What is Motion?
Look around you. A flying bird, a moving car, blood flowing in veins—these are in motion. A table, a house, or a tree are at rest. But how do we define this scientifically?
Tip
Definition: An object is in motion if its position changes with respect to a reference point with time.
If the position does not change with respect to the reference point, the object is said to be at rest.
The Importance of a Reference Point
Motion is relative. Whether an object is moving depends on who is looking at it (the observer) or what point we compare it to (the reference point).
The Bus Example: Imagine Deepa is sitting in a moving bus.
- With respect to the bus (Reference Point): Deepa is not changing her position. She is at rest relative to the seat.
- With respect to a tree outside (Reference Point): Deepa’s position is changing. She is in motion relative to the tree.
The Kilometre Stone Example: When Padma travels to Delhi, she sees milestones: “Delhi 70 km”, then “Delhi 60 km”.
- Reference Point: Delhi.
- Observation: Her distance from Delhi is changing. Therefore, she is in motion.