1.1
physical quantities
1.1.1 physical quantities and units
in physics, every measurable quantity is described using two parts. one part tells us how large the quantity is, and the other part tells us the standard used to measure it.
- a physical quantity always has a numerical magnitude
- it must also have an associated unit
- the numerical value gives size, the unit gives meaning
common examples include:
- length measured in
m - mass measured in
kg - time measured in
s - electric current measured in
a
1.1.2 estimating physical quantities
estimation is used when an exact measurement is unnecessary or impossible. instead of precise values, sensible approximations are used.
- estimation depends on basic real-world experience
- reasonable values are chosen and substituted into equations
- the final answer is an approximate calculated result
useful reference values
- the resistivity of copper is about
1.68 × 10−8 Ω m - the young modulus of steel is about
1.9 × 1011 pa
estimating density using water
water is often used as a reference substance when estimating density. by deciding whether an object floats or sinks, its density can be compared to that of water.
- objects that float have a density lower than water
- objects that sink have a density greater than water
- density of water is approximately
1000 kg m−3 - density of water is approximately
1 g cm−3
during estimation, values are often rounded to 1 significant figure
to simplify calculations.