Motion in a straight line

Reaction time : When a
situation demands our immediate
action, it takes some time before we
really respond. Reaction time is the
time a person takes to observe, think
and act. For example, if a person is
driving and suddenly a boy appears on
the road, then the time elapsed before
he slams the brakes of the car is the
reaction time. Reaction time depends
on complexity of the situation and on
an individual.
You can measure your reaction
time by a simple experiment. Take a
ruler and ask your friend to drop it
vertically through the gap between
your thumb and forefinger (Fig. 2.8).
After you catch it, find the distance d
travelled by the ruler. In a particular
case, d was found to be 21.0 cm.
Estimate reaction time.