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Chapter: 9th Science : Motion

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 |


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