"Mechanical Efficiency" Simple Machine Work PPT Courseware 2

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"Mechanical Efficiency" Simple Machine Work PPT Courseware 2

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.

Please judge

When a bucket is used to lift water from a well, the useful work done is _________, the additional work is _________, and the total work is _________;

If a bucket falls into a well, and when the bucket is fished out of the well, some water is brought into the bucket. The useful work is ________, the extra work is ________, and the total work is ________.

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----(work done by power F when using machinery)

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

Mechanical efficiency

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

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

2. Mechanical efficiency η

1. Mechanical efficiency: The ratio of useful work to total work is called mechanical efficiency.

2. Calculation formula: η = W useful/W total × 100%

Understand: a:η is often expressed as a percentage;

b: eta is less than 100%;

c: eta has no unit.

Mechanical efficiency of some machines:

The mechanical efficiency of the crane is 60%. What does it mean?

The ratio of useful work to total work done when using a crane to lift heavy objects is 60%. It can also be said that useful work accounts for 60% of the total work, and the other 40% is extra work.

3. How to improve mechanical efficiency?

Improve the structure of machinery and reduce its own weight

For example, if the movable pulley is too heavy, the mechanical efficiency of the pulley group is very low. Replacing it with a lighter pulley can improve its mechanical efficiency.

Reduce friction between machines.

For example: strengthen the lubrication of mechanical moving parts.

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 ( )

Exercise 1. Use the pulley set as shown in the figure to lift an object weighing 180N 1m at a constant speed under the action of a pulling force of F=80N. The weight of the rope and friction are not taken into account.

Find: (1) W total =?

(2) W is useful =?

(3) W extra =?

(4) η=?

(5)G move=?

Exercise 2. Xiao Ming uses a force of 100N to pull one end of the lever down 2m in the vertical direction, and the other end of the lever raises the 320N weight by 0.5m.

(1) How much useful work did he do?

(2) What is the total work?

(3) What is the mechanical efficiency of the lever?

1. Measure the mechanical efficiency when using the same pulley set to lift different objects

2. Measure the mechanical efficiency when using different pulley sets to lift the same object.

Experiments show: 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)

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Update Time: 2024-10-03

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