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Class 9, Science, Chapter-7, Lecture-1, Motion & Rest (Notes)

MECHANICS:

The branch of science which deals with the study of motion is called mechanics.

Two branches of mechanics:

(i) Kinematics – deals with the description of motion (Galileo)

(ii) Dynamics – deals with the cause of motion (Newton)

MOTION:

An object is said to be in motion if it is changing its position with respect to a reference object with the passage of time.

REST:

An object is said to be at rest if it is not changing its position with respect to a reference object with the passage of time.

Motion and Rest are Relative terms.

Justification:

Two passengers in a moving train sitting quietly do not change their position with respect to each other and hence they are at rest w.r.t. each other but their position changes with respect to an observer standing outside the train. Thus they are in motion w.r.t. the third person.

This justifies that motion and rest are relative.

ORIGIN:

A fixed point with respect to which the position of an object is described is called the origin.

Information Required To Describe The Position of an Object:

(i) Distance of the object from a reference point (origin)

(ii) Direction of the object w.r.t. the origin. (The angle made by the line joining the origin and the object, with the principal axis)

SCALAR QUANTITY:

A physical quantity that is completely described by its magnitude alone is called a scalar quantity.

Examples – Distance, Work, Energy, Mass, Temperature, Time, Speed, Power, Charge.

VECTOR QUANTITY:

A physical quantity that requires magnitude, as well as direction to be completely described and which follows the same rules of addition as displacements do is called a vector quantity.

Examples – Displacement, Velocity, Acceleration, Force

Graphical Representation of Vector:

A vector is graphically represented by a straight line with an arrowhead in which the direction of the arrow indicates the direction of the vector the length of the tail indicates its magnitude on a suitable scale.

DIFFERENCES BETWEEN:

SCALAR

VECTOR

  • Scalar quantities are described by the magnitude of size only
  • Vector quantities are described by both magnitude as well as direction.
  • Scalar quantities change with a change in magnitude only
  • Vector quantities change either with the change in magnitude or with the change in direction or both.
  • Scalar quantities can be added or subtracted using ordinary rules of algebra. 
  • Vector quantities cannot be added or subtracted using ordinary rules of algebra.
  • Scalar quantities are represented by ordinary letters.
  • Vector quantities are represented by letters having arrows over them.

DISTANCE:

The total length of the path travelled by the body in a certain interval of time is called the distance.

Distance is denoted by 's'

Distance is a SCALAR quantity

SI unit of distance is metre (m)

DISPLACEMENT:

The shortest distance between the final and the initial positions of a body in a specific direction is called displacement.

Displacement is denoted by an arrow over 's'

Displacement is a VECTOR quantity.

SI unit of displacement is metre (m)

DIFFERENCES BETWEEN:

DISTANCE

DISPLACEMENT

  • It is the total length of the path covered 
  • It is the shortest distance between the final and initial positions.
  • It is scalar
  • It is a vector
  • Distance can never be negative.
  • Displacement can be negative.
  • The distance covered by a moving object can never be zero.
  • The displacement of a moving object can be zero.
  • Distance always increases with motion.
  • Displacement may decrease with motion.