Page 84 - Understanding NCERT Science 09

7.1.1 MOTION ALONG A STRAIGHT LINE = 60 km + 25 km = 85 km while the magnitude
of displacement = 35 km. Thus, the magnitude
The simplest type of motion is the motion of displacement (35 km) is not equal to the path
along a straight line. We shall first learn to length (85 km). Further, we will notice that the
describe this by an example. Consider the magnitude of the displacement for a course of
motion of an object moving along a straight motion may be zero but the corresponding
path. The object starts its journey from O distance covered is not zero. If we consider the
which is treated as its reference point object to travel back to O, the final position
(Fig. 7.1). Let A, B and C represent the position concides with the initial position, and therefore,
of the object at different instants. At first, the the displacement is zero. However, the distance
object moves through C and B and reaches A. covered in this journey is OA + AO = 60 km +
Then it moves back along the same path and 60 km = 120 km. Thus, two different physical
reaches C through B. quantities—the distance and the displacement,

Fig. 7.1: Positions of an object on a straight line path

The total path length covered by the object are used to describe the overall motion of an
is OA + AC, that is 60 km + 35 km = 95 km. object and to locate its final position with
This is the distance covered by the object. To reference to its initial position at a given time.
describe distance we need to specify only the
numerical value and not the direction of Activity ______________ 7.3
motion. There are certain quantities which
• Take a metre scale and a long rope.
are described by specifying only their • Walk from one corner of a basket-ball
numerical values. The numerical value of a court to its oppposite corner along its
physical quantity is its magnitude. From this sides.
example, can you find out the distance of the • Measure the distance covered by you
final position C of the object from the initial and magnitude of the displacement.
position O? This difference will give you the • What difference would you notice
numerical value of the displacement of the between the two in this case?
object from O to C through A. The shortest Activity ______________ 7.4
distance measured from the initial to the final
position of an object is known as • Automobiles are fitted with a device
the displacement. that shows the distance travelled. Such
Can the magnitude of the displacement be a device is known as an odometer. A
equal to the distance travelled by an object? car is driven from Bhubaneshwar to
New Delhi. The difference between the
Consider the example given in (Fig. 7.1). For
final reading and the initial reading of
motion of the object from O to A, the distance
the odometer is 1850 km.
covered is 60 km and the magnitude of • Find the magnitude of the displacement
displacement is also 60 km. During its motion between Bhubaneshwar and New Delhi
from O to A and back to B, the distance covered by using the Road Map of India.

MOTION 73

Rationalised 2023-24