Motion Class 9

Physics

Physics

Study of laws of nature

Physical Quantities

Measurable quantities Eg: time, density etc.

Units

Used to measure physical quantities.

Scalar quantity

a quantity that can be described by magnitude only and has no direction. Eg: Mass, time

Vector quantity

a quantity that has both magnitude and direction. Eg: Velocity, acceleration, pressure.

Body at rest definition

A body is said to be at rest, if it does not change its position with respect to a fixed point in its surroundings and time.

BODY IN MOTION

A body is said to be in motion, if it changes its position continuously with respect to a fixed point in its surroundings and time.

Rest and motion are relative terms.

A car driver is at rest with respect to a person sitting beside him, but is in motion with
respect to a person on the road.

Distance

The total path covered by a moving body from the initial position to the final position. It's denoted by 'S' and is a scalar quantity.

SI unit of distance

meter

CGS unit of distance

centimeter

Can distance be zero

NO

Displacement

The shortest distance between the initial and final position of an object. It's a vector quantity.

SI/CGS unit of displacement

meter/centimeter

Can displacement be zero, positive/negative?

Yes

Can displacement be greater than distance?

No

Speed

Distance travelled by a body in unit time. It's a scalar quantity

SI unit of speed

m/s

Bigger unit/CGS unit

(km/h)/(m/s)

Can speed be negative or zero?

NO

1 km/h

5/18 m/s

1 m/s

18/5 km/h

Velocity

Displacement per unit time. It's a vector quantity

SI unit of velocity

m/s

CGS unit

cm/s

Can velocity be positive/negative/zero?

YES

UNIFORM MOTION

If a body travels equal distances in equal intervals of time, then the body is said to be in uniform motion.

NON-UNIFORM MOTION

If a body travels unequal distances in equal intervals of time or vice versa, the body is said to have non-uniform motion

AVERAGE SPEED

It is the ratio of total distance travelled to the total time taken.

AVERAGE VELOCITY

It is the ratio of total displacement to the total time

If the velocity of the object changes at uniform rate, then

(u+v)/2

INSTANTANEOUS SPEED

The rate at which an object is moving at a given moment in time

ACCELERATION

The rate of change of velocity of a moving body. (v-u)/t. It's a vector quantity.

Can it be +ve/-ve/0?

YES

RETARDATION

Negative acceleration or Deceleration (r = -a)

Positive acceleration

an increase in velocity

Negative acceleration

decrease in velocity

Body with uniform motion (Graph)

The graph is a straight line making an angle with x - axis

Body with non - uniform motion (Graph)

The graph of a body moving with a non - uniform speed is a curve

Zero acceleration (Graph)

The graph shows a straight line parallel to x - axis

Uniform acceleration (Graph)

The graph shows a straight line sloping upwards and passing through the origin.

A body having initial velocity and moving with uniform acceleration. (Graph)

The graph shows a straight line sloping upwards, having some initial velocity.

Deceleration (Graph)

The graph shows a straight line sloping downwards i.e. uniform retardation.

VELOCITY - TIME RELATIONSHIP

v = u+at

POSITION - TIME RELATIONSHIP

S = ut + 1⁄2 at^2

POSITION - VELOCITY RELATIONSHIP

v^2 - u^2 = 2as

UNIFORM CIRCULAR MOTION

When an object is moving on a circular path with a constant speed, the motion of the object is said to be in uniform circular motion.

In circular path, v =

(2𝜋r)/t

When a body is moving in a circular path,

the direction of the velocity constantly changes. The direction of the velocity is along the tangent to the circle at any point in its motion.