"Gong" Work and Mechanical Energy PPT Courseware 4

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"Gong" Work and Mechanical Energy PPT Courseware 4

Please tell me the meaning of "gong" in the following words:

meritorious deeds—

Done, get twice the result with half the effort——

Have we ever learned the physical concept of "work" in physics?

Electrical work - the process of converting electrical energy into other forms of energy using electrical appliances.

1. Work in mechanics

If a force acts on an object and the object moves a certain distance in the direction of the force, the effect of the force is effective. In mechanics, it is said that the force has done work on the object.

Ask the students to observe which forces do work on the objects in the previous pictures? Analyze how many forces act on objects, cars, and cars? Under the action of what force and in what direction did they move a certain distance? What force does work on them?

In these two cases, are the rocks and buckets affected by the force? Do they move a certain distance in the direction of the force? Is there any work done by the force moving the stone in this direction and the force lifting the bucket upward?

There are two necessary factors for success:

1. The force acting on an object;

2. The distance the object moves in the direction of the force.

"Two necessary factors" means "one cannot exist without the other"

1. Are you tired when you are holding a basin of water without moving? Did you do your job? Why?

2. A football is kicked with the foot and the ball rolls on the ground. During the rolling process, does the person do any work on the ball?

3． When a person pushes a wooden box forward, does the box's gravity and supporting force do any work? Why?

Ask students to analyze the following situations: In which cases did the force do work on the object, and in which cases did it not?

①Hold heavy objects by hand and stay still in place

②A small ball rolling on a horizontal table

③ If you hold the brick in your hand and move it horizontally, does gravity do any work?

④Pushing a car stuck in the mud without pushing it

⑤ Heavy objects are hung on the steel cables of the crane

⑴When the weight is stationary, does the tension of the steel rope do work? Does gravity do work?

⑵When the crane lifts the cargo to a certain height, it moves it in the horizontal direction. During the movement, does gravity do work? Does the pulling force do work?

Learn and practice immediately

In the following situations, the force that does work on the object is ( )

A. The ice ball slides at a constant speed on the smooth ice surface

B. The child pushed the box hard, but the box was not pushed.

C. The student is standing on a car traveling at a constant speed on a level road with his schoolbag on his back.

D. The cleaner lifted a bucket of water from the ground

2. Calculation of work

1. Definition of work: In physics, the product of a force and the distance moved in the direction of the force is called work.

That is: work = force × distance moved in the direction of force

2. Calculation formula of work: W = F·S

W— Work—Joules (J) F— Force—Newtons (N)

S - Distance - Meter (m) 1 Joule = 1 N × 1 meter

Meaning: The work done by using a force of 1 Newton to move an object 1 meter in the direction of the force is 1 Joule.

The principle of gong:

When using machinery, the work done by people will not be less than the work done without machinery. That is to say, using any machinery cannot save work. That is FS≥Gh

1. A person uses a movable pulley to lift an object weighing 100N. If friction and the weight of the movable pulley are ignored, how much work does the person do on the rope to lift the object 2m? What is the pulling force on the rope?

Since friction and the weight of the pulley are ignored, the work done by a person on the rope is equal to the work done directly by hand without the pulley W1=W2, that is, Gh=Fs

Therefore, 100N×2m=F×2×2m F=50N

2. Someone uses a pulley to lift an object weighing 100N and lifts the object 2m. If the pulling force on the rope is 60N, how much work does the person do on the rope? At this time, compared with directly raising the 100N object by 2m, how much more work does the person do? When you use machinery, do you want more or less work? Why?

Solution: W1=Gh=100N×2m=200J

W2=Fs=f·2h=60N×2×2m=240J

△W=W2－W1=240J－200J=40J

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Update Time: 2024-07-29

This template belongs to Physics courseware People's Education Edition Physics for Grade 8, Volume 2 industry PPT template

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