# Motion Class 9 Notes Science

# Chapter 8

**Facts that Matter**

An object is said to be in motion when its position changes with time.

We describe the location of an object by specifying a reference point. Motion is relative. The total path covered by an object is said to be the distance travelled by it.

The shortest path/distance measured from the initial to the final position of an object is known as the displacement.

**Uniform motion:** When an object covers equal distances in equal intervals of time, it is said to be in uniform motion.

**Non-uniform motion:** Motions where objects cover unequal distances in equal intervals of time.

**Speed:** The distance travelled by an object in unit time is referred to as speed. Its unit is m/s.

**Average speed:** For non-uniform motion, the average speed of an object is obtained by dividing the total distance travelled by an object by the total time taken.

**Velocity:** Velocity is the speed of an object moving indefinite direction. S.I. unit is m/s.

**Acceleration:** Change in the velocity of an object per unit time.

**Graphical representation of motions**

** (i) Distance-time graph**

For a distance-time graph, time is taken on x-axis and distance is taken on the y-axis.

[**Note:** All independent quantities are taken along the x-axis and dependent quantities are taken along the y-axis.]

**(ii) Velocity-time graph**

**Equation of motion by graphical methods**

** (i)velocity-time relation:**

**(ii) The equation for position-time relation:**

**(iii) Equation for position-velocity relation:**

**Uniform circular motion:** When a body moves in a circular path with uniform speed, its . motion is called uniform circular motion.

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