Motion Class 9 Physics Lecture 01

Motion

Rest

 1. A body is said to be at rest if it does not change its position with respect to other body lying in its surrounding wit time.

 2. Examples

 A book lying on the table is at rest because it does not change its position with respect to table,

 A person sitting in a chair is also an example of rest.

 3. The absolute rest is not possible because everything in the universe is in motion according to Big Bang Theory.

 Example: 

A building is at rest with respect to its surrounding but it is in motion along with the earth around the sun. So, absolute rest is not possible.

.

 Motion

 1. A body is said to be in motion if it changes its position with time, with respect to other bodies of the surrounding.

 2. Examples

 A bus running on the road is in motion because it is changing its position with time , with respect to its surrounding .

 A bird flying in air, a train moving on rails, a ship sailing on water, a man walking on the road are examples of motion visible to eyes.

 Rest and Motion are relative

 It means an object in one situation can be at rest but in another situation the same object can be in motion.

 Example

 A person sitting in a moving train is at rest with respect to his fellow passengers but he is in motion with respect to the objects lying outside the train.

 A person sitting in his house is at rest with respect to the earth but he is in motion with respect to other planets and the sun.

 Scalar Quantities

 

1. Those physical quantities which have magnitude only but no direction, are called scalar quantities.

 Examples :

 Mass, length, time, distance, speed, temperature, work, power, energy e t c are examples of scalar quantities.

 2. The scalar quantities can be added, subtracted, multiplied and divided by ordinary laws of algebra.

 3. A scalar is specified by mere number and unit where number represents magnitude. 

4. A scalar may be positive or zero.

 5. A scalar can be represented by a single letter.

 Vector Quantities

 1. Those physical quantities which have magnitude as well as direction, are called vector quantities. Examples

 Displacement, velocity, acceleration, force, torque, gravitational intensity e t c are examples of vector quantities.

 2. Vector quantities can not be added, subtracted, multiplied and divided by ordinary laws of algebra.

 3. Vector quantities can be added, subtracted and multiplied by laws of vectors.

 4. The division of a vector by another vector is not valid.

 5. A vector can be represented either by a single letter in bold face or by a single letter with an arrow head on it i.e. A or A.

Distance

 1. The actual path moved by a body is called distance.

 2. It is also known as a path length.

 3. The distance travelled by a body is equal to the product of the speed of the body and time taken to travel the distance.

 i.e. Distance = Speed x Time 

4. Distance travelled by a body is generally denoted by ‘s’ or ‘x’ or ‘y’ or ‘z’. i.e. s=u .t 

Here,

 s= distance travelled by the body

 u = Speed of the body

 t = Time taken to travel the distance

 5. Distance is a scalar quantity.

 6. S.I. Unit of distance is metre (m).

 7. The other units of distances are

 (a) Kilo metre(km)

 (b) Light year

 (c) Astronomical unit (A.U.)

 (d) Parsec

 8. It is always positive. It can never be negative

 Displacement

 1. The distance travelled by a body in a certain direction is called displacement.

 2. In other words, “The shortest distance between two points is called displacement. “ 

3. It is generally denoted by s or x or y or z.

 4. The displacement is equal to the product of velocity and time taken to travel.

 Displacement= Velocity x Time

 S = u.t

 Here,

 S = displacement

 u= velocity

 t= time 

5. Displacement is a vector quantity.

 So, in vector form it is given by

 S = u.t

 Here bold face represents vector.

 6. S.I. Unit of displacement is metre (m). 

7. It can be positive or negative or zero.

By Shivanand Choudhary

For Class 9th Physics

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