In which of the following examples of motion, can the body be considered approximately a point object:
(d) a tumbling beaker that has slipped off the edge of a table.
The size of a beaker is comparable to the height of the table from which it slipped. Hence, the beaker cannot be considered as a point object.
What is the most essential thing to study the motion of a body?
The most essential thing to study the motion is reference frame. Rest and motion are relative terms. It means that an object in one situation can be in rest but in another situation, the same object will be in motion.