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# CBSE Class 9 Physics Notes for Chapter 1 Motion

MOTION ALONG A STRAIGHT LINE

When a body moves in a straight line, it is said to exhibit motion along a straight line.

Distance: The total path length covered by an object is the distance travelled by it.

Displacement: The shortest distance measured from the starting point to the final point is called the displacement of the object. If the final position of the body is the same as the initial position, displacement is said to be zero.

UNIFORM MOTION

If an object travels equal distances in equal intervals of time, it is said to exhibit uniform motion.

NON-UNIFORM MOTION

If an object travels unequal distances in equal intervals of time, it is said to be in non-uniform motion.

SPEED

Speed is the rate at which the body moves i.e. how fast or slow it moves. The speed of the body is the distance travelled by it divided by the time taken to cover that distance. The SI unit of speed is metre per second.

Speed=  (Distance travelled)/(Time taken)

VELOCITY

The quantity that specifies both the speed and direction of the motion is called velocity.

Average velocity=  (Initial velocity+final velocity)/2

ACCELERATION

The rate of change of velocity is called acceleration.

Acceleration (a) =(Change in velocity)/(Time taken)

The SI unit of acceleration is ms-2.

EQUATIONS OF MOTION

There are three equations of motion:

• v = u + at
• s = ut + at2
• 2as = v2 – u2

UNIFORM CIRCULAR MOTION

If an object moves in a circular path with uniform speed, its motion is called uniform circular motion. If the radius of the circle is r,

v=(2 π r)/t