Which of the two definitions of force, f = Ma; F = DP / DT, is accurate? The second one is used by default in special relativity. Why? From the derivation of F = DP / DT, when the direction of force is not consistent with the direction of initial velocity, there is a certain difference between the direction of acceleration and the direction of force, so f = ma is wrong, even if it is instantaneous. I mean in special relativity

"When the direction of the force is not consistent with the direction of the initial velocity, there is a certain difference between the direction of acceleration and the direction of the force". In Newtonian mechanics, for a single object (with constant mass), when you say this, first reflect on yourself (do you think that F = DP / dt does not hold in high school uniform circular motion?) as for the theory of relativity, it is f = DP / dt, And Newton's earliest formula was f = D (MV) / dt
If you put f = DP / dt in another form, it is FDT = DP, which is the theorem of momentum. Now it is generally believed that conservation of momentum and conservation of energy always hold as the embodiment of space-time symmetry
There is a lot to say about this, but in the final analysis, whether it is Newton's genius or Einstein's strong physical intuition, or the symmetry of time and space, it is not equal to the fact that the conclusion of special relativity after using this formula is consistent with the experiment

What is the meaning of F = ma of traction force

It's not just a formula for traction, it's a formula derived from Newton's second law
F is the force, M is the mass of the object, and a is the acceleration of the object with mass m under the action of force F

How to calculate the effective value of sinusoidal alternating current

If it is sinusoidal alternating current, simply divide the peak by 1.414 or multiply by 0.707 to get the effective value

Why is the average value of alternating current twice the root of the effective value

Wrong, the maximum is twice the root of the RMS. RMS = √ 2 / 2 max, average = 1 / π max, Effective value: the work done by alternating current in half a cycle is the same as that of direct current with effective value. As for how to get the above relationship, you need to calculate the integral. You should be a high school student, so you don't need to know the intermediate process for the time being, Just know their physical meaning

Why is the ratio of the maximum value and the effective value of the sinusoidal AC voltage root 2?

This is calculated according to the definition of effective value. If an AC voltage is applied to a resistor, at a certain time (take a cycle), the energy consumed by it is the same as that consumed by adding DC voltage UDC, then the effective value of the AC voltage U = UDC
Because the power of alternating current is variable, the energy consumed should be calculated by integral, so the effective value is equal to 1 / 2 of the root of the maximum value

Is the effective value of cosine AC equal to the maximum divided by the root 2

Yes. High school physics knowledge!

Why is the ratio of the maximum value and the effective value of the sinusoidal AC voltage root 2? It seems that it can be proved by the method of mathematical integration,

I hope to give you some points

Why is the relationship between the maximum value and the effective value in alternating current root 2

Let a periodic current I (T) pass through a resistance R. because the current is variable and the instantaneous power I ^ 2R is different, the heat generated in a very short time DT is I ^ 2rdt, and the heat generated in a cycle T is ∫ t I ^ 2rdt. If the current passes through a resistance R, the magnitude of the DC current generating equal heat after time t is I,
Then ∫ t I ^ 2rdt = I ^ 2rt,
The effective value of current I = [(1 / T) ∫ t I ^ 2DT] ^ (1 / 2)
For sinusoidal quantities, let I (T) = imsin (WT + ∮)
I={1/T∫T Im^2SIN^2(wt+∮)dt}^(1/2)
Because sin ^ 2 (WT + ∮) = (1 / 2) [1-cos ^ 2 (WT + ∮)]
So I = {(IM ^ 2 / 2t) ∫ t [1-cos ^ 2 (WT + ∮] DT} ^ (1 / 2)
={Im^2/2T[t]T}^(1/2)
=(Im^2/2)^(1/2)
=Im/[2^(1/2)]=0.707Im

Derivation of several typical effective value calculation formulas of alternating current

The effective value of alternating current is defined according to the thermal effect of current. Let alternating current and direct current respectively pass through the resistance with the same resistance value, if they produce equal heat in the same time, This DC value is called the AC effective value. That is to say, the average energy effect of AC current is replaced by DC which has the same thermal effect as AC current, But when calculating the RMS of non sinusoidal alternating current (such as rectangular wave, sawtooth wave, etc.), the RMS formula of sinusoidal full wave alternating current can not be applied, and can only be solved according to the definition of RMS, In order to deepen the understanding of the definition of the effective value of the alternating current. 1. The calculation formula of the effective value of the sine wave alternating current. 1, Because the magnitude and direction of the alternating current change periodically with time, the effect at different time is generally different, that is, the instantaneous power can not reflect the actual effect of the alternating current, The average power is usually used to describe the average effect of alternating current in a period. The current of sinusoidal alternating current (I = imsin ω T) and that of pure resistance R are (3 pages in total) [read on]

Finding the effective value of alternating current
The effective value is the maximum value multiplied by the root 2, but how to calculate if the frequency is different?
Sorry, wrong number. The effective value must be smaller than the maximum value. It should be the maximum value divided by the following number 2,

The magnitude of RMS is independent of frequency
That is to say, whether it is 50 Hz AC or 100 Hz AC, its effective value is the maximum value divided by the root sign 2
As for the reason, we need to use the knowledge of calculus to explain, so don't go into it