"Mechanical Efficiency" Simple Machinery PPT Courseware 3

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"Mechanical Efficiency" Simple Machinery PPT Courseware 3

The three functions of machinery

To raise G, this pulley must also be raised. At this time, you have to overcome the gravity and friction of the pulley and do extra work, otherwise you will not be able to complete the task. This skill has no use value but has to be done, it is extra work.

To achieve the goal and complete useful work, power F must be applied to the pulley to do work, which is the total work

Lifting heavy object G is a purpose, has utilization value, and is useful.

Read this section of the textbook carefully, highlight the key points, and complete the following questions

1. Understand the three kinds of gong

2. Understand mechanical efficiency

1. Three kinds of power

1. Useful work (Wyou): work that is useful to people.

W has = Gh - lifting object

2. Extra work (W amount): work that people do not need but have to do.

3. Total work (W total): the sum of useful work and extra work

W total = FS----- (the work done by power F when using machinery)

4. The relationship between the three: W total = W has + W amount

2. Mechanical efficiency

The ratio of useful work to total work is called mechanical efficiency

Formula: η = W has / W total × 100%

Mechanical efficiency is generally expressed as a percentage and has no unit.

Look at the data just now and think about whether the mechanical efficiency will be greater than 1? Why?

The useful work is always less than the total work, so the mechanical efficiency is always less than 1

Please judge: Is the following statement correct?

1. The more useful work is done, the higher the mechanical efficiency ( )

2. The less the extra work, the higher the mechanical efficiency ( )

3． The slower the object does work, the lower the mechanical efficiency ( )

4. The more total work done, the lower the mechanical efficiency ( )

5. To do the same useful work, the less the extra work, the higher the mechanical efficiency ( )

6． When doing the same total work, the more useful work, the higher the mechanical efficiency ( )

7. The higher the mechanical efficiency, the more labor-saving ( )

Experimental steps:

1. According to the textbook picture 9-30, install the pulley group and scale, measure the value of hook code G, and record it in the table.

2. Pull the spring dynamometer vertically at a constant speed to make the hook code G rise, read the value of the pulling force F shown on the spring dynamometer, and read the distance h the hook code rises and the distance the spring dynamometer moves from the scale. S, record the data in a table.

3. Calculate the useful work, total work and mechanical efficiency respectively. Record the results.

4. Install another pulley set and repeat the above experiment.

Analysis and discussion

1. What are the differences between the two pulley sets when compared twice? What is the difference in mechanical efficiency?

2. Analyze what factors affect the mechanical efficiency of the pulley block?

Factors affecting the mechanical efficiency of pulley blocks

1. Friction between pulley block wheel and shaft and wheel and rope

(Reducing friction can reduce additional work and improve mechanical efficiency)

2. The weight of the object lifted

(As the weight of objects increases, mechanical efficiency also increases)

3. The dead weight of the moving pulley

(Reduce the gravity of the moving pulley and improve the mechanical efficiency)

Practice and consolidate in a clear and dignified manner

1. Please judge: Is the following statement correct?

1. The more useful work is done, the higher the mechanical efficiency ( )

2. The less the extra work, the higher the mechanical efficiency ( )

3． The slower the object does work, the lower the mechanical efficiency ( )

4. The more total work done, the higher the mechanical efficiency ( )

5. To do the same useful work, the less the extra work, the higher the mechanical efficiency ( )

6． When doing the same total work, the more useful work, the higher the mechanical efficiency ( )

7. The higher the mechanical efficiency, the more labor-saving ( )

2. As shown in the figure: If the object weighs 500N, the frictional resistance is 0.2 times the gravity, and the object moves at a constant speed of 0.2m/s for 5s under the action of a pulling force of 120N, find:

1. Useful work, 2. Total work, 3. Mechanical efficiency

3. As shown in the figure: use a pulling force of 80N to lift an object of 180N 1m high at a constant speed, ignoring the weight of the rope and friction.

Find: (1) W total =?

(2) W is useful =?

(3) W extra =?

(4))η=?

(5) G action=?

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Update Time: 2024-11-06

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

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