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# Speed, Velocity and Acceleration

Speed is the rate of change of distance or the distance travelled in unit time.

Speed, Velocity and  Acceleration

## 1. Speed

Speed is the rate of change of distance or the distance travelled in unit time. It is a scalar quantity. The SI unit of speed is ms-1. Thus,

Speed = Distance travelled / time taken

## 2. Velocity

Velocity is the rate of change of displacement. It is the displacement in unit time. It is a vector quantity. The SI unit of velocity is ms-1. Thus,

Velocity = Displacement / time taken

## 3. Acceleration

Acceleration is the rate of change of velocity or it is the change of velocity in unit time. It is a vector quantity. The SI unit of acceleration is ms-2.

Acceleration = Change in velocity/time

= (Final velocity – initial velocity)/time

a = (v–u)/t

Consider a situation in which a body moves in a straight line without reversing its direction.

Case 1: From the above equation if v > u, i.e. if nal velocity is greater than initial velocity, the velocity increases with time and the value of acceleration is positive. Case 2: If v < u, i.e. if nal velocity is less than initial velocity, the velocity decreases with time and the value of acceleration is negative. It is called negative acceleration. If the acceleration has a value of -2 ms-2 when we say that the retardation is 2 ms-2 or deceleration is 2 ms-2.

Case 3: If v = u, then a = 0. This means that the acceleration is zero when the final velocity is equal to initial velocity

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9th Science : Motion : Speed, Velocity and Acceleration |