1. Understanding Position
Position tells us the exact place where an object is located. It answers the question, “Where is the object?”
If you say, “The ball is near the door,” you are describing its position. In science, we try to describe position more clearly using measurements and reference points.
1.1. Position as a Location
An object does not have a fixed position on its own. We always describe position by comparing it with something else, like a wall, a tree, or a building.
1.2. Position Can Change With Time
If an object moves, its position changes. We can write position as a simple function of time:
\( x(t) \)
This means the value of position depends on what time it is.
2. Need for a Reference Point
To describe the position of an object clearly, we must choose a fixed point. This fixed point is called a reference point.
Without a reference point, position has no meaning because we won’t know where the object is being measured from.
2.1. What Is a Reference Point?
A reference point is a point that we treat as fixed and stable. We compare the object’s position relative to this point.
Example: If you say, “My school is 500 meters from my house,” your house is the reference point.
2.2. Why Reference Points Are Important
Different people may choose different reference points. So the same object may have different positions for different observers.
Example: A person inside a car may say a bag is ‘next to them’. But a person outside the car may say the bag is ‘20 meters away on the road’. Both are correct with their own reference points.
3. Choosing a Suitable Reference Point
A good reference point is one that is easy to identify and does not move. Common examples include a lamp post, a building, a tree, or a fixed corner of a room.
3.1. Different Observers, Different Reference Points
Two people looking at the same event may choose different reference points. That is why their descriptions of position may differ.
For example, if a bus is passing by:
- A person standing on the road sees the bus moving.
- A person inside the bus sees the road moving backward.
Both descriptions are correct relative to their chosen reference points.
3.2. Reference Point and Rest or Motion
Whether an object is in rest or motion depends on the reference point. An object may appear at rest relative to one point but in motion relative to another.
4. Position as a Function of Time
The position of an object changes as time passes when it is in motion. We describe this change using a simple expression.
4.1. Representation of Position
In science, we often write position as:
\( x(t) \)
This means “position x at time t”.
4.2. Example of Position Changing With Time
If a car starts from home and moves along a straight road, we can track how far it is from home at different times:
- At 0 seconds: \( x = 0 \) m
- At 5 seconds: \( x = 20 \) m
- At 10 seconds: \( x = 45 \) m
This shows its position changing with time.