Notes 8. Motion - Distance and Displacement | Class 9 Science - Toppers Study

8. Motion

(Science - IX)

Motion: when an object changes its position respect to its surrounding with time, it is said to be the object is in motion. 

Example: motion of flying birds, motion of car, motion of blood in arteriesand veins etc. 

Most motions are complex. Some objects may move in a straight line, others may take a circular path. Some may rotate and a few others may vibrate. There may be situations involving a combination of these.

Origin: we need to specify a reference point that describes the position of an object is called the origin. 

Distance: This is total path covered by the body between its initial and final position is called distance.

Displacement: The shortest distance measured from the initial to the final position of an object is known as the displacement.

Magnitude: The numerical value of a physical quantity is its magnitude.

Physical Quantities: Quantities in physics which can be measured are called physical quantities like mass, time, area, velocity, area, density, volume, force etc. 

scalars  and vectors quantities:

Scalars quantities: The quantities that require magnitudes only to specify them are called scalars quantities or scalars. e.g Mass, time, area, speed, distance, temperature, density, volume etc. 

Vector quantities: Quantities that require both magnitudes and direction to specify them are called vector quantities or vectors. e.g Displacement, velocity, force, momentum, weight etc. 

Differences between Displacement and Distance. 

Use of distance and displacement:

(i) The displacement and distance are used to describe the overall motion of an object.

(ii) They are used to locate its final position with reference to its initial position at a given time.

Motion along a straight line: 

The simplest type of motion is the motion along a straight line.

When an object moves on a straight path, such a motion is called motion along a straight line or Linear motion. 

Examples:

1. A bus moving on a straight road.
2. A train moving on straight road.                         
3. A ball rolling along a straight path.

Type of Motion: 

(i) Uniform Motion: As the object covers equal distance in equal intervals of time it is said to be in uniform motion.

Example: 

1. Motion of car.
2. Flying of a bird.
3. Motion of a sparing.

(ii) Non-uniform Motion: As the object does not covers equal distance in equal intervals of time it is said to be in uniform motion.

Uniform Motion:

When an object covers equal distance in equal intervals of time, it is said that the object is in uniform motion. 

e.g: See table

 Object A covers equal distance in equal time interval, while object B does not cover equal distance in equal time interval. Therefore it is said that object A is in uniform motion and object B is in non-uniform motion. 

Finding rate of motion:

Rate of motion is found by the speed of an object. 

Speed: The distance travelled by the object in per unit time is called speed. 

The SI unit of speed is metre per second. symbolically it is written as m s^-1 . 

Avarage speed: The average speed of an object is obtained by dividing the total distance travelled by the total time taken.

If an object travels a distance s in time t then its speed v is,