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9th Class Physics Chapter 2 Notes

A body is said to be at rest, if it does not change its position with respect to its surroundings.

A body is said to be in motion, if it changes its position with respect to its surroundings.

Rest and motion are always relative.There is no such thing as absolute rest or absolute motion.

Motion can be divided into the following three types.

Translatory motion: In which a body moves without any rotation.

Rotatory motion: In which a body spins about its axis.

Vibratory motion: In which a body moves to and fro about its mean position.

Physical quantities which are completely described by their magnitude only are known as scalars.

Physical quantities which are dscribed by their magnitude and direction are called vectors.

Position means the location of a certain place or object from a reference point.

The shortest distance between two points is called the displacement.

The distance travelled in any direction by a body in unit time is called speed.

If the speed of a body does not change with time then its speed is uniform.

Average speed of a body is the ratio of the total distance covered to the total time taken.

We define velocity as rate of change of displacement or speed in a specific direction.

Average velocity of a body is defined as the ratio of its net displacement to the total time.

If a body covers equal displacements in equal intervals of time, however small the interval may be, then its velocity is said to be uniform.

The rate of change of velocity of a body is called acceleration.

A body has uniform acceleration if it has equal changes in its velocity in equal intervals of time, however small the interval may be.

Graph is a pictorial way of describing information as to how various quantities are related to each other.

Slope of the distance-time graph gives the speed of the body.

Distance – time graphs provide useful information about the motion of an object. Slope of the displacement-time graph gives the velocity of the body.

Distance covered by a body is equal to area under speed – time graph.

Speed-time graph is also useful for studying motion along a straight line.

The distance travelled by a body can also be found from the area under a velocity – time graph if the motion is along a straight line.

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