Page 88 - Understanding NCERT Science 09

–1
To answer such a question, we have to attain a velocity of 6 m s in 30 s. Then
introduce another physical quantity called he applies brakes such that the velocity
acceleration, which is a measure of the of the bicycle comes down to 4 m s in
-1
change in the velocity of an object per unit the next 5 s. Calculate the acceleration
time. That is, of the bicycle in both the cases.

change in velocity Solution:
acceleration =
time taken In the first case:
If the velocity of an object changes from initial velocity, u = 0 ;
–1
an initial value u to the final value v in time t, final velocity, v = 6 m s ;
the acceleration a is, time, t = 30 s .
From Eq. (8.3), we have
v – u
a = (7.3) v – u
t a =
t
This kind of motion is known as
accelerated motion. The acceleration is taken Substituting the given values of u,v and
t in the above equation, we get
to be positive if it is in the direction of velocity
and negative when it is opposite to the ( 6m s –1 – 0m s –1 )
direction of velocity. The SI unit of a =
acceleration is m s . 30 s
–2
If an object travels in a straight line and
= 0.2 m s –2
its velocity increases or decreases by equal
amounts in equal intervals of time, then the In the second case: –1
acceleration of the object is said to be initial velocity, u = 6 m s ;
–1
final velocity, v = 4 m s ;
uniform. The motion of a freely falling body
time, t = 5 s.
is an example of uniformly accelerated
motion. On the other hand, an object can ( 4m s –1 – 6m s –1 )
travel with non-uniform acceleration if its Then, a =
velocity changes at a non-uniform rate. For 5 s
example, if a car travelling along a straight
–2
= –0.4 m s .
road increases its speed by unequal amounts
The acceleration of the bicycle in the
in equal intervals of time, then the car is said first case is 0.2 m s and in the second
–2
to be moving with non-uniform acceleration.
–2
case, it is –0.4 m s .
Activity ______________ 7.8
• In your everyday life you come across uestions
a range of motions in which
(a) acceleration is in the direction of 1. When will you say a body is in
motion, Q (i) uniform acceleration? (ii) non-
(b) acceleration is against the uniform acceleration?
direction of motion, 2. A bus decreases its speed from
(c) acceleration is uniform, 80 km h –1 to 60 km h –1 in 5 s.
(d) acceleration is non-uniform. Find the acceleration of the bus.
• Can you identify one example each
3. A train starting from a railway
for the above type of motion?
station and moving with uniform
acceleration attains a speed
–1
Example 7.4 Starting from a stationary 40 km h in 10 minutes. Find its
position, Rahul paddles his bicycle to acceleration.

MOTION 77

Rationalised 2023-24