What is the formula for maximum static friction and sliding friction?

What is the formula for maximum static friction and sliding friction?

Ff=u*Fn
(1) FF: maximum static friction
u: Maximum static friction coefficient
FN: positive pressure
(2) FF: sliding friction
u: Dynamic friction coefficient
FN: positive pressure

The formula of friction
What are the formulas for static friction and sliding friction

Sliding friction formula F = UN
Where n is the pressure, n = mg at level ground
U is the sliding friction coefficient, which is related to the material
friction
1. Friction
(1) Definition: two objects in contact with each other, when they are about to move or have moved relative to each other, will produce a force on the contact surface to hinder the relative movement, which is called friction
(2) There are three conditions for friction between objects
First, objects contact and squeeze each other
Second, the contact surface is not smooth
Third, there is a tendency or relative motion between objects
2. Sliding friction
(1) Definition: when an object is sliding on the surface of another object, the friction is called sliding friction
(2) An experiment to study the relationship between the sliding friction and which factors: why do we use the spring scale to pull the wood block to move in a uniform and straight line? This is because the spring scale measures the tensile force rather than the friction force. When the wood block moves in a uniform and straight line, the horizontal tensile force of the wood block and the friction force of the board against the wood block are a pair of balance forces, A large number of experiments show that the sliding friction is only related to the pressure and the roughness of the contact surface. The greater the pressure is, the greater the sliding friction is; the rougher the contact surface is, the greater the sliding friction is
(3) Sliding friction is the force that hinders the relative movement of objects in contact with each other, not necessarily the force that hinders the movement of objects, It should be clear that the object in contact with each other is used as the reference to hinder the "relative motion". The "object motion" may use other objects as the reference. For example, in the experiment, when a weight is placed on the wood block and the spring scale is used to pull the wood block to move at a uniform speed in a straight line, The specific situation is: when the wood block is pulled forward from static, the weight will slide backward relative to the wood block, and the wood block will give the weight a friction force to prevent it from sliding backward. The direction of the friction force is forward. Therefore, the weight does not slide relative to the wood block, The friction at this time is static friction
(4) The sliding friction has nothing to do with the speed of motion and the contact area between objects
(5) When studying practical problems, in order to simplify, we often adopt the "idealized" method. For example, if one object is placed on the smooth surface of another object, the "smooth" means that there is no friction between two objects when they are in relative motion
3. Static friction
Static friction is produced by the interaction of two mutually stationary objects. The direction of static friction can be the same as or opposite to the direction of force
4. Rolling friction
When an object rolls on the surface of another object without sliding or tends to roll, the rolling friction is caused by the deformation of two objects in the contact part

A formula for calculating friction in Physics
What about the unit?

If you're only in grade three, you don't need to deal with the formula of friction. I'm also in grade three. It's not very difficult to solve the problem of friction because the explanation of force is too clear, which often involves high school knowledge, including parallelogram rule, f = UN, etc, And junior high school often out of the question type is often very simple, nothing more than a uniform linear motion of an object, known traction to strive for friction. Or use how to improve or reduce friction to a few short answer questions, are not very difficult, so don't worry, as long as the things in the book review thoroughly, there will be a great improvement
————————————————————To solve the next problem, because friction is a kind of force after all, and the unit of force is cattle, so the unit of friction is cattle
————————————————————Finally, I wish you progress in your study!

A super difficult high school mathematics problem
It is known that a + B + C + D + e = 8, the square of a + the square of B + the square of C + the square of D + the square of E = 16 (a, B, C, D, e, f are real numbers)
Find the maximum value of E
Hope to provide convincing reasons

Is it very simple? Anyone who has studied high school competition should be able to do it
Firstly, we know that when e is the largest, e must be greater than 0 and e must be less than 4
Then use Cauchy: (a ^ 2 + B ^ 2 + C ^ 2 + D ^ 2) * (1 + 1 + 1) > = (a + B + C + D) ^ 2
The inequality about e is obtained: (16-e ^ 2) * 4 > = (8-e) ^ 2
Solution and substitution
We obtain the condition that e is 16 / 5 if and only if a = b = C = D = 6 / 5

An electric iron is marked with 220V, 100W. When the voltage at both ends of the electric iron is 110V, what is its actual power

First of all, the internal resistance R of the soldering iron is required
R=U²/P=220²/100=484Ω
The change of resistance without considering the change of voltage
When u = 110V, the solvable P = u & # / r = 25W

Simple calculation: (0.4 + 4 / 25 divided by 1.6) divided by 3.5

（0.4+4/25÷1.6）÷3.5
=（0.4+0.1）÷3.5
=0.5÷3.5
=1/7

When a motor works normally, the voltage at both ends is 220 V, and the current through the coil is 10 A. if the coil resistance is 3 Ω
So the heat generated in one minute is___ The efficiency of this motor to convert mechanical energy is____ %
The first empty answer is 18000, which is calculated by q = uit. Why can we calculate different results by q = u ^ 2T / R

The coil of motor is equivalent to inductance and resistance in series, and the current passing through is the same, so the electric energy consumed by resistance is calculated by I * I * r * t. 220 V is shared by inductance and resistance, not loaded on resistance

25 times 78 minus 78 by skillful calculation

=25×（80-2）-78
=25×80-25×2-78
=2000-50-（80-2）
=1950-80+2
=1870+2
=1872

Which of the following ways is to convert chemical energy into electrical energy () a, dry cell B, photocell C, nuclear power plant D, Three Gorges power station

A. Dry cell
Photocell converts light energy into electricity, nuclear power station converts nuclear energy into electricity, and Three Gorges water station converts water energy into electricity

80%[(1+40%)x]=224

80℅[(1+40℅)x]=224
0.8×1.4x=224
1.12x=224
x=224÷1.12
x=200