D1

point mass/gravitational field - assumption of gravitational force: The objects are point mass.

Kepler first law: Each planet orbits the Sun along an elliptical path. The Sun is located at a focus point of the elliptical orbit.

Kepler second law: Joining the line between the planet and the sun, the area swept is the same in equal time

Kepler third law: \(\text{period}^2 \propto \text{distance}_{\text{between the sun and the planet (average)}}^3\)

SI unit of gravitational field strength = acceleration (\(m/s^2\))

![[_resources/Pasted image 20251112105258.png]]

keywords: what is centripetal force: perpendicular force against velocity -> change in direction

centripetal force是合力

D2

==conventional current’s direction is in reverse of electron==

electric charge: property of matter; conserved; quantised quantity

conductor: lots of free electrons insulator: few free electrons

methods to charge a body: - friction - electrostatic induction 感应起电！

Coulomb law (in formula booklet)

electric field strength (vector): force experienced per unit charge by a positive charge - electric field lines

The Milikan oil drop experiment
![[_resources/Pasted image 20260222211236.png]]
conclusion: charge is quantised quantity / discrete

magnetic field magnetic flux density: vector of magnetic field

magnetic field lines around
![[_resources/Pasted image 20260222220339.png]]	magnetic bar
![[_resources/Pasted image 20260222220435.png]]	straight wire carrying current
![[_resources/Pasted image 20260222220454.png]]	solenoid
![[_resources/Pasted image 20260222220631.png]]
![[_resources/Pasted image 20251112122247.png]]	two wires scenerio

direction: out of paper(dot) or into paper(X, 屁眼)

牛逼定律：右手大拇指(正方向)指向N（当你觉得一个人很NB，竖起大拇指指向NB！），而且大拇指永远是正粒子移动方向，因为竖起大拇指是正能量！

right hand rule (this is the way)

![[_resources/Pasted image 20260222221440.png]]

given planet radius and density ratio, 老老实实算质量和radius