Learning objective
Use SI units, significant figures, dimensions and scientific notation with confidence.
CLASS 11 PHYSICS
Units and Measurements
Build the measurement language used in every Physics chapter.
State Board to NCERT bridge
From measuring familiar objects to checking whether a Physics equation makes sense.
Tamil concept bridge
Alavu, unit, accuracy and precision-ai purindhukondaal ella Physics calculations-um thelivaga seyyalam.
FULL LESSON · UNITS AND MEASUREMENTS
Learn it step by step.
By the end, you can:
- ✓ choose appropriate SI units
- ✓ use scientific notation and prefixes
- ✓ explain accuracy and precision
- ✓ check an equation with dimensions
Physics measures properties that can be described with a number and a unit. A bare number is incomplete: 5 could mean 5 metres, 5 seconds or 5 kilograms.
Tamil concept bridge
Alavu sollumbodhu number mattum podhathu. Unit-um serndhaal dhaan meaning thelivaga irukkum: 5 m, 5 s, 5 kg.
- ✓Base quantities are independent: length, mass, time, electric current, temperature, amount of substance and luminous intensity.
- ✓Derived quantities are made from base quantities: velocity (m s^-1), force (kg m s^-2) and density (kg m^-3).
- ✓Use SI units in calculations unless the question asks for another unit.
Worked example
A runner covers 100 m in 20 s. The values are meaningful only because each number has its unit.
QUICK CHECK
Check your understanding before you move on.
1. Which is an SI base unit?
2. 0.00072 in scientific notation is:
3. Which statement is correct?
LAST-MINUTE REVISION
Unit
Number + standard reference.
Number + standard reference.
Accuracy
Close to accepted value.
Close to accepted value.
Precision
Repeated values agree.
Repeated values agree.
Dimensions
Both sides must match.
Both sides must match.
VISUAL REVISION NOTES · CLASS 11 PHYSICS
Units and Measurements
Measure clearly. Calculate carefully. Check intelligently.
1. Physical quantity
Every answer needs a number + unit. Writing only “5” is incomplete.
5 m · 20 s · 2 kg
2. SI base units
Length · mMass · kgTime · sCurrent · A
Derived example: force = kg m s^-2
3. Powers of ten
kilo = 10^3
milli = 10^-3
micro = 10^-6
nano = 10^-9
4. Accuracy vs precision
Accuracy → close to accepted value
Precision → repeated readings agree
Precise readings can still be inaccurate if an instrument has a systematic error.
5. Significant figures
- • Non-zero digits are significant.
- • Leading zeros are not significant.
- • Match the least precise input.
2.34 × 1.2 = 2.8
two significant figures
two significant figures
6. Dimensions check
Both sides of a valid equation must have the same dimensions.
[F] = [M][LT^-2] = MLT^-2
Worked check
Is s = ut + 1/2 at^2 dimensionally valid?
[ut] = [LT^-1][T] = [L] and [at^2] = [LT^-2][T^2] = [L]. Both terms are displacement.
Avoid these mistakes
- ✕ Dropping units in the final answer
- ✕ Treating accuracy and precision as the same
- ✕ Keeping every calculator digit
- ✕ Using dimensions to find a numerical constant
VISUAL NOTES
Units and Measurements at a glance
Physical quantities
Write every measurement as a number with a unit.
Measurement quality
Separate accuracy, precision, errors and significant figures.
Dimensions
Use dimensions to verify the physical form of an equation.
Formula / fact focus
Force dimensions
[F] = MLT^-2
Last-minute memory cues
- 1. ✓SI base units
- 2. ✓significant figures
- 3. ✓dimensional analysis
- 4. ✓unit conversion
NEXT IN THIS CHAPTER
Practise this chapter with its complete 80-question MCQ bank, then use the video and revision notes to strengthen any weak areas.