# NCERT CLASS IX - Science Textbook Chapter 8 - Motion Distinguish between speed and velocity.

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by swapnilShanker

2015-05-16T15:12:18+05:30

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speed is a scalar quantity.  It is the distance moved in a unit time.  Its units are   meters/sec  in SI system.  It is direction less.  Speed is always non-negative.    It depends on the path length of the curve that the body moves on.

This is instantaneous speed at any instant.

Average speed = total distance traveled / total time duration
= path length traveled / duration of time

Velocity is the change of displacement in unit time.   Since displacement is a vector, the velocity of a body is also a vector.  It has a direction and magnitude.  Velocity follows vector laws of addition.

Since displacement depends only on the initial and final positions,  the velocity vector also depends on the initial and final positions and not the path length which the body traces.

velocity average
=  (final displacement -  initial displacement ) / total duration of time

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