"Mechanical Efficiency" Simple Machinery PPT Courseware

"Mechanical Efficiency" Simple Machinery PPT Courseware

think:

What is the useful work of a person who draws water from a well with a bucket? What is extra work?

The work done on the water is useful work, and the work done on the bucket is extra work.

If a bucket falls into water and the bucket filled with water is picked up, what is the useful work? What is the extra work?

The work done on the bucket is useful work, and the work done on the water is extra work.

2. Mechanical efficiency

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

2.Expression: η=Whas/Wtotal

Note: Because mechanical efficiency is the ratio of two works, it has no unit.

think:

1) Is there any machine with 100% mechanical efficiency? Why?

No! Because useful work is always less than total work, mechanical efficiency is always less than 1.

2) How can we improve the efficiency of machinery?

Reduce extra work

Improve the structure to make it reasonable and lightweight

Regular maintenance and regular lubrication

A crane lifts a load weighing 3600 N by 4 m. If the additional work is 9600 J, how much useful work is done by the crane? What is the total work? What is the mechanical efficiency? In what ways does the crane consume extra work?

Solution: The force of the crane lifting the object is equal to the gravity of the object

F=G=3600 N

The useful work done by a crane in lifting heavy objects

W has=Fh=Gh=3600 N×4 m=14400 J=1.44×104 J

The total work done by the crane

W total=W has+W amount=14400 J+9600 J=2.4×104 J

Therefore, the mechanical efficiency of the crane

η= W has/W total=1.44×104 J/2.4×104 J=60%

Explore the relationship between the mechanical efficiency of the inclined plane and the slope of the inclined plane

1. Measure the weight of small wooden blocks and record the data;

2. Use a spring dynamometer to pull the small wooden block upward at a constant speed along the incline; record the pulling force F, the distance s the block moves along the incline, and the height h the block rises to;

3. Change the inclination of the bevel and repeat step 2.

3. Mechanical efficiency of the inclined plane

The mechanical efficiency of the inclined surface is related to the smoothness of the inclined surface and the slope of the inclined surface. The smoother the inclined surface and the larger the slope, the higher the mechanical efficiency.

Note: Methods to improve the mechanical efficiency of inclined planes

1) Increase the inclination of the slope

2) Reduce the roughness of the bevel

4. Mechanical efficiency of pulley block

Using the same pulley set, the heavier the object lifted, the higher the mechanical efficiency. It has nothing to do with the winding method of the pulley block

Note: Methods to improve the mechanical efficiency of the pulley block

Classroom training

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

A. Mechanical efficiency is always less than 1

B. The more work a machine does per unit time, the higher the mechanical efficiency.

C. The more work a machine does, the higher its mechanical efficiency will be.

D. The more labor-saving machinery, the higher the mechanical efficiency.

2. The mechanical efficiency of a certain machine is 75%, which means that this device is used to do work ( )

A.The total work is 3 times the additional work

B. The useful work is 3 times the extra work

C. Total work is 3 times the useful work

D. The extra work accounts for 3/4 times of the total work

3. In order to improve the mechanical efficiency of the pulley block, which of the following practices is correct ( )

A. Lift heavy objects at a fast and constant speed

B. Lift the heavy object at a slow speed and uniform speed

C. Increase the number of rope strands that bear the weight of the object

D. Make the pulley lighter and reduce the friction during the rotation of the pulley.

Class summary

1. Useful work, extra work, total work

2. Mechanical efficiency: η = W/W total

3. The mechanical efficiency of the inclined surface is related to the slope and smoothness

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