Motion, Class 9 Science Notes
What is Motion?
The change of position of an object with time is called motion. If something moves from one place to another, it is said to be in motion. For example, the running of a car on the road or the rotation of a fan shows the motion.
Physical Quantities: Scalars and Vectors
In physics, any measurement is referred to as a physical quantity. These are divided into scalars and vectors. Scalars have only magnitude (like time, mass, and speed), while vectors have both magnitude and direction (like velocity and force).
| Type | Description | Examples |
|---|---|---|
| Scalar | Has only magnitude | Time, mass, distance |
| Vector | Has magnitude and direction | Velocity, acceleration, force |
Motion and Rest
An object is said to be in motion if its position changes with time. It is said to be at rest if it remains in the same position. Interestingly, motion and rest are relative. For instance, when you sit in a moving bus, you are at rest with respect to the bus but in motion with respect to the road.
Distance and Displacement
The total path covered by an object is known as distance. Displacement refers to the shortest path between two points. Distance is a scalar while displacement is a vector.
| Basis | Distance | Displacement |
|---|---|---|
| Type | Scalar | Vector |
| Path | Total path covered | Shortest path |
| Value | Always positive | Can be zero or negative |
| Example | Car goes 10 m forward and 10 m back – distance = 20 m | Displacement = 0 m |
Speed and Velocity
Speed tells how fast an object moves. Velocity tells both how fast and in which direction. Speed is scalar, while velocity is vector.
Speed = Distance ÷ Time
Velocity = Displacement ÷ Time
Acceleration
Acceleration means how fast the velocity of an object changes with time. If a car increases its speed, it has positive acceleration. If it slows down, it has negative acceleration (deceleration).
Acceleration = (Final velocity – Initial velocity) ÷ Time
Graphical Representation of Motion
Graphs help us understand motion easily.
- A distance-time graph shows how far an object travels in a given time.
- A velocity-time graph shows how velocity changes with time.
| Graph Type | Axis Used | What It Shows |
|---|---|---|
| Distance-Time | Distance vs. Time | Speed of motion |
| Velocity-Time | Velocity vs. Time | Acceleration or deceleration |
Motion is one of the most important topics in physics. Regular practice of formulas and graphs helps in mastering the topic of motion.