Important Points: Motion | Science Class 9 PDF Download

Rest 

A body is said to be in a state of rest when its position does not change with respect to a reference point. For instance, a mountain can be said to be 'at rest'.

Object at Rest

Motion

A body is said to be in a state of motion when its position changes continuously with reference to a point.

Sometimes, motion is inferred through indirect evidence, such as:

• The movement of dust indicates the motion of air.
• The swaying of leaves and branches shows that the air is moving.

Motion can be of different types depending upon the type of path by which the object is going through.

Object in Motion

• Circulatory Motion / Circular Motion – In a circular path.
• Linear Motion – In a straight line path.
• Oscillatory / Vibratory Motion – To and fro path with respect to the origin.

Physical Quantity

There are seven basic physical quantities. Each physical quantity is expressed in two parts: its magnitude (numerical value) and its unit (standard measurement). For example: If we say a length is 4 meters, 4 is the magnitude, and meters (m) is the unit.

Physical Quantities can be grouped into two:

Scalar Quantities

• These are physical quantities that have only magnitude and no direction.
• Examples include speed, distance, mass, volume, time, and temperature. The SI unit of speed is metres per second (m/s).

Vector Quantities

• These physical quantities have both magnitude and direction.
• Examples include velocity, force, momentum, and displacement.

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Distance 

The actual path or length travelled by an object during its journey from its initial position to its final position is called the distance. 

• It is a scalar quantity which requires only magnitude but no direction to explain it.
• Example: Ramesh travelled 65 km. (Distance is measured by odometer in vehicles.)

Displacement 

• Displacement is a vector quantity requiring both magnitude and direction for its explanation.
• Example: Ramesh travelled 65 km southwest from Clock Tower.
• Displacement can be zero (when the initial point and final point of motion are the same) Example:- circular motion.
• Distance and displacement are denoted by ‘S’.

Difference between Distance and Displacement 

Uniform Motion

• When a body travels an equal distance in an equal interval of time, then the motion is said to be uniform motion.
• Example: movements of hands of a clock rotation and revolution of the earth.

Non–uniform Motion

• In this type of motion, the body will travel unequal distances in equal intervals of time. 
• Example: motion of a car on a busy road.

Types of Non–uniform Motion

Non–uniform motion is of two types 

Accelerated Motion: When the motion of a body increases with unequal time.
De–accelerated Motion or Non-uniform Retardation: When the motion of a body decreases with unequal interval of time.

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Speed

• The term speed refers to how fast an object moves, measured as the distance covered in a set amount of time. It is represented by the symbol v.
• The standard unit for speed is metres per second, abbreviated as m/s. Other units include centimetres per second (cm/s) and kilometres per hour (km/h).
• Speed can be calculated using the formula: Speed = Distance travelled / Time taken
• If an object covers a distance s in time t, its speed v can be expressed as: v = s/t
• When an object moves at a steady rate, it is said to be in uniform motion, meaning its speed is constant. However, in many cases, the speed may change, indicating non-uniform motion.
• In non-uniform motion, the average speed gives a general idea of how fast the object is moving overall, but it does not mean the speed remains the same throughout the journey.
• Average speed = Total distance travelled / Total time taken
• For instance, if a car covers a distance of 100 km in 2 hours, its average speed is calculated to be 50 km/h.

Velocity

• Velocity is the speed of an object moving in a specific direction.
• Velocity can be calculated using the formula: Displacement/Time.
• Velocity is a vector quantity, meaning it can change if either its speed or direction changes. It is often represented by the letter v.
• When an object moves in a straight line at varying speeds, we can define the average rate of motion using average velocity. This is determined by total displacement rather than just distance.
• Average velocity is calculated as: Total displacement / Total time.
• The velocity of an object may be either uniform or variable. Changes can occur in the object's speed, direction, or both.
• For a velocity that changes uniformly, the average velocity is calculated with the formula: average velocity = (initial velocity + final velocity) / 2.
• V(avg) = (u + v) / 2, where u is the initial velocity and v is the final velocity.

Acceleration

• Acceleration: Change in velocity/Time
  Acceleration: Final Velocity Initial/Velocity Time
• a = v-u/t
• where, v = final velocity, u = initial velocity 
• If v > u, then ‘a’ will be positive (+ve).

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Retardation/Deacceleration

• Deacceleration is seen in uniform motion during a decrease in velocity with time. It has the same definition as acceleration.
• => Change in velocity/time
• A=V-u/t
• Here v < u, ‘a’ = negative (-ve).
• The unit of acceleration and deacceleration is m/s² or ms^-2 

Uniform Circular Motion

• If a body is moving in a circular path with uniform speed, It is motion is called uniform circular motion. 
• In such a motion the speed may be the same throughout the motion but its velocity (which is tangential) is different at each and every point of its motion due to continuous change in direction. Thus, uniform circular motion is an accelerated motion. 
• So, the velocity of an object in a circular motion is
  V=2πr/t, where r represents the radius of the circular path and t is the time taken to complete one full revolution.