Page 190 - Physics

Physics for Secondary Schools
(b) average velocity for the journey,
assuming that the journey is in a
straight line.
4. An object with an initial velocity
of 20 m/s moves due to north
at an acceleration of 8.0 m/s 2
for 5 seconds. What is the total
displacement during that time?
Reaction time and stopping distance
In the realm of linear motion,
understanding the concept of reaction
time and its impact on stopping distance
is crucial for safety and effi cient
transportation.
Reaction time
Reaction time refers to the time it takes
for an individual to perceive a stimulus,
such as an obstacle in the road, and then
initiate a response, typically applying the
brakes in the context of driving. During
this reaction time, an object in motion
continues to move, and this motion is
described by the principles of inertia and
linear momentum. Reaction time depends
on, the person making the decision and
the situations/surroundings in which the
reaction time is taking place.
Stopping distance
The stopping distance of a moving
vehicle is directly infl uenced by the
reaction time of the driver. To delve into
the physics behind this relationship, we
can apply the equations of linear motion.
The stopping distance (d) of a vehicle is
determined by two primary factors: the
initial velocity (v) of the vehicle and the
184
Physics Form 1 Final.indd 184
stopping capacity/braking capacity of the
vehicle, that is, how fast the vehicle can
decelerate.
Block
Stopping Thinking distance
distance
Figure 8.25
The relationship can be expressed as:
s  ut  1 at 2
2
Where: s is the stopping distance, v is
the initial velocity of the vehicle, t is the
driver’s reaction time, a is the deceleration
or braking force applied to the vehicle.
The fi rst term, ut , represents the distance
travelled by the vehicle during the driver’s
reaction time, while the second term,
1 2
2 at , represents the distance travelled
during the actual braking process, where
a is negative to signify deceleration.
Understanding this physics equation
emphasizes the critical importance of
reducing reaction time through increased
alertness and attention while driving,
as well as the signifi cance of having
effi cient braking systems to minimize
stopping distances. Ultimately, grasping
the interplay between reaction time and
stopping distance is fundamental in the
realm of linear motion, promoting safer
and more effective transportation.
Deriving formula for stopping distance
At the stopping point, the vehicle has
applied the brakes, and now it has to
Student’s Book Form One
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