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What are the rules of mechanical efficiency of pulley block? The more hook yards, the greater the mechanical efficiency
1. When the pulley block pulls the object along the horizontal direction, the gravity work of the moving pulley is ignored
When the pulley block is used to pull an object along the horizontal direction, the total work is wtotal = f * s, and the active work is wyou = f * L. if the object moves along the horizontal direction at a uniform speed and in a straight line, the object is under the action of the balance force, which should be: F = f, and S = NL. At this time, the mechanical efficiency of the pulley block is & para; = wyou / wtotal * 100% = f / (n * f) * 100%
2. When the pulley block is used to pull the object vertically:
(1) Considering all the extra work, such as the dead weight of the moving pulley, the friction resistance and the weight of the rope, w total = f * s. The active work is w you = f pull * l, because the object is lifted uniformly along the vertical direction, then the object is subject to the balance force, f pull = g object, s = NH. So there is & para; = w you / W total * 100% = g object / (n * f) * 100%
(2) In this case, the object is lifted uniformly along the vertical direction, and the object is subjected to the balance force, that is, the effective work of the pulley block is w you = g object * h, and the total work is w total = w you + W amount, so there is & para; = w you / W total * 100% = w you / (w you + W amount) * 100% = g object / (g object + G dynamic) * 100%
(3) In the case of ignoring the additional work done by the dead weight of the movable pulley, the friction resistance between the axles and the weight of the rope: at this time, the useful work done by the pulley block w has = g object * h, w amount = 0, w total = f * s = n * f * h, so w total = w has, n * f = g object, f = g object / h,
The mechanical efficiency of pulley block is 100%
Note: (1) when different pulley blocks lift the same weight, the greater the number of rope strands, the greater the G movement, which will reduce the mechanical efficiency of the pulley block
(2) When the same pulley block is used to lift different weights, due to a certain amount of extra work, the more useful work the pulley block does, the higher the mechanical efficiency
(3) There is a certain geometric relationship between the free end of the rope and the moving distance of the moving pulley. Generally, s = NL or S = NH (where s is the moving distance of the free end of the rope, l is the moving distance of the moving pulley or object in the horizontal direction, n is the number of strands of the rope pulled by the power, and H is the height of the object rising)
In the experiment of "measuring the mechanical efficiency of pulley block", if there is no scale, can the mechanical efficiency of pulley block be measured? How to obtain the rising height of hook code and the moving distance of spring dynamometer?
Answer: count the number of segments of the rope connecting the movable pulley n
Because s = NH, so
η = w useful / W total = GH / Fs = GH / (FNH) = g / (NF)
Therefore, G and f measured in the experiment can be substituted into the calculation
In order to explore whether the mechanical efficiency of the lever is related to the weight of the hook, how to design the experiment
① Use the adjusted spring dynamometer to measure the gravity G1 of a hook code and record it in the table
② Lift the hook up with a pulley block
③ Use the same spring dynamometer and pulley block to measure the gravity g 2, and repeat steps 1 and 2 to record the data in the table
④ The conclusion is drawn from the mechanical efficiency = g / G + G and the measured data
Factors affecting mechanical efficiency of leverage
Use a 2-N lever to do the same test as above The results show that: 1. The mechanical efficiency of the lever is related to the gravity of the lever (the greater the gravity, the lower the efficiency); 2. It is related to the position of the fulcrum
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