Class 9 Science

**Uniform Motion:** When an object covers equal distance in equal interval of time, the motion is called uniform motion. For example – if a moving vehicle covers a distance of 10 km every hour, the motion of the vehicle is called uniform motion.

**Non-uniform Motion:** When an object covers unequal distance in equal interval of time, the motion is called non-uniform motion. For example – If moving vehicle covers a distance of 10 km in the first hour, covers a distance of 20 km in the second hour, covers a distance of 5 km in the third hour, etc. the motion of the vehicle is called non-uniform motion.

Distance covered by a moving object in unit time is called distance.

Speed = `(text{Distance covered})/(text{Total time taken})`

`=>v=s/t`

Where, v = speed, s = distance, t = total time.

SI unit of speed is meter per second (m/s).

The average distance covered in unit time by a moving object is called average speed. Average speed is the ratio of total distance covered and total time taken.

Average speed =`(text{Total distance covered})/(text{Total time taken})`

`=>v=s/t`

Where, v = Average Speed, s = Total distance covered, t = total time taken.

SI unit of average speed is meter per second (m/s).

The speed of a moving object in particular direction is called velocity. Velocity has both magnitude and direction while speed has only magnitude and no direction.

Velocity of an object is the distance covered in particular direction in unit time.

`text{Velocity}=(text{Distance covered})/(text{Time taken})`

`=>v=s/t`

*SI unit of velocity is meter per second.*

**Uniform Velocity:** Uniform speed of an object in same direction is called uniform velocity.

**Non-Uniform Velocity:** Velocity of an object is changed in following two conditions.

- When speed is changed
- When direction is changed.

Thus, non uniform speed of a moving object in same direction, or non-uniform speed in different directions or uniform speed in different directions is called non-uniform velocity.

**Example:** If a vehicle is moving on a circular path with uniform speed, then its velocity is said to be non-uniform, because on a circular path the direction of moving body changes along with direction of curve.

If a vehicle moving with uniform speed on a jig-jag path, the velocity of the vehicle will be non-uniform because direction of vehicle is changed with the change of direction of path.

The arithmetic mean of velocity of an object moving along a straight line is called the average velocity.

`text {Average velocity}` `=(text{Initial velocity + Final velocity})/2`

`v_(av)=(u+v)/2`

Where, u is the initial velocity and v is the final velocity.

The displacement of a moving object in unit time is also called the average velocity.

`text{Average velocity}` `=(text{Displacement})/(text{Total time taken})`

The rate of change in velocity is called acceleration. Acceleration is generally denoted by `a` or `f`

Thus, Acceleration `=(text{Change in velocity})/(text{Time taken})`

`=>a=(text{Final velocity - Initial velocity})/(text{Time taken in change in velocity})`

`=>a=(v-u)/t`

Where, ‘a’ is acceleration, ‘v’ is final velocity, ‘u’ is initial velocity and ‘t’ is time taken for change.

A positive sign of the magnitude of acceleration shows increase in velocity and a negative sign show decrease in velocity. If there is decrease in acceleration, it is called Retardation. This means, rate of decrease in velocity is called Retardation.

The SI unit of velocity is meter /second

The SI unit of time is second.

Thus, SI unit of acceleration (a) `=(m//s)/s` `=m//s^2 or m//s^(-2)`

In the case of uniform velocity, the speed or direction of a moving object is not changed and thus there is no change in acceleration. Therefore, in the case of uniform velocity acceleration will be zero.

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