bus.

(D) When the boy is travelling in an open car moving at a constant velocity,both the boy and the ball possess the same horizontal velocity as the car due to the law of inertia.
When the ball is thrown vertically upward,its horizontal velocity remains unchanged because there is no horizontal force acting on it (ignoring air resistance).
Since the car continues to move at the same constant velocity,the horizontal distance covered by the ball and the car in the same time interval is identical.
Therefore,the ball will fall exactly back into the boy's hand.