How many Types of Force are there in a classical mechanical system?

Real Forces (Forces that must be considered first)
1) Pressure
2) Tension
3) Gravitational Force
4) Frictional Force (Results from pressure that causes surface molecules to collide)

Imaginary Forces (Exists only because real forces exist)
1) Derived Forces (Gravitational Force must be seperated into 2 directions at a frictionless slope, and itself perished)
2) Centripetal Force

Opposing Force (Must act on different objects and must be opposite in direction)
1) Pressure - Pressure
2) Tension - Tension
3) Gravity - Attract - Attract
4) Friction - Rub - Rub

Will an astronaut feel an opposing force if
he stands on the ground and pushes an orange?
he floats in space and pushes an orange?
he floats in space and pushes the moon?
He wont feel an opposing force when he is at space, but he will accelerate at the opposite direction of the force he apply, which maybe will make him think that he successfully pushes the moon.

External Force and Internal Force
1) External Force is the force that is not paired up within the system, and will eventually cause the system to accelerate or if the external forces sum to 0, the system stays still.
2) Internal Force will never cause any changes to the "SYSTEM", in terms of displacement, velocity, acceleration, or energy.
3) If something does not move or is in an equilibrium, the external force must always be 0. (Applies even to water)[Pressure and Gravity is the 2 forces that shapes all liquids]

Physic Questions

External Force is the force that is not paired up and thus is the only one that can make the system suffer changes, or maybe not suffer changes if the dot product of the external forces sum to 0.

Gravitational Force MUST BE CHANGED to 2 resultant force at a frictionless slope,
ORIGINAL Gravitational Force MUST BE DELETED.

 Why vectors and trigonometry can be apply to Forces, Acceleration, Velocity, Displacement?
They are all derived from Displacement, which is metres. (PERSONAL OPINION)

What keeps us standing?
Gravity and pushing force from ground which sums to 0.

What keeps water surface constantly parallel to xxx?
Gravity + pushing force from water molecules that sums to 0.

Why water going down a slope will become diagonal?
the pushing force of water molecules at the direction of motion becomes 0.

If an astronaut pushes the moon/an orange in space, will he feel pressure?

What is the cause that we feel our own weight?
We dont feel our own weight, we feel the pressure exherted by the ground to our feet.

What actually increases when the lift rises up with an acceleration?
The force used to push us up is increase, our weight does not, the resultant force

Force and Motion

These kind of questions always frustrate me.

The upper scale shows 5N Initially, and when 5N of force is exherted at the bottom, the upper scale shows a reading of 5N but the lower scale shows a reading of 10N. Why is it so?

For these type of problem,
1)Every force across one rolling wheel as an individual force.
2) Newton's Third Law when a string is pulling something
3) Seperate everything you can into free body diagrams.

Fundamentals of rolling wheels,
1) A 180degree pull with one rolling wheel ----> Force does not change, only changes direction
2) A 0degree straight pull with the string at the other end attach to something, draw out all the forces individually and consider the total force at each direction. (Dirty Trick : use logic to find out the factor of the length of the string to be pulled compared to a 180degree pull)

Dot, Line, Plane, Body

What can we actually represent with 1D,2D and 3D?

For 1 Dimension,
1 Axis
1 Dot = Dot on a line
2 Dot = Line on a line

For 2 Dimension
2 Axis
1 Dot = Dot on a plane
2 Dot = Line on a plane
>= 3 Dot Area on a plane

For 3 Dimension
3 Axis
1 Dot = Dot on a Body
2 Dot = Line on a Body
3 Dot = Area on a Body
>=4 Dot = Volume on a Body

AC Circuit Analysis, RL,RC,RLC circuits

Impedence (Z)
A way to represent the reactance + resistance in an ac circuit in order to do calculations for V=IR
Note : j stands for imaginary (square root of -1)

RC circuit :
Z = 1/RCw j
RL circuit :
Z = 1/RLw
RLC circuit :
[to be added]

There are many ways to represent V=IR in an ac circuit Z are always used to replace R in ac circuit analysis and the calculation of the imaginary part is always seperated from the calculation of the real part.

1) V = Vmax [cos(wt + x)+sin(wt+x)]
2) V = ax + b
3) V = Vmax (e^(wt+x))
4) V = Vmax _wt+x

Kirchoff's Rule, Superposition, Thevenin's and Norton's Theorem

I was in this PALSI meeting where senior students guide the freshmens on homework and everything, trying to understand the connection between these 4 rules when people are trying to say, " Its enough that you know how to do it, just learn how to do it and get the right answer.". I was like "WTF? Are you guys even learning physics at all?"

It took me quite not a few hours to prove that they are related in some manner.

Kirchhoff's Rule is powerful enough to solve almost all circuits with batteries and resistors, how about if a current source is present?

From "Basic Electronics For Scientist" I get to know that alot of the formulas we get will stack with each other, so we must any how determine which one is useless and eliminate it. A common way to get through this is not to loop 2 loops individually that originate from the same battery

We just ignore the current source when we are calculating Total V in a specific loop, any current source that is encountered can be ignored.

200V = 47I3 + 4I1 + 23R4

And for The 2nd law of Kirchhoff where current never changes at any Nodes,
20A + I1 = I4