Chapter 8: Motion - Class 9 NCERT Notes
1. Introduction to Motion
Motion: An object is said to be in motion if it as position changes with respect
to a fixed point (reference point or origin) over time.
Rest: An object is said to be at rest if its position does not change with respect
to a reference point over time.
Reference Point (Origin): A fixed point or object used to describe the
position or motion of another object. Motion is relative.
Types of Motion: Can be linear (straight line), circular, oscillatory, etc. This
chapter focuses mainly on motion along a straight line (rectilinear motion).
2. Describing Motion: Physical Quantities
Scalar Quantity: A physical quantity that has only magnitude (size).
Examples: Distance, Speed, Mass, Time.
Vector Quantity: A physical quantity that has both magnitude and direction.
Examples: Displacement, Velocity, Acceleration, Force.
Distance:The total actual path length covered by an object during its motion.
Scalar quantity.
Always positive or zero.
, SI Unit: meter (m).
Displacement:
The shortest distance between the initial and final positions of an
object, along with direction.
Vector quantity.
Can be positive, negative, or zero.
SI Unit: meter (m).
Note: If an object returns to its starting point, its displacement is zero,
even if it has covered a significant distance.
3. Uniform and Non-Uniform Motion
Uniform Motion:
An object travels equal distances in equal intervals of time, no matter
how small the intervals are.
The object moves with constant velocity (constant speed in a straight
line).
Acceleration is zero.
Non-Uniform Motion:
An object travels unequal distances in equal intervals of time.
The object's velocity changes (either speed or direction or both change).
The motion is accelerated.
, 4. Rate of Motion: Speed and Velocity
Speed:The rate at which an object covers distance.
Speed = Distance / Time
Scalar quantity.
SI Unit: meters per second (m/s or ms⁻¹).
Average Speed: Total Distance Covered / Total Time Taken. Used when speed varies.
1. Introduction to Motion
Motion: An object is said to be in motion if it as position changes with respect
to a fixed point (reference point or origin) over time.
Rest: An object is said to be at rest if its position does not change with respect
to a reference point over time.
Reference Point (Origin): A fixed point or object used to describe the
position or motion of another object. Motion is relative.
Types of Motion: Can be linear (straight line), circular, oscillatory, etc. This
chapter focuses mainly on motion along a straight line (rectilinear motion).
2. Describing Motion: Physical Quantities
Scalar Quantity: A physical quantity that has only magnitude (size).
Examples: Distance, Speed, Mass, Time.
Vector Quantity: A physical quantity that has both magnitude and direction.
Examples: Displacement, Velocity, Acceleration, Force.
Distance:The total actual path length covered by an object during its motion.
Scalar quantity.
Always positive or zero.
, SI Unit: meter (m).
Displacement:
The shortest distance between the initial and final positions of an
object, along with direction.
Vector quantity.
Can be positive, negative, or zero.
SI Unit: meter (m).
Note: If an object returns to its starting point, its displacement is zero,
even if it has covered a significant distance.
3. Uniform and Non-Uniform Motion
Uniform Motion:
An object travels equal distances in equal intervals of time, no matter
how small the intervals are.
The object moves with constant velocity (constant speed in a straight
line).
Acceleration is zero.
Non-Uniform Motion:
An object travels unequal distances in equal intervals of time.
The object's velocity changes (either speed or direction or both change).
The motion is accelerated.
, 4. Rate of Motion: Speed and Velocity
Speed:The rate at which an object covers distance.
Speed = Distance / Time
Scalar quantity.
SI Unit: meters per second (m/s or ms⁻¹).
Average Speed: Total Distance Covered / Total Time Taken. Used when speed varies.
