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Authoritative PPT Summary
"Mechanical Efficiency" Simple Machine Work PPT Courseware
learning target
(1) Know the concepts of useful work, extra work, and total work;
(2) Master the concept and calculation formula of mechanical efficiency;
(3) Understand methods to increase mechanical efficiency.
1. Useful work and extra work
Experiment as shown in the figure
Use a spring dynamometer to lift the weight to the same height at a constant speed and compare the work done by the three pulling forces.
W C > W B > W A. Although the use of machinery can bring conveniences such as labor saving or changing the direction of force, additional work is always required due to the influence of mechanical weight or friction.
Total merit
To achieve a goal, the force that must be exerted, the work done by the motive force
extra work
The extra work that has to be done to overcome the pulley's dead weight and friction
Useful
Lifting heavy objects is the purpose and doing valuable work
Several concepts of gong
1. Useful work: "W is useful"
The valuable work done by machinery to the outside world
2. Extra work: "W extra"
The useless work done by a machine to overcome its own gravity and friction
3. Total merit: “W total”
The work done by the power on the machine
The relationship between the three: W total = W useful + W extra
2. Mechanical efficiency
Thoughts and Summary
We always want useful work to account for a greater share of total work
This leads to the question of mechanical efficiency. We call the ratio of useful work to total work mechanical efficiency.
Formula: η=?Have/?Total×100%
η: mechanical efficiency
?Yes: Useful
?Total: total work
Example: A movable pulley is used to lift a 200N object by 2m at a constant speed. The pulling force F used is 120N. What is the mechanical efficiency of the movable pulley?
Known: G=200N, h=2m, F=120N, s=2h=4m
Solution: W has =Gh=200N×2m=400J
W total=Fs=120N×4m=480J
W has/W total×100%=400J/480J×100%≈83.3%
Analysis and Discussion
The use of any machinery inevitably requires extra work, so mechanical efficiency is always less than 100%.
Things like air friction are always inevitable, so mechanical efficiency cannot reach 100%.
Determine the mechanical efficiency of an inclined plane
For an inclined surface with the same roughness, will the mechanical efficiency of the inclined surface be the same when its inclination is changed? Get your hands dirty and complete the table below.
Ways to Improve Machinery Efficiency
(1) Improve the structure of the machine to make it more reasonable and lighter
(2) During use, perform regular maintenance and regular lubrication
(3) Within the tolerance range, the weight of the object can be appropriately increased
Summary of this lesson
What did you gain from studying this class?
1. Mechanical efficiency
(1) Three concepts of work: total work, extra work, and useful work
(2) Mechanical efficiency: The ratio of useful work to total work is called mechanical efficiency
Formula: η＝Whas/Wtotal×100%
2. Variability of mechanical efficiency
(1) For the same machine, different lifting weights have different mechanical efficiencies. The heavier the weight, the greater the mechanical efficiency.
(2) The mechanical efficiency of any machine cannot reach 100%.
Practice in class
1. The mechanical efficiency of machine A is higher than that of machine B, which means ( )
A.A’s useful work is more than B’s
B. To do the same total work, A’s useful work is less than B’s useful work
C.A’s total work is less than B’s total work
D. To do the same total work, A’s useful work is more than B’s useful work
2. Someone uses a force of 100N to push down the end of the lever's power arm by 2 meters in the vertical direction, and the other end of the lever lifts a 320N weight 0.5 meters up. The useful work he does is _____J. The lever's Mechanical efficiency is _____.
3. Use a fixed pulley to lift an object with a mass of 20kg at a constant speed. The pulling force used is 220N, and the height of the object raised is 2m. Find the useful work, total work and mechanical efficiency of the fixed pulley in this process. (g=10N/Kg)
4. Since the friction when using a lever is very small, the lever is a simple machine with very high mechanical efficiency. If a lever with a mechanical efficiency of 90% is used to lift an 18Kg object 50cm, how much total work is required? (g=10N/Kg)
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