What is Linear motion?
When an object moves along a straight line, its motion is called linear motion.
For example, a train moving along a straight railway track from one station to another shows linear motion.
While observing a moving train, we may notice that its speed does not always remain the same. When the train starts, it may move slowly, then become faster, travel at a steady speed, and finally slow down before stopping.
Therefore, linear motion can be classified as uniform linear motion or non-uniform linear motion depending on whether the speed remains constant or changes.
Uniform Linear motion:
An object is said to be in uniform linear motion when it moves along a straight line with constant speed.
In uniform motion, the object covers equal distances in equal intervals of time.
Example:
Suppose a train covers:
- \(10\ km\) in the first \(10\ minutes\)
- \(10\ km\) in the next \(10\ minutes\)
- \(10\ km\) in the next \(10\ minutes\)
- \(10\ km\) in the next \(10\ minutes\)
The train covers the same distance in every equal time interval. Therefore, its motion is uniform.
Non-uniform Linear motion:
An object is said to be in non-uniform linear motion when it moves along a straight line with changing speed.
In non-uniform motion, the object covers unequal distances in equal intervals of time.
Example:
Suppose a train covers:
- \(8\ km\) in the first \(10\ minutes\)
- \(12\ km\) in the next \(10\ minutes\)
- \(15\ km\) in the next \(10\ minutes\)
- \(10\ km\) in the next \(10\ minutes\)
The time intervals are equal, but the distances covered are different. Therefore, the train is in non-uniform motion.
How can we identify the type of Motion?
We can determine whether motion is uniform or non-uniform by comparing the distances covered during equal intervals of time.
Follow these steps:
Step 1: Look at the time intervals
Check whether the given time intervals are equal.
Step 2: Find the distance covered in each interval
Subtract the previous distance from the next distance.
For example:
| Time | Distance |
|---|---|
| \(0\ min\) | \(0\ km\) |
| \(10\ min\) | \(10\ km\) |
| \(20\ min\) | \(20\ km\) |
| \(30\ min\) | \(30\ km\) |
Distance covered in each 10-minute interval:
- \(0–10\ min\) \(\to\) \(10\ km\)
- \(10–20\ min\) \(\to\) \(10\ km\)
- \(20–30\ min\) \(\to\) \(10\ km\)
The distances are equal. Therefore, the motion is uniform. If the distances were different, the motion would be non-uniform.
Comparing Two Trains:
The distances travelled by two trains in equal time intervals of \(10\ minutes\) is given. The important thing is not simply to look at the total distance travelled. We need to compare how much distance each train covers in every \(10-minute\) interval.
Train X:
Suppose Train X covers:
\(20\ km\), \(20\ km\), \(20\ km\), \(20\ km\), \(20\ km\), \(20\ km\)
in successive \(10-minute\) intervals.
Since it covers the same distance in every \(10-minute\) interval, Train X has uniform motion.
Train Y:
Suppose Train Y covers:
\(15\ km\), \(20\ km\), \(15\ km\), \(25\ km\), \(20\ km\), \(25\ km\)
in successive \(10-minute\) intervals.
Since the distances are different in equal \(10-minute\) intervals, Train Y has non-uniform motion.
| Uniform Motion | Non-uniform Motion |
| Equal distances in equal time intervals | Unequal distances in equal time intervals |
| Speed remains constant | Speed changes |
| Straight line with constant speed | Straight line with changing speed. |
A Train's Motion can change during its journey:
A single object does not necessarily have the same type of motion throughout its entire journey.
Consider a train travelling from station A to D.
- From A to B, the train may increase its speed.
- From B to C, it may travel at a constant speed.
- From C to D, it may decrease its speed and finally stop at D.
Therefore:
A–B \(\to\) Non-uniform motion
B–C \(\to\) Uniform motion
C–D \(\to\) Non-uniform motion
This shows that an object's motion can change from one part of its journey to another.
Is Uniform Motion Common in Everyday Life?
Uniform motion is an idealisation. This means that it is useful for understanding and studying motion, but objects in everyday life seldom move with constant speed for long periods.
For example, consider a car travelling through a city.
It may:
- speed up when the road is clear,
- slow down because of traffic,
- stop at a traffic signal, and
- speed up again when the signal turns green.
What is Time?
Time helps us describe and compare motion.
When we say that a train travels a certain distance in \(10\ minutes\), we are comparing the distance travelled with the time taken.
Time is therefore an important quantity when studying motion because it helps us determine how quickly an object changes its position.
We commonly measure time using units such as:
- \(second (s)\)
- \(minute (min)\)
- \(hour (h)\)
The \(second (s)\) is the SI unit of time.
Ancient people also developed different methods of measuring time. Devices such as the ghatika or water clock were used in India to measure the passage of time.