Numbers, Units, and Measurement
Why Units Matter
Measurement in physics
Physics begins with measurement. To compare ideas with reality, we need numbers and units, such as meters for length, seconds for time, and kilograms for mass.
Key idea
A number without a unit is incomplete in physics. means nothing by itself; and are very different.
Base quantities
Common base quantities include length , time , mass , temperature, electric current, and more. Many other quantities are built from these.
Common units
| Quantity | SI unit |
|---|---|
| Length | meter |
| Time | second |
| Mass | kilogram |
| Speed | meter per second |
Which quantity is measured in meters per second?
Speed is distance divided by time, so its unit is .
Correct answer: Speed
Why do physicists always include units with numbers?
Units give meaning to numerical values.
Correct answer: Because numbers without units are ambiguous and cannot be compared correctly.
Order of magnitude
When exact values are not necessary, physicists often estimate using powers of ten. For example, means one thousand, and means one thousandth.
What does represent?
A negative exponent means a reciprocal power of ten, so .
Correct answer: 0.001