"Gong" Work and Mechanical Energy PPT Courseware 2

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"Gong" Work and Mechanical Energy PPT Courseware 2

1. Work in mechanics

1. Definition: When a force acts on an object, and the object moves a certain distance in the direction of the force, it is said in mechanics that this force has done work.

The child's pulling force F pulls the puppy to move a distance s. We say that the child's pulling force has done work on the puppy, or simply that the child has done work on the puppy.

The lifting force F of the forklift moves the goods a distance s. We say that the lifting force of the forklift has done work on the goods, or simply that the forklift has done work on the goods.

The force F of the hammer on the nail moves the nail a distance s. We say that the force of the hammer does work on the nail, or simply that the hammer does work on the nail.

2. Two necessary factors for doing work

necessary factors

(1) A force acts on an object F

(2) The object moves a distance s in the direction of the force

3. The situation when the object does not do work

(1) The object is acted upon by a force but remains at rest

s=0, no effort

(2) The object is not affected by force in the direction of movement and moves due to inertia.

F=0, no effort, no result

(3) The object is acted upon by a certain force, but the direction of motion is always perpendicular to the direction of the force.

F⊥s, hard work without success

Decide whether the following statements are correct and tell why.

1. When a weightlifter lifts a barbell, work is done by the lifting force.

2. The traction force of the car makes the car move forward horizontally, and the traction force does work on the car.

3． When an apple falls from the tree, gravity does work on the apple.

4. Pull the car to move a certain distance on the horizontal plane, and the support force of the table does work on the car.

5. Kick a football with your feet and the ball rolls on the ground. During the rolling process, the person does work on the ball.

think

You push the wall hard and the wall doesn't move. Have you done any work on the wall?

A force acts on an object, but the object does not move a certain distance in the direction of the force, so the force does no work.

I throw the chalk in my hand horizontally and the chalk flies in the air. Have I done any work on the chalk?

There is no force on the object, it moves by inertia, and the force does no work.

2. Calculation of work

1. Work is equal to the product of the force and the distance the object moves in the direction of the force.

2. Calculation formula: W = Fs

3. The unit of work: joule, referred to as joule, symbol J

1J = 1N·m

If a force of 1N moves an object a distance of 1m in the direction of the force, the force will do 1J of work on the object.

principle of mechanical work

When working with machinery, sometimes you can save effort, sometimes you can save distance, but saving effort will inevitably cost distance, and saving distance will inevitably require effort. It is impossible to save both labor and distance. In other words, the use of any machinery cannot save energy. This conclusion is called the principle of mechanical work.

Classroom training

1. An object weighing 100 N moves forward 4m at a constant speed on a smooth horizontal surface due to inertia. The work done by the external force is ____J and the work done by gravity is ____J.

2. After school, a classmate walked 200 m along the level road carrying a schoolbag weighing 40 N, and then climbed to the fourth floor about 10 m high before returning home. Then the work he did on the schoolbag during his return home is approximately ( )

A.0J B.400J C.2000J D.2400J

3. At the Olympic Games, Chinese athlete Ding Meiyuan won the gold medal in the women's weightlifting category above 75 kg. Her clean and jerk score was 165 kg. Estimate the work she did on the barbell during the entire clean and jerk process as ( )

A.3×102 J B.1×103 J

C.3×103 J D.5×103 J

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Update Time: 2024-12-02

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

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