Can kinetic friction not do work? Please help with an example

Of course, you can do it, you should first make the concept of force work clear, 3l 4l is misleading people's children, 1l 2l is right, but it is not clear.

The definition in Cheng Jiafu mechanics is: the product of the displacement of the force and the point of action of the force in the direction of the force The key is the point of action of the force, which is the point on the object.

1. Yes. With the example of 3L:

Let's say you take a small rock and slide it on top of a big rock.

The big stone does not move, so although it is subjected to sliding friction, the displacement of the application point is 0, so the friction does not do work on it, only negative work on the small stone above, and from another angle, the kinetic energy of the small stone is indeed completely converted into internal energy, and the kinetic energy of the big stone is 0

2. Yes. You hold a pen in your hand and accelerate the horizontal movement to the right, and the static friction force is doing negative work on your hand, because the direction of the force is to the left, and the hand is moving to the right.

In fact, dynamic friction and static friction are only defined by whether there is relative sliding between the acting objects, in other words, looking at the problem with another object as a frame of reference. And in the definition of work, the displacement of the action point of the force in the direction of the force is relative, and different reference frames have different displacements, and there are different work done, so dynamic friction static friction can do any kind of work.

The definition is fundamental, and we must return to the definition

The upstairs example is a bit problematic.

1. The so-called sliding friction.

That is, the object is subjected to force and has relative motion.

So be sure to do the work.

Let's say you take a small rock and slide it on top of a big rock.

Although the big rocks are not moving, there is no kinetic energy changing.

But the heat produced by sliding friction.

Therefore, work must be done.

2. Static friction can do work.

Think about how you walk.

It is the static friction between the sole of your shoe and the ground that makes you move.

Sliding friction does work.

When you pull the box on the ground, the box slides, and the dynamic friction force does negative work.

Kinetic friction does not do positive work, but static friction can do positive work.

The flatbed truck pulls you to accelerate, and the static friction of the soles of your feet does the right work.

Yes. I'm sure I can.

It's hard to say for example, but you have to use voice... Anyway, it must be, depending on your comprehension, for example, two wooden blocks on a smooth horizontal plane, they have a lot of relative motion, and there are different situations at this time, which include all the physical problems of the slider.

I think the score situation is discussed, and if the various parts are considered, I think he has to do negative work, because it has relative displacement (kinetic friction must slide relative to each other, etc.).

If we consider the whole system, if it belongs to the part of the internal system, we can ignore the problem of starting work.

The same goes for question 2.

I feel that static friction does not do work because the object is not displaced under its action

Well, depending on the object, ls is the correct solution.

<> case where friction does not work: Friction does not work when it is stationary or when friction does centripetal force.

A few examples about friction:

1. People walk forward because the ground exerts a frictional force on the feet to move forward, and at this time, it is the sliding friction force that does positive work.

2. When a horizontal force is applied to an object, the object slides on the ground, and then the sliding friction force does negative work.

3. Sliding friction always has to do work.

4. Static friction does not do work, when pushing a box, when the box does not move, because there is no displacement, static friction does not do work.

Sliding friction does not necessarily do work.

For example, if your hand is scratching on a wall, there is friction between your hand and the wall, and this friction does work on your hand, but it does not work on the wall, because the force has a force on the wall but does not move in a circle.

The work done by sliding friction force must have relative sliding, and there must be friction to generate heat, and this part of the energy is only related to the relative sliding distance.

The kinetic energy of the change in the velocity of the object does not necessarily change because the velocity is a vector quantity and may only change in direction with the same magnitude; When the kinetic energy changes, the velocity must change, because it must be the magnitude of the velocity that changes.

There is no objection that the static friction can do work positively, negatively, or zero. Dynamic sliding friction generally does negative work (the textbook "Theoretical Mechanics" of Chinese and foreign universities generally says so), because the dynamic friction does work for the system, but considering that both have motion, and there are also two objects in relative motion, from the perspective of an object, the friction force does positive work, such as a uniform horizontal transmission belt suddenly put an object, the front speed of the object is small, and the same speed as the belt is obtained later, the kinetic energy increases, and the friction of the object can only do positive work on the object. The friction force of the object on the belt makes it do negative work on the belt, and the sum of the two must be negative, which shows that the teaching materials are generally negative in terms of total work. Since the purpose of considering this function is to use the kinetic energy theorem.

Some people say that the object slides on the rough horizontal ground, and the frictional force on the object does negative work on the object, and the friction force on the ground does not do work because the ground does not move, so that the dynamic sliding friction force can not do work, this statement is really meaningless, is the analysis problem stupid enough to use the kinetic energy theorem on the ground? It's pure play on words!

Don't discuss whether the work done by the dynamic sliding friction force is zero, it has no practical significance, and the college entrance examination must not test this kind of meaningless multiple-choice question!

In addition, in the textbook "Theoretical Mechanics" in college, there is a classification called the division of dynamic friction and static friction (this may be the same as the middle school name), and there is also a classification: sliding friction includes dynamic sliding friction, static sliding friction, and maximum static friction. What's the point of this kind of ambiguous questioning?

Even if there are exceptions, what problems can be solved? Must not take the test, boring people's questions, engage in word games.

Regarding the work done by friction, the following statement is correct ( )aStatic friction always does positive work, and sliding friction always does negative work bStatic friction does not necessarily do work on an object, and sliding friction must do work on an object

Static friction must do work on an object, and sliding friction may not do work on an objectBoth static and sliding friction may not do work on the object, and the answer should be d aStatic friction can not do work, this is a good example, I don't need to give it, right?

Sliding friction can do positive work, e.g. when a conveyor belt carries a cloth belt to a high place.

b.Sliding friction can do no work. For example, if your hand is scratching on a wall, there is friction between your hand and the wall, and this friction does work on your hand, but not on the wall, because the force has a force on the wall but does not displace.

c.Don't use it, right?

d.Don't use it, right?

Let me explain to you again under what circumstances static friction does positive work. If you do it on a car, your speed will increase when the car accelerates, and the force between you is static friction, and it will do positive work on you.

If there is relative motion in force, then it must be done.

You may not understand these concepts well and are a little confused.

Takeaway: Work is a measure of energy conversion.

It means that "the process of force doing work corresponds to the process of energy transformation, and the value of work is equal to the value of energy conversion".

In the kinetic energy theorem, the total work is equal to the increment (or change) of the kinetic energy of an object (when it is a unit of an object).

This total work refers to the algebraic sum of all the forces on the object to do work (regardless of concepts such as energy conversion and heat generation), and this law itself is to "see" the problem from the perspective of "work", so there is no need to pay attention to energy conversion of these things.

If we "see" the problem from the perspective of "energy transformation", we don't need to pay attention to the work done, but directly look at which energy increases and which energy decreases, and then lists the equation by the energy conservation relation.

The above two research questions have different perspectives, but the results are consistent. Because the "value of the work" is equal to the "value of the energy conversion".

For example, when the work done by sliding friction corresponds to the conversion of mechanical energy into heat energy, if the kinetic energy theorem is used, only the work of friction (regardless of the increased heat) is listed, and if it is solved by energy conversion, only the increased heat (regardless of the work of friction) is listed.