"Mechanical Efficiency" Simple Machinery PPT

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"Mechanical Efficiency" Simple Machinery PPT

Part One: Preview Perception

1. Useful work and extra work

1. Useful work: the work that must be done to achieve the goal.

2. Extra work: In order to achieve a certain purpose, the work must be done to overcome factors such as the gravity and friction of the machine itself.

3. Total work: the sum of useful work and extra work.

4. Relationship: W total = W has + W amount.

2. Mechanical efficiency

1. Definition: The ratio of useful work to total work.

2. Formula: η=Whas/Wtotal.

3. Rule: Because the useful work is less than the total work, the mechanical efficiency is always less than 1.

4. Factors affecting the mechanical efficiency of the pulley block: (1) The weight of the lifted object; (2) The gravity of the moving pulley; (3) The friction between the pulley and the rope.

Mechanical efficiency PPT, part 2: interactive classroom understanding

Understanding mechanical efficiency

(1) Mechanical efficiency is the proportion of useful work in total work, which is a ratio.

(2) Because W has a total less than W total, the mechanical efficiency η is always less than 1.

(3) Mechanical efficiency is a physical quantity that describes the quality of mechanical properties. The mechanical properties with high mechanical efficiency are excellent. The level of mechanical efficiency has nothing to do with whether and how much effort the machine saves when working; it has nothing to do with the amount of work completed when the machine works.

(4) When using a pulley block to lift a heavy object, W has = Gh, W total = Fs, η = W_"have" / W_"total" =Gh/Fs=Gh/Fnh=G/nF, where G=G object + G moves.

(5) Mechanical efficiency is variable. From the formula η=W_"有" /W_"全" =W_"有" /(W_"有" +W_"的" ), it can be seen that when the additional work is constant, the more useful work the machine does, the higher the mechanical efficiency. . When the useful work done by the machine is constant, the less extra work the machine does, the higher the mechanical efficiency.

[Example] Use the pulley block as shown in the figure to slowly lift the cargo 3 m. It is known that the cargo weighs 850 N, and the pulling force F used is 300 N (not counting the effects of friction and rope weight). beg:

(1) There is useful work.

(2)The total work is W total.

(3) Amount of additional work.

(4) Gravity of the moving pulley.

(5) Mechanical efficiency of the pulley set η.

Mechanical efficiency PPT, part three: try application

1. Useful work and extra work

1. When Li Ling was on duty, she carried water from the first floor to the classroom on the third floor with a bucket. What she did usefully was ()

A. Work done on the barrel

B. Work done on water

C. Work done on the water and bucket

D. Work done on water, bucket and person

2. The teacup seller picks up the teacup from the shelf; the tea drinker picks up the teacup from the table. Based on the work done by the two people to overcome the gravity of the teacup, which of the following statements is correct ()

A. They are all useful

B. It’s all extra work

C. It is useful for tea drinkers

D. It is useful to those who sell teacups

2. Mechanical efficiency

3. Regarding mechanical efficiency, which of the following statements is correct ()

A. The more work the machine does, the higher the mechanical efficiency.

B. The more total work a machine does, the higher the mechanical efficiency.

C. The more extra work is done, the higher the mechanical efficiency is.

D. The greater the proportion of extra work in the total work, the lower the mechanical efficiency.

4. Crane A has higher mechanical efficiency than crane B. When they lift objects of the same mass to the same height at a constant speed, then ()

A. The motor of crane A does more useful work

B. The motor of crane B does more extra work

C. The motor of crane A does more total work

D. The total work done by the motors of two cranes A and B is the same.

5. As shown in the figure, the master worker uses a force of 250 N to lift an object weighing 400 N by 1 m at a constant speed. During this process, the mechanical efficiency of the pulley set is ()

A.80%　B.62.5%

C.125% D.20%

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For more information about the PPT courseware "Mechanical Efficiency of Simple Machines", please click the PPT Mechanical Efficiency of Simple Machines tab.

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Update Time: 2024-10-20

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

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