UY1: Basics For Mechanics

This post serves to provide you with the basics for the learning of Mechanics. The content on this page should be familiar to you.

Quantities & Units

Experiments require measurements of physical quantities. Every measurement give a number whose value depends on the units that goes with it.

There is a need to agree on certain important basic physical quantities and standard units. However, it is unavoidable that there are other units than the SI units.

If you are part of a policy making group in the non-SI countries, please push for the use of SI units in your local scientific establishment.

Standard units evolve over time from physical items to atomic and invariant standards, such as the frequencies of certain atomic transitions, speed of light.

Seven Base Quantities & Units

LengthmDistance light traveled in vacuum for $\frac{1}{299792458}$ seconds
MasskgMass of a specific platinum-iridium alloy
Times9192631770 cycles of radiation of cesium-133
CurrentAThat flows in 2 parallel wires resulting in force of $2 \times 10^{-7} \, \text{N m}^{-1}$ on the wires
TemperatureK$\frac{1}{273.15}$ of thermodynamic temperature of triple point of water
Amount of substancemolThat contains equal number of fundamental entities as 0.012 kg of carbon-12
Luminous intensitycdOf a source that emits monochromatic radiation of frequency $540 \times 10^{12}$ Hz and that has a radiant intensity of $\frac{1}{683}$ watt/steradian

Unit Prefixes

Physics deal with quantities that have values that spans over many orders of magnitudes.

Some of the Greek prefixes and their symbols to indicate decimal sub-multiples and multiples of the SI units are:

Number Prefix
$10^{-9}$nano (n)$10^{-1}$deci (d)
$10^{-6}$micro ($\mu$)$10^{3}$Kilo (K)
$10^{-3}$milli (m)$10^{6}$Mega (M)
$10^{-2}$centi (c)$10^{9}$Giga (G)

Scientific Notation

Scientific notation bypasses the use of prefixes. For instance, 734 mm can be written as $7.32 \times 10^{-1}$ m.

Uncertainty In Measurements

Uncertainty in measurement depends on:

  • the quality of the apparatus;
  • skill of the experimenter (or robustness of the methodology employed);
  • number of measurements performed.

There is a constant NEED for an accurate and precise measurement.

Systematic and random errors:

  • Systematic: Repeatable measurements but could differ from instruments to instruments, or with different methods. Averaging with the same instrument do not help.
  • Random: Deviations due to conditions that do not remain the same. Averaging helps.


Next: 1-D Kinematics

Back To Mechanics (UY1)

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