Physical Quantities and Units Flashcards
What is a physical quantity
All physical quantities consist of a numerical magnitude and a unit
To represent a physical quantity, it must contain both a numerical value and the unit in which it was measured
e.g. speed, velocity, volume
Estimate the mass of an adult
Estimate gravitational field strength
Estimate the mass of a car
Estimate the wavelength of visible light
Mass of an adult = 70 kg
Gravitational field strength, g = 10 m s-2
Mass of a car = 1500 kg
Wavelength of visible light = 400 nm (violet) – 700 nm (red)
Name all the SI base units
mass = kg
length = m
time = s
current = A
temperature = K
amount of substance = mol
Derived unit of force
Derived unit of energy
Derived unit of pressure
The Newton (N), the unit of force, is defined by the equation:
Force = mass × acceleration
N = kg × m s–2 = kg m s–2
Therefore, the Newton (N) in SI base units is kg m s–2
The Joule (J), the unit of energy, is defined by the equation:
Energy = ½ × mass × velocity2
J = kg × (m s–1)2 = kg m2 s–2
Therefore, the Joule (J) in SI base units is kg m2 s–2
The Pascal (Pa), the unit of pressure, is defined by the equation:
Pressure = force ÷ area
Pa = N ÷ m2 = (kg m s–2) ÷ m2 = kg m–1 s–2
Therefore, the Pascal (Pa) in SI base units is kg m–1 s–2
How to check the homogeneity of equations
To check the homogeneity of physical equations:
- Check the units on both sides of an equation
- Determine if they are equal
- If they do not match, the equation will need to be adjusted
Powers of 10
tera = 10^12
giga = 10^9
mega = 10^6
kilo = 10^3
centi = 10^-2
milli = 10^-3
micro = 10^-6
nano = 10^-9
pico = 10^-12
What is a scalar and vector quantity?
Scalar is a quantity that only has magnitude
Vector is a quantity that has magnitude and direction
How to do the triangle method of adding vectors
To combine vectors using the triangle method:
Step 1: link the vectors head-to-tail
Step 2: the resultant vector is formed by connecting the tail of the first vector to the head of the second vector
How to do the parallelogram method of adding vectors
Step 1: link the vectors tail-to-tail
Step 2: complete the resulting parallelogram
Step 3: the resultant vector is the diagonal of the parallelogram
Coplanar forces
forces that act on the same plane
Conditions for equilibrium in vectors
In equilibrium, these are closed vector right-angled triangles. The vectors, when joined together, form a closed path
How to calculate vertical and horizontal component of vectors
For the horizontal component, Fx = Fcosθ
For the vertical component, Fy = Fsinθ
True value
a perfect measurement which reflects quantity being measured with no errors
Uncertainty
an estimate of the difference between a measurement reading and the true value
- Random and systematic errors are two types of measurement errors which lead to uncertainty
Random error
Random errors cause unpredictable fluctuations in an instrument’s readings as a result of uncontrollable factors
This affects the precision of the measurements taken
To reduce random error: repeat measurements several times and calculate an average from them
Systematic error
Systematic errors arise from the use of faulty instruments used or from flaws in the experimental method
This type of error affects the accuracy of all readings obtained
To reduce systematic errors: instruments should be recalibrated or the technique being used should be corrected or adjusted
Zero error
a type of systematic error which occurs when an instrument gives a reading when the true reading is zero
Precision and accuracy
Precision: how close a measured value is to each other
Accuracy: how close a measured value is to the true value
Absolute uncertainty
where uncertainty is given as a fixed quantity
Fractional uncertainty
where uncertainty is given as a fraction of the measurement
Percentage uncertainty
where uncertainty is given as a percentage of the measurement
percentage uncertainty = absolute / measured x 100%
