Page 94 - Understanding NCERT Science 09
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4. A racing car has a uniform straight parts AB, BC, CD and DA of the track.
acceleration of 4 m s . What In order to keep himself on track, he quickly
-2
distance will it cover in 10 s after changes his speed at the corners. How many
start? times will the athlete have to change his
5. A stone is thrown in a vertically direction of motion, while he completes one
upward direction with a velocity
-1
of 5 m s . If the acceleration of round? It is clear that to move in a rectangular
the stone during its motion is 10 track once, he has to change his direction of
–2
m s in the downward direction, motion four times.
what will be the height attained Now, suppose instead of a rectangular
by the stone and how much time track, the athlete is running along a
will it take to reach there? hexagonal shaped path ABCDEF, as shown
in Fig. 7.8(b). In this situation, the athlete will
7.6 Uniform Circular Motion have to change his direction six times while
he completes one round. What if the track
When the velocity of an object changes, we say was not a hexagon but a regular octagon,
that the object is accelerating. The change in
with eight equal sides as shown by
the velocity could be due to change in its ABCDEFGH in Fig. 7.8(c)? It is observed that
magnitude or the direction of the motion or as the number of sides of the track increases
both. Can you think of an example when an
the athelete has to take turns more and more
object does not change its magnitude of
often. What would happen to the shape of the
velocity but only its direction of motion?
track as we go on increasing the number of
sides indefinitely? If you do this you will
notice that the shape of the track approaches
the shape of a circle and the length of each of
the sides will decrease to a point. If the athlete
moves with a velocity of constant magnitude
(a) Rectangular track (b) Hexagonal track along the circular path, the only change in
his velocity is due to the change in the
direction of motion. The motion of the athlete
moving along a circular path is, therefore, an
example of an accelerated motion.
We know that the circumference of a circle
of radius r is given by p2 r . If the athlete takes
t seconds to go once around the circular path
(c) Octagonal shaped track (d) A circular track of radius r, the speed v is given by
Fig. 7.8: The motion of an athlete along closed tracks 2 pr
v =
of different shapes.
t
Let us consider an example of the motion
(7.8)
of a body along a closed path. Fig 8.9 (a)
shows the path of an athlete along a When an object moves in a circular path
rectangular track ABCD. Let us assume that with uniform speed, its motion is called
the athlete runs at a uniform speed on the uniform circular motion.
MOTION 83
Rationalised 2023-24
