### Main points:-

A reference point or origin is required to explain the position of an object. To one observer, an object may appear to be moving while to another, it appears to be motionless.

For instance, a passenger on board a bus perceives the other passengers to be at rest, yet an observer outside the bus perceives the people to be moving.

A convention, or a standard reference point or frame, is required to make observations easier. The reference frames of all objects must be the same.

## Distance and Displacement

The magnitude of the length covered by a moving object is called distance. It has no direction.

Displacement is the shortest distance between two points or the distance between the starting and final positions with respect to time. It has magnitude as well direction.

Displacement can be zero, but distance cannot.

**Magnitude**

Magnitude is the size or extent of a physical quantity. In physics, we have scalar and vector quantities.

Scalar quantities are only expressed as magnitude. E.g: time, distance, mass, temperature, area, volume

Vector quantities are expressed in magnitude as well as the direction of the object. E.g: Velocity, displacement, weight, momentum, force, acceleration, etc.

## Time, Average Speed and Velocity

**Time and speed**

Time is the duration of an event that is expressed in seconds. Most physical phenomena occur with respect to time. It is a scalar quantity.

Speed is the rate of change of distance. If a body covers a certain distance in a certain amount of time, its speed is given bySpeed=DistanceTimeSpeed=DistanceTime

**Uniform motion and non-uniform motion**

When an object covers equal distances in equal intervals of time it is in uniform motion.

When an object covers unequal distances in equal intervals of time it is said to be in non-uniform motion.

**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.