Measurement of Length and Motion · Lesson 2 of 5
Measuring Length Accurately
“Learn how to choose measuring tools, read scales correctly, handle broken scales and measure curved lengths with confidence.”
• Choose an appropriate tool for different kinds of length measurements. • Place a scale and position the eye correctly when taking a reading. • Measure with a scale whose zero end is damaged by subtracting two readings. • Record measurements using correct unit notation. • Measure curved lengths using a flexible tape or thread.
Choosing the Right Measuring Tool
Accurate measurement begins before a reading is taken. The measuring tool must suit the object. A short ruler works well for a pencil, a metre scale can measure larger straight lengths, and a flexible measuring tape is better for a chest, tree trunk or other curved surface.
| What is being measured? | Useful tool | Why it suits the task |
|---|---|---|
| Pencil | 15 cm or 30 cm ruler | Small, straight object |
| Height of a room | Measuring tape or metre scale | Longer length |
| Chest or tree girth | Flexible measuring tape | Wraps around curved surfaces |
| Curved line on paper | Thread, then ruler | Thread follows the curve |
How to Read a Scale Correctly
Even a perfectly marked ruler can give a poor measurement if it is placed badly or read from the wrong angle. The scale should lie along the length being measured, and the reading should be viewed from directly above the relevant mark.
- Place the scale in contact with the object and along its length.
- Align one end of the object with the zero mark when the zero end is usable.
- Look vertically down at the mark beside the other end of the object.
- Read the value carefully and include the unit.
If your eye is to the left or right of the mark instead of directly above it, the reading can appear shifted. This viewing error is avoided by keeping the eye vertically above the endpoint.
Measuring with a Broken Scale
A ruler can still be useful even if its zero edge is broken or unclear. Instead of starting at zero, begin at any clearly marked value and subtract that starting reading from the reading at the other end.
Problem
One end of an object is placed at 1.0 cm and the other end reaches 10.4 cm. Find the object's length.
- 1.The initial reading is 1.0 cm.
- 2.The final reading is 10.4 cm.
- 3.Length = final reading − initial reading.
- 4.Length = 10.4 cm − 1.0 cm = 9.4 cm.
- 5.So the object is 9.4 cm long.
People who cannot rely on sight can use measuring scales with raised markings that can be felt by touch.
Recording Measurements Clearly
A measurement should be written so another person can interpret it immediately. Unit names such as kilometre, metre, centimetre and millimetre are normally written in lowercase. Their symbols km, m, cm and mm are also lowercase, are not made plural, and are separated from the number by a space.
Problem
Which is clearer: 8cms or 8 cm?
- 1.The standard symbol for centimetre is cm.
- 2.Unit symbols are not made plural by adding s.
- 3.A space is left between the number and the symbol.
- 4.Therefore, write 8 cm.
Measuring Curved Lengths
A rigid ruler cannot bend along an arch, the curved base of a bottle or another irregular path. A flexible tape can follow the curve directly. If no flexible tape is available, place a thread carefully along the curve, mark the required section, straighten the thread, and measure that straightened length with a ruler.
When several students measure the same object, small differences can still occur. They may place the scale slightly differently, choose different endpoints, or read a mark from a slightly different angle. Good technique reduces these differences.
Quiz
Which tool is most suitable for measuring the girth of a tree?
Where should your eye be when reading the endpoint of an object on a ruler?
An object starts at 2.0 cm and ends at 8.7 cm on a ruler. What is its length?
Which is the best way to measure the length of a curved line on paper?
Which notation is written correctly?
Practice Problems
- Describe the correct way to place a ruler while measuring a pencil.
- Explain why the eye should be directly above the reading mark.
- A broken ruler shows 3.2 cm at one end of a stick and 14.6 cm at the other. Find the stick's length.
- Measure three small objects at home or school and record each result with the correct unit symbol.
- Explain how you would measure the curved rim of a tumbler using thread.
- Why is a flexible measuring tape more suitable than a rigid ruler for measuring the chest?
- List two reasons why two careful students might still obtain slightly different measurements of the same object.
Key Takeaways
• Match the measuring tool to the object and the kind of length being measured. • Keep the scale along the object and the eye directly above the reading mark. • With a broken zero end, subtract the initial reading from the final reading. • Write measurements with both a number and the correct unit symbol. • Flexible tape or thread makes curved lengths measurable.