Page 86 - Understanding NCERT Science 09
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Look at the situations given in Fig. 7.2. If
Total distance travelled
–1
the bowling speed is 143 km h in Fig. 7.2(a) Average speed =
what does it mean? What do you understand Total time taken
from the signboard in Fig. 7.2(b)? 32 m
Different objects may take different = = 5.33 m s –1
6 s
amounts of time to cover a given distance.
Some of them move fast and some move Therefore, the average speed of the object
–1
is 5.33 m s .
slowly. The rate at which objects move can
be different. Also, different objects can move
at the same rate. One of the ways of 7.2.1 SPEED WITH DIRECTION
measuring the rate of motion of an object is
to find out the distance travelled by the object The rate of motion of an object can be more
in unit time. This quantity is referred to as comprehensive if we specify its direction of
speed. The SI unit of speed is metre per motion along with its speed. The quantity that
second. This is represented by the symbol specifies both these aspects is called velocity.
–1
m s or m/s.The other units of speed include Velocity is the speed of an object moving in a
centimetre per second (cm s ) and kilometre definite direction. The velocity of an object
–1
–1
per hour (km h ). To specify the speed of an can be uniform or variable. It can be changed
object, we require only its magnitude. The by changing the object’s speed, direction of
speed of an object need not be constant. In motion or both. When an object is moving
most cases, objects will be in non-uniform along a straight line at a variable speed, we
motion. Therefore, we describe the rate of can express the magnitude of its rate of
motion of such objects in terms of their motion in terms of average velocity. It is
average speed. The average speed of an object calculated in the same way as we calculate
is obtained by dividing the total distance average speed.
travelled by the total time taken. That is, In case the velocity of the object is
Total distance travelled changing at a uniform rate, then average
average speed =
Total time taken velocity is given by the arithmetic mean of
initial velocity and final velocity for a given
If an object travels a distance s in time t then
its speed v is, period of time. That is,
initial velocity + final velocity
s average velocity =
v = (7.1) 2
t
Let us understand this by an example. A
u + v
car travels a distance of 100 km in 2 h. Its Mathematically, v = (7.2)
av
–1
average speed is 50 km h . The car might 2
not have travelled at 50 km h all the time. where v is the average velocity, u is the initial
–1
av
Sometimes it might have travelled faster and velocity and v is the final velocity of the object.
sometimes slower than this. Speed and velocity have the same units,
that is, m s or m/s.
–1
Example 7.1 An object travels 16 m in 4 s Activity ______________ 7.6
and then another 16 m in 2 s. What is
the average speed of the object?
• Measure the time it takes you to walk
Solution: from your house to your bus stop or
the school. If you consider that your
Total distance travelled by the object = average walking speed is 4 km h ,
–1
16 m + 16 m = 32 m estimate the distance of the bus stop
Total time taken = 4 s + 2 s = 6 s or school from your house.
MOTION 75
Rationalised 2023-24
