Class 9 Physics Motion Speed & Velocity

Speed & Velocity

• Speed: Distance travelled by the object in unit time.
• The SI unit of speed is metre per second. This is represented by the symbol m s–1 or m/s.
• Speed is a scalar quantity; it only has direction.

• Velocity: Velocity is the speed of an object moving in a definite direction.
• The SI unit of velocity is also metre per second.
• Velocity is a vector quantity; it has both magnitude and direction.

• In most cases, objects will be in non-uniform motion. Therefore, we describe the rate of motion of such objects in terms of ‘average’.

Example: Consider yourself going to a shop on a cycle, you stop cycle many times on the way and then you come from the shop to your home. Distance from shop to home = 3 km, time taken from home to shop = 1 hour and time taken from shop to home = 2 hour.

• Average speed = total distance / total time

= (3+3) / (1+2) = 2 km/hr

• Average velocity = displacement / total time

= (3-3) / (1+2)   = 0 km/hr

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