Chapter 7: Motion
In everyday life, we see some objects at rest and others in motion. Birds fly, fish swim, blood flows through veins and arteries, and cars move. Atoms, molecules, planets, stars and galaxies are all in motion.
Discuss whether the walls of your classroom are at rest or in motion.
7.1 Describing Motion
We describe the location of an object by specifying a reference point. For example, to describe the position of a school in a village, we might say it is 2 km north of the railway station. Here, the railway station is the reference point or origin.
7.1.1 Motion Along a Straight Line
The simplest type of motion is the motion along a straight line. The shortest distance measured from the initial to the final position of an object is known as the displacement.
Automobiles are fitted with a device that shows the distance travelled. Such a device is known as an odometer.
7.1.2 Uniform and Non-Uniform Motion
If an object covers equal distances in equal intervals of time, it is said to be in uniform motion. If not, it is non-uniform motion (e.g., a car in traffic).
- An object has moved through a distance. Can it have zero displacement? If yes, support your answer with an example.
- A farmer moves along the boundary of a square field of side 10 m in 40 s. What will be the magnitude of displacement at the end of 2min 20s?
7.2 Measuring the Rate of Motion
The SI unit of speed is metre per second (m sā»Ā¹). Velocity is the speed of an object moving in a definite direction.
| Time | Distance (Obj A) | Distance (Obj B) |
|---|---|---|
| 9:30 am | 10 m | 12 m |
| 9:45 am | 20 m | 19 m |
| 10:00 am | 30 m | 23 m |
7.3 Rate of Change of Velocity
Acceleration is a measure of the change in the velocity of an object per unit time.