Why do we need standard units?
Long ago, people measured with body parts: a handspan, a cubit (elbow to fingertip), a foot or a pace. The problem is that these differ from person to person. A table that measures 5 handspans for a child might be only 4 handspans for an adult.
To avoid confusion, the world agreed on standard units that are the same everywhere. The modern system is the International System of Units (SI units).
Quantity | SI unit | Symbol |
|---|---|---|
Length | metre | m |
Mass | kilogram | kg |
Time | second | s |
Units of length
Unit | Equal to |
|---|---|
1 centimetre (cm) | 10 millimetres (mm) |
1 metre (m) | 100 cm |
1 kilometre (km) | 1000 m |
Use small units for small things (the thickness of a coin in mm) and large units for long distances (Delhi to Mumbai in km).
Measuring length correctly
When you use a ruler:
Place the ruler along the length to be measured, touching the object.
If the end of the ruler is broken or worn, start from a clear mark such as 1 cm and subtract it from the final reading.
Keep your eye directly above the point you are reading. Looking from the side gives a wrong reading, called a parallax error.
Example. One end of a pencil is at the 1.0 cm mark and the other end is at 13.5 cm. The length of the pencil is 13.5 − 1.0 = 12.5 cm.
Measuring a curved line
A ruler cannot follow a curve. Lay a thread carefully along the curved line, mark where it ends, then straighten the thread and measure it with the ruler.
What is motion?
An object is in motion if its position changes with time. If its position stays the same, it is at rest.
Whether something is moving depends on what you compare it with. A passenger sitting in a moving bus is at rest compared with the bus seat, but in motion compared with a tree outside.
Types of motion
Type | Description | Examples |
|---|---|---|
Rectilinear (linear) | moving along a straight line | a soldier marching, a ball dropped from a height |
Circular | moving along a circular path, staying the same distance from a centre | the tip of a clock's hand, a stone whirled on a string |
Periodic | motion that repeats itself after a fixed time | a swinging pendulum, a child on a swing, your heartbeat |
One object can show more than one type of motion at the same time.
A bicycle wheel turns in a circle around its axle while the bicycle moves forward in a straight line.
The Earth spins on its axis and also goes around the Sun, and both motions repeat, so they are periodic too.
Summary
Standard units let everyone get the same measurement. The SI unit of length is the metre.
Read a ruler with your eye straight above the mark to avoid parallax error.
Motion is a change in position with time, judged against something else.
The main types of motion are rectilinear, circular and periodic.