The trolley moves in a straight line with uniform acceleration to the right, and the block m sticks to the left wall of the trolley, and is relatively static with the left wall. When the acceleration of the trolley increases, the following statement is correct 1. The friction force on the block remains unchanged; 2. The elastic force on the block remains unchanged; 3. The friction force on the block increases; 4. The external force on the block increases

The trolley moves in a straight line with uniform acceleration to the right, and the block m sticks to the left wall of the trolley, and is relatively static with the left wall. When the acceleration of the trolley increases, the following statement is correct 1. The friction force on the block remains unchanged; 2. The elastic force on the block remains unchanged; 3. The friction force on the block increases; 4. The external force on the block increases

1、4
The static friction on the block is always equal to gravity;
When the acceleration of the block and the car increases, the external force becomes larger

When the car moves at a uniform acceleration a, an object a can just slide down along the vertical wall of the car at a uniform speed, and the elastic force between the object and the wall of the car is? Object and car
What is the dynamic friction factor?

It can slide down the vertical wall of the car at a constant speed
The acceleration of the object is also a. the force analysis shows that the horizontal direction of the object only receives the elastic force from the car
Let the mass of object a be m
Then the elastic force FN = ma
Because of the uniform sliding speed, the force in the vertical direction is balanced
Mg=μFn=μMa
So the dynamic friction coefficient μ = g / A

If 1 / r = 1 / R1 + 1 / R2, then. A: r = R1 + R2, B: r = R1 + R2 / R1 + R2, C: r = r1r2 / r2 + R1, D: r = r1r2 / R1-R2 ask for the help of God
/It's a division sign

C：R=R1R2/（R2+R1）

A problem of maximum coefficient in binomial expansion
The method in the book is to set up a system of inequalities and solve the terms greater than or equal to the two adjacent terms. But this method defaults that the sequence of coefficients of each term is first increased and then decreased, but why? It can't directly show that there are no multiple non adjacent solutions. The binomial coefficient increases first and then decreases. That's right, but when it is multiplied by another sequence, it doesn't have to be!
I have doubts about the principle of this method!
Please read the question carefully before you answer it!

1. The largest binomial coefficient is the middle term or the middle two terms;
2. The term with the largest coefficient is not necessarily the term with the largest binomial coefficient, so it is solved by using the coefficient of T (R + 1) greater than or equal to the coefficient of T (R) and the coefficient of T (R + 1) greater than or equal to the coefficient of T (R + 2)
3. Binomial coefficient and coefficient are two things!

How to calculate a + B = 5, ab = 3 for a + B

(Wuhan, 2007) if 2 is a root of the quadratic equation x2 = C, then the constant C is ()
A. 2B. -2C. 4D. -4

Substituting x = 2 into the equation x2 = C, we can get C = 4

If the solution of the equation 3 (AX-1) = 3x-2 about X is negative, then the value range of a is zero
RT

3(ax-1)=3x-2
3ax-3=3x-2
3ax-3x=1
(3a-3)x=1
x=1/(3a-3)
The solution is negative
x

A1.a2.a3 is an n-dimensional vector. The vector group A1 + a2.a2 + a3.a1 + a3 is linearly independent. It is proved that the vector group a1.a2.a3 is linearly independent

How to solve 1.5x + x = x + 12?

1.5X+X=X+12
2.5X－X＝12
1.5X＝12
X＝12÷1.5
X＝8

To explore the real number root of the equation (b-X) ^ 2-4 (A-X) (C-X) = 0 about X (a, B, C are all real numbers)

1. Because (A-X) ^ 2-4 (b-X) (C-X) = 0, so 3x ^ 2 + (2a-4b-4c) x + (4bc-a ^ 2) = 0, so () 1 = (2a-4b-4c) ^ 2-12 (4bc-a ^ 2) = 16 [a ^ 2 - (B + C) a + (b ^ 2 + C ^ 2-bc)], Let f (a) = a ^ 2 - (B + C) a + (b ^ 2 + C ^ 2-bc), so (discriminant) 2 = (B + C) ^ 2-4 (b ^ 2 + C ^ 2-bc) = - 3 (B-C) ^ 2 =