Events and segments

An event is a significant instant of the motion: the moment a body starts, brakes, changes direction, or arrives at its destination. If the motion has $n$ events, then it has $n-1$ segments, that is, intervals in which the type of motion does not change.

Events and segments

Event = key instant of the motion Segment = interval between two events $n$ events $\Rightarrow$ $n-1$ segments

This distinction is the key to the working method of all kinematics: a complicated motion is broken down into simple segments, in each of which the acceleration is constant, and the equations of motion are applied to each segment.

It is useful to collect all the data in an event table, with one row per event and the four quantities in columns:

Event	$t$ (s)	$s$ (m)	$v$ (m/s)	$a$ (m/s²)
A	$t_A$	$s_A$	$v_A$
				$a_{AB}$
B	$t_B$	$s_B$	$v_B$

The acceleration $a_{AB}$ is written between events A and B, because it is constant over that segment: it belongs to the interval, not to a single instant. Each segment has its own acceleration, and in the table it occupies the intermediate row between the two events that bound it.

Links

Topics: Kinematics Concepts: Uniformly accelerated motion Skills: Choice of axes

Related exercises: Carlo’s journey · Stopping distance (reaction and braking) · Proof of the time-free equation