# Kinematics (1D) III- Speed, Velocity and Acceleration

### Speed

Speed comes into picture when a particle is in motion. It is another useful parameter. As we know that when the particle is in motion, it covers a particular distance in some amount of time. The distance that particle covers in a particular time period is called speed. It is a scalar quantity. Just like distance. Speed is always positive.

**S = total distance / total time taken**

Speed is usually calculated in metres/second. It can have other units such as Kilometres/second, Kilometres/hour, centimetre/minute etc.

For example a man runs 10 metres in 2 seconds. The speed of the man will be 10 / 2 = 5 metres/second.

### Velocity

Velocity is similar to speed but has one difference. Velocity has a defined direction (therefore it is a vector). The formula for velocity is **displacement/time**. The units used for velocity are the same as speed.

The velocity of an object is 0 when an object returns to the point where it started from. The Velocity is negative when the object travels opposite to the direction which is considered to be positive.

For example, imagine the number line again.

If the object travels from 0 to 3 in 3 seconds, it will have a positive velocity of 1 meter/second. If it travels from 0 to -3 in 3 seconds, it will have a negative velocity of -1 metres/second. When the object moves from 0 to 3 and then from two to zero in 6 seconds, distance traveled is 6 units but the displacement is 0. therefore, the speed is 1 m/s and the velocity is 0.

### Acceleration

Acceleration is a very common word. It refers to change in speed per unit time. In common language Acceleration means increase in speed of an object. When an objects speed increases with time it is called acceleration. It is also a vector. When an object’s speed decreases with time, acceleration is negative. if an object’s speed is constant, then its acceleration is 0.

**Formula => (change in Speed)/time**

**Change in speed= (final velocity) – (initial velocity**)

**SI unit- (metres) / (second^2)**

**Example- **An objects speed decreases from 6 m/s to 4/s in 1 second. the acceleration is given by change in speed/time **= (4-6)/1 = -2 m/s^2**

**Important point- Acceleration does not give direction of motion**

For example an object is moving towards right (positive direction). The acceleration can then be along the direction of motion (positive acceleration, speed increases) or opposite to direction of motion ( negative acceleration, speed decreases).