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

**Speed, Velocity and Acceleration**

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*

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*

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

**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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