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Advance Physics For You: Mechanics I - Coggle Diagram
Advance Physics For You: Mechanics I
1: Basic Ideas
SI Base Units internationally agreed up
Name | Base unit
time t | second s
length l | metre m
mass m | kilogram kg
temperature T | kelvin K
electric current I | ampere A
amount of substance n | mole mol
luminous intensity | candela cd
Derived Units
Are derived by combining base units
E.g. speed = Distance/Time = ms^-1
Homogeneity of equations
The units on each side of the equation i.e. derived
and base must be the same
E.g. Kinetic energy (kg m^2 s^-2)
Base units 1/2 * (m s^-1)^2
Note: 1/2 is a pure number with not units
Prefixes
Use to summaries very large or very small
numbers
Prefix | Symbol | Multiplier
tera | T | 10^12
giga | G | 10^9
mega | M | 10^6
kilo | k | 10^3
deci | d | 10^-1
centi | c | 10^-2
milli | m | 10^-3
micro | µ | 10^-6
nano | n | 10^-9
pico | p | 10^-12
femto | f | 10^-15
Significant figures
Any calculation involving those measurements
will only be accurate to one significant figure
more that the least number of significant figures in any measurement
Alternatively, if some figures are given less precisely than others, then round to the lowest number of significant figures
Vectors and scalars
Vectors have magnitude (represent as length of arrow) and direction (represent as angle or N, E, S, W direction)
Scalers have only magnitude
Scalars | Vectors
distance | displacement
speed | velocity
mass | weight
pressure | force
energy | momentum
temperature | acceleration
volume | electric current
density | torque
Adding vectors involves taking account of direction
unlike scalars.
E.g. Adding 2 forces or velocities in the same direction results in a larger force called the resultant.
Resultant = F1 + F2
E.g. Adding 2 forces or velocities with one in the opposite direction involves changing the sign before adding
Resultant = F1 + (-F2)
For vector arithmetic:
Up and right are taken by convention as positive
Left and down are taken by convention as negative
Change in vector or scalar quantity is:
New - Old
The resultant of 2 vectors acting perpendicular to each
other can be found by:
Magnitude using Pythagoras theorem:
x^2 + y^2 = r^2
The direction (angle to the horizontal) can be found by:
tanθ = opposite/adjacent = vector 1/vector 2
θ = inv tan(vector 1/vector 2)
Vector drawing
With more than 2 vectors the resultant is worked
out by linking each vector head to tail then joining a line from start to end point (for addition of vectors).
For subtraction of vectors draw the negative vector in reverse and join head to tail
This technique also works with finding the resultant using the Pythagorean technique for forces acting perpendicular to each other
Resolving vectors
A resultant can also be broken down into it's component vectors by using the principles of trigonometry
Opposite (Vector 1), Adjacent (Vector 2) can be resolved by:
Sinθ × Hypotenuse = Opposite
Cosθ × Hypotenuse = Adjacent
The
resultant force of multiple vectors
acting on
a single point can also be resolved using same principles e.g. 3 vectors acting on a single point:
Resolve non-right-angled forces into horizontal and vertical forces
Get rid of the non-right-angled force.
Combine the resolved horizontal and vertical forces with any other horizontal and vertical force
Finally, find the resultant by combining horizontal and vertical vectors
2: Looking at Forces
Objects interact by forces
Three main forces in the universe:
Gravitational: Objects with mass exert gravitational forces upon other objects. Weight (different from mass) is the effect of gravitational forces acting downwards and is given by
W = mass × g
Note: g and mass are scalar
g is the gravitational field strength and is the force acting on 1 kg of mass
Electromagnetic: Interaction between negative and positive charges. Act at the atomic scale holding atoms and molecules together. Also, electromagnetic forces play a big role at the macro scale repulsion between same type of charge e.g. electrons in your hand and on a trolley repel each other creating motion
Nuclear (weak and strong): The strong nuclear forces hold together protons in the nucleus.
Forces from the basic forces:
Normal contact forces: Occurs between solid objects. Stops objects from merging into each other due to electromagnetic forces. Always acts perpendicular to surfaces.
Tension force: attractive forces cause objects to be elastic and try and pull back or repulsive forces cause objects to push back when squeezed together. This again is due to the attractive and repulsive nature of electromagnetic forces
Frictional forces: Is a type of resistance to motion. It always acts to the opposite direction of motion and parallel along the two objects surfaces. It is the force required to overcome the contact points between two objects. Without friction would be impossible to walk!
Motive force: Is a force that drives something forward due to friction. It can be considered primary to the say forces occurring in the engines to move a car forward for example because without friction the wheels will just spin without any motion
All forces come in pairs but act on
different objects (Newtons 3rd law i.e. equal and opposite reaction)
Fluid resistance
Friction in fluid is called drag and occurs between the molecules of the fluid and the solid surface
Factors affecting drag:
i) Speed (more molecules moving past each other per second)
ii) Viscosity (how easy the fluid flows). Could include air too. Drag increases with higher viscosity
iii) Temperature. High temperature in liquids resorts in less viscosity i.e. the heat increases speed of molecules and therefore decreases intermolecular forces. But in gases it's the opposite as the heat actually increases intermolecular collisions
3: Turning Effects of Forces
4: Describing Motion