What does f-mg = ma mean?

Newton's second law f = ma (F is the resultant force on an object, M is the mass of the object, and a is the acceleration of the object in motion)
The formula of f-mg = ma body in free fall~
That's about it==~

Σ f = ma or F combination = ma
What does f stand for and what does Ma stand for

It means that the resultant force of various forces (such as gravity, friction and elastic force) on an object is equal to its own weight m times its own acceleration a

On the mass in F = ma
A 20kg object is pulled by F = 40n on the ground, and a 20kg rectangular steel block is placed on it. The object Accelerates along the plane, a = 0.5m/s2. When calculating the friction force F = f-ma, why is the mass calculated by 20kg instead of 40kg
Why is it 20kg? It's not 40kg as a whole. What's the friction force if 40kg is used? Detailed reasons. I'm not particularly satisfied with the above answers

According to the force analysis, the resultant force of the object is f-f. according to the second law of Newton, the resultant force of the object is Ma, so f-f = ma. Ma is the force of the object, so the mass is 20kg
If the friction is 40kg, it is the friction of the ground to the object

A problem about Coulomb's law
A small ball with mass m carries the same amount of electric charge. It is suspended at a point by a line of length L. when it is in equilibrium, the two suspension lines and the vertical direction form an angle A. the solution is as follows: 1) the tensile force on the suspension line; 2) the amount of electric charge on each small ball

T=mg/cosA
F=mgtanA=kq^2/(2lsinA)^2
Q = 2lsin a (mgtana / R)

If one of the two point charges keeps a certain distance in vacuum increases by 12, but the interaction force between them remains unchanged, the electric quantity of the other point charge must decrease ()
A. 12B. 13C. 14D. 124

According to the formula F = kq1q2r2 of Coulomb's law, if the Coulomb force remains unchanged, then Q2 ′ = 23q2, that is, the other charge decreases by 13. Therefore, B is correct and a, C and D are wrong. Therefore, B is selected

What is tangent length theorem?
What is the tangent length? What does it mean to make two tangents of a circle from a point outside the circle? Because my textbook didn't teach?

Two tangent lines of a circle from a point outside the circle are equal in length. The line between the center of the circle and this point bisects the angle between the two tangent lines
Two tangents of a circle leading from a point outside the circle point to this point and make two tangents tangent to the circle respectively (there is one on both sides). In other words, they are two straight lines perpendicular to the radius of the tangent point

PA and Pb are tangent to a and B respectively, and intersect with the tangent of o at C and D respectively. PA = 7cm is known
1. Calculate the perimeter of △ PCD. 2. If ∠ P = 46 °, calculate the degree of ∠ cod
(connect OC and OD after drawing)

1. Let CD tangent circle o be at point E, then CA = CE, DB = De
CΔPCD=PC+PD+CD=PC+PD+CE+DE=PC+CA+PD+DB=PA+PB=2PA=14cm
2. ∵ the inner angle of the quadrilateral is 360 ° and ∠ Pao = ∠ PbO = 90 ° and ∵ AOB = 134 °
∵ CA = CE and Ca ⊥ OA, CE ⊥ OE, ∵ OC bisection ∠ AOE, similarly, OD bisection ∠ BOE
∴∠COD=1／2∠AOB=67°

A topic of tangent length theorem in the third grade of junior high school
As shown in the figure, point I is the inner center of △ ABC, and point O is the outer center of △ ABC. If ∠ BOC = 140 °, calculate the degree of ∠ BIC

Angle BIC = 140

Proof: the inverse theorem of tangent length theorem
As shown in the figure (of course, it will be clearer after clicking this figure to open)
It is known that O is the center of ⊙ o, a and B are on the circle, PA = Pb, ∠ apo = ∠ BPO
Verification: OA ⊥ AP, ob ⊥ BP
PS. if the certificate is not available, please explain the reason and add some high school knowledge. If it is true, the score will still be given
The symbol provides:
Vertical ⊥ parallel ∥ angle ⊙ circle ⊙
First, give 100 points, and then give 100 points for a good answer (write down the detailed answer process)

&It is known that O is the center of ⊙ o, a and B are on the circle, OA ⊥ AP, ob ⊥ BP & nbsp; to prove: PA = Pb, ⊥ apo = ⊥ BPO & nbsp; above is the "tangent length theorem", which is true of course

The meaning of element symbol
Microcosmic: express?
On the macro level: how to express?

Microcosmically, it represents atoms and some molecules, such as he;
Macroscopically, it represents elements, some chemical formulas, such as Na

What does f-mg = ma mean?