We describe the location of an object by specifying a reference point.
Let us assume that a school in a village is `2` km north of the railway station. We have specified the position of the school with respect to the railway station.
In this example, the railway station is the reference point. We could have also chosen other reference points according to our convenience. Therefore, to describe the position of an object we need to specify a reference point called the origin.
`ul"MOTION ALONG A STRAIGHT LINE"`
The simplest type of motion is the motion along a straight line. We shall first learn to describe this by an example.
Consider the motion of an object moving along a straight path. The object starts its journey from O which is treated as its reference point (Fig. 8.1).
Let A, B and C represent the position of the object at different instants. At first, the object moves through C and B and reaches A. Then it moves back along the same path and reaches C through B.
The total path length covered by the object is `OA + AC`, that is ` 60 km + 35 km = 95 km`. This is the distance covered by the object.
To describe distance we need to specify only the numerical value and not the direction of motion. There are certain quantities which are described by specifying only their numerical values.
The numerical value of a physical quantity is its magnitude. From this example, can you find out the distance of the final position C of the object from the initial position O?
This difference will give you the numerical value of the displacement of the object from O to C through A. The shortest distance measured from the initial to the final position of an object is known as the displacement.
Can the magnitude of the displacement be equal to the distance travelled by an object? Consider the example given in (Fig. 8.1).
For motion of the object from O to A, the distance covered is `60 km` and the magnitude of displacement is also `60 km`.
During its motion from O to A and back to B, the distance covered `= 60 km + 25 km = 85 km` displacement, are used to describe the overall motion of an object and to locate its final position with reference to its initial position at a given time.
Activity ______________ `8.3`
♦ Take a metre scale and a long rope.
♦ Walk from one corner of a basket-ball court to its oppposite corner along its sides.
♦ Measure the distance covered by you and magnitude of the displacement.
♦ What difference would you notice between the two in this case?
Activity ______________ `8.4`
♦ Automobiles are fitted with a device that shows the distance travelled. Such a device is known as an odometer. A car is driven from Bhubaneshwar to New Delhi. The difference between the final reading and the initial reading of the odometer is `1850 km`.
♦Find the magnitude of the displacement between Bhubaneshwar and New Delhi by using the Road Map of India.
`ul"UNIFORM MOTION AND NONUNIFORM MOTION"`
Consider an object moving along a straight line. Let it travel `50 km` in the first hour, `50 km` more in the second hour, `50 km` in the third hour and `50 km` in the fourth hour. In this case, the object covers `50 km` in each hour.
As the object covers equal distances in equal intervals of time, it is said to be in uniform motion.
The time interval in this motion may be small or big. In our day-to-day life, we come across motions where objects cover unequal distances in equal intervals of time, for example, when a car is moving on a crowded street or a person is jogging in a park.
These are some instances of non-uniform motion.
Activity ______________ `8.5`
♦ The data regarding the motion of two different objects A and B are given in Table 8.1.
♦ Examine them carefully and state whether the motion of the objects is uniform or non-uniform.