Page 174 - Physics

Physics for Secondary Schools
Example 8.1
Halima runs twice round a fi eld track of
length 500 m.
(a) What distance does she cover?
(b) What is her displacement from the
starting point?
Solution
(a) Distance in actual ground covered;
Distance
(b) Displacement means how far she
is from the starting point.
Since she ends the race at the
starting point, the displacement is
zero metres (0 m).
Speed and velocity
Suppose you and your friend are walking
to school in the morning. It takes 45
minutes for you to get to school, but
it takes 1 hour and 10 minutes for your
friend to get there. If each of you travelled
the same distance, 1.5 km, it means you
travelled faster than your friend.
Speed is defi ned as the rate of change
in distance. Average speed is the rate
at which an object travels.
Speed can be determined as follows:
Distance (s)
Speed (v) =
Time (t)
s
v =
t Velocity is the rate of change of
So, in your case,
s = 1.5 km or 1 500 m; t = 45 minutes
or 2 700 s.
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Physics Form 1 Final.indd 168
Therefore, your speed is:
1500 m
v = 2 700 s
= 0.56 m/s
For the case of your friend,
t = 1 hr 10 min = 4 200 s. Therefore,
the average speed at which your friend
travelled is:
1500 m
v =
4 200 s
0.36 m/s=
Note: During your journey, you do not
maintain the same speed at all times.
That is why the term ‘average speed’
is normally used. So, if you walked to
school in 45 minutes, your average speed
was 0.56 m/s.
Speed may also be expressed in kilometres
per hour (km/hr) or in centimetres per
second (cm/s).
However, the term speed is usually
confused with the term velocity. These
terms do not mean the same thing. Velocity
is a measure of the speed in a given
direction. For example, the speed of a car
is 120 km per hour, but its velocity is 120
km per hour in a south-west direction. If a
car is moving at a constant speed around a
bend, its direction changes continuously.
Hence, its velocity will also be changing
even if the speed remains constant.
distance moved in a specified
direction (i.e., the rate of change of
displacement).
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16/10/2024 20:58