Does friction do work on an object? What are the examples of frictional work?

In this case, the frictional force and the direction of motion of the object are not perpendicular to the work.

1. Static friction does work.

Static friction can do positive work, negative work, or no work.

In the process of doing work by static friction, there is only the mutual transfer of mechanical energy (static friction plays the role of transferring mechanical energy), and there is no conversion of mechanical energy into other forms of energy.

2. Sliding friction does work.

This condition is most common and when the sliding friction force hinders the movement of the object or the object moves against the sliding friction force, it does negative work on the object.

Summary. 1: Look at the effect.

For example, if you let go of an object in the air and let it fall freely, then in the process of falling, only gravity does the work, what is the result? The velocity of the object is increasing, that is to say, the kinetic energy is increasing.

So where does this increased kinetic energy come from? The positive work done by gravity is transformed. On the contrary, if an object is thrown vertically, the velocity of the object will slowly decrease under the action of gravity, that is, the kinetic energy will become smaller and smaller, which is because the negative work done by gravity cancels out the kinetic energy.

2: Look at the angle. This angle is the angle between the direction of the force and the direction of the object's motion, if the angle is greater than 90, negative work is done; If it is less than 90, do positive work.

Take the above example to say that in the free fall motion, the direction of motion of the object is downward, and the direction of gravity is also downward, then the angle is 0 degrees, so it is a positive work. The vertical upward throwing motion, the direction of movement is upward, the gravity is downward, and the angle is 180 degrees, so negative work is done. Also, when you talk about friction, it's a very special force.

The definition of this force is that the direction is opposite to the relative motion trend (that is, it is a force that wants to hinder the motion of the object), so the direction of the frictional force is (in general) opposite to the direction of the object's motion, that is, the angle is 180 degrees, so (in general) the frictional force is done negative work.

1: Look at the effect. For example:

If you let go of the object in the air and let it do free fall, then in the process of falling, only gravity does the work, what is the result? The velocity of the object is increasing, that is to say, the kinetic energy is increasing. So where does this increased kinetic energy come from?

The positive work done by gravity is transformed. On the contrary, if an object is thrown vertically, the velocity of the object will slowly decrease under the action of gravity, that is, the kinetic energy will become smaller and smaller, which is because the negative work done by gravity cancels out the kinetic energy. 2：

Cavity Sen looks at the angle. This angle is the angle between the direction of the force and the direction of the object's movement, if the angle is greater than 90, do the negative ruler Xungong; If Xiaoling is round at 90, do the right work. Take the above example to say that in the free fall motion, the direction of motion of the object is downward, and the direction of gravity is also downward, then the angle is 0 degrees, so it is a positive work.

The vertical upward throwing motion, the direction of movement is upward, the gravity is downward, and the angle is 180 degrees, so negative work is done. Also, when you talk about friction, it's a very special force. The definition of this force is that the direction is opposite to the relative motion trend (that is, it is a force that wants to hinder the motion of the object), so the direction of the frictional force is (in general) opposite to the direction of the object's motion, that is, the angle is 180 degrees, so (in general) the frictional force is done negative work.

The above is the frictional force to judge the work done.

I'll just ask you a question, if v0 is multiplied by time, isn't it equivalent to the square being stationary? But this block is obviously still slow to guess the speed of v1. Scramble.

I just feel that this x is not right, there is a problem, and I also know to choose D.

Question 9 is chosen BC

The eighth question I asked was 4d, and this x is the displacement of friction, but he is directly multiplied by v0, and I don't feel right. Can you clarify what I'm asking?

Okay, you can.

Understand. That's right, it's VO multiplied by time.

<> total work: w fs, tensile force multiplied by bevel length;

Useful work: w has gh, the weight of the object is multiplied by the vertical height of the inclined plane;

Extra work: w amount w total w has fs gh;

Friction: The extra work is the work done to overcome the frictional force, that is: w fs gh fs, f is the friction force, s is the length of the inclined plane;

Simplified: f f gh s.

The force that hinders the relative motion (or relative tendency of motion) of an object is called friction. The direction of frictional force is opposite to the direction of the relative motion (or relative tendency of motion) of the object. There are three types of friction: static friction, rolling friction, and sliding friction.

Work is: the product of the force and the distance of the object in the direction of the force.

If the direction of velocity (pulling direction) of the object's motion is the specified direction, then the direction of friction is opposite, and the work done by friction is negative. This means that work is done to overcome friction.

The distance s in the formula for the work done by friction is the displacement of the frictional force.

You may be confused as follows: the sliding friction force does work, part of which is converted into heat, and the other part is converted into the kinetic energy of the object being rubbed.

For example, when a stationary object is placed on a conveyor belt, there is sliding friction between the object and the conveyor belt for a period of time, and the object accelerates until the object has the same speed as the conveyor belt, and then remains relatively stationary. In this process, part of the work done by friction is converted into kinetic energy of the object, and the other part is converted into heat.

The total work of friction: w=fscos

where s is the displacement acting on friction.

The heat produced q=fl where l is the relative displacement between the two objects where the sliding friction occurs, i.e., in the example, the displacement of the objects relative to the conveyor belt.

The work of friction: w=q+e e is the mechanical energy gained by the object, which is the kinetic energy obtained by the object in the example.

Pick B. An object with an initial velocity of zero is placed on a conveyor belt, and at first it slides relatively, and the frictional force does positive work on physics, causing the object to be displaced. The brake device is also sliding friction and does negative work.

If the two objects do not slide relative to each other, the static friction force will do positive work on the objects above. Two stacked blocks have an initial velocity, pull the block below to slow down, if the two objects do not slide relative to each other, the static friction force will do negative work on the block above.

The correct option is B, the friction force on the driving wheel of the car is positive work, and the friction force on the other wheels is negative work.