"End of Chapter Summary and Improvement" Simple Machinery PPT

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"End of Chapter Summary and Improvement" Simple Machinery PPT

Part One: Key Breakthroughs

Power and mechanical efficiency

Example 1: Among the following statements, the correct one is ( )

A. The higher the mechanical efficiency, the faster the mechanical work must be done

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

C. When machinery does work, the shorter the working time, the greater the power must be

D. You can save effort when using machinery, but you cannot save distance at the same time.

[Analysis] The higher the mechanical efficiency, it means that the ratio of useful work to total work is greater, but it does not necessarily mean that the work is done faster. Item A is wrong; the higher the mechanical efficiency, it means that the ratio of useful work to total work is large, and it has nothing to do with whether the machine is labor-saving or not. , Item B is wrong; the amount of power is related to the amount of work done and the time. The work done is certain. The shorter the time, the greater the power. Item C is wrong. When using machinery to save labor, distance must be spent. Item D is correct.

【Answer】D

Typical example 2: Using the inclined plane device as shown in the figure, an object weighing 500 N can be pulled to the top of the inclined plane at a constant speed in 10 s. The inclined plane is 4 m long, 2 m high, and the actual pulling force is 300 N. beg:

(1) If the pulling force along the direction of the inclined plane is F, the weight of the object is G, the length of the inclined plane is L, and the height is H, please use the relationship between total work, useful work and extra work to derive the expression of friction force f;

(2) What is the power of the pulling force?

(3) What is the mechanical efficiency of this ramp device?

End-of-Chapter Summary and Improvement PPT, Part 2: Experimental Activities

Experiment 1: Explore the equilibrium conditions of levers

1. Experimental equipment: iron stand, lever, hook code, etc.

2. Experimental steps

(1) Hang different numbers of hook codes on both sides of the lever, and move the position of the hook codes to balance the lever in the horizontal position again. At this time, the force on both sides of the lever is equal to the gravity on the respective hooks.

(2) Assume that the pulling force of the right hook code on the lever is the power F1, and the pulling force of the left hook code on the lever is the resistance F2. Measure the power arm l1 and the resistance arm l2 when the lever is balanced, and put F1, F2, l1, l2 Enter the values ​​into the table.

(3) Change the size of power F1 and power arm l1, adjust resistance F2 and resistance arm l2 accordingly, and do several more experiments.

3.Experimental form

4. Experimental conclusion: The balance condition of the lever is power × power arm = resistance × resistance arm (or F1 × l1 = F2 × l2).

Typical example 1: Xiao Ming conducted an experiment to “explore the conditions for lever balance”. Every hook code is exactly the same. Before the experiment, Xiao Ming adjusted the balance nuts at both ends of the lever to balance the lever in a horizontal position to eliminate the influence of the lever's own weight on the experiment.

(1) To make the lever in Figure A horizontally balanced, __________ hook codes should be hung at a;

(2) Some students came to the following conclusion based on their own experimental data: power × distance from the fulcrum to the point of action of the power = resistance × distance from the fulcrum to the point of action of the resistance. This conclusion is inconsistent with the equilibrium condition of the lever because there was no _______ during the experiment;

A. Conduct experiments multiple times B. Change the size of the force

C.Change the direction of the force D.Change the point of action of the force

(3) Xiao Ming changed the experimental method, as shown in Figure B. Point C, the center of gravity, is no longer the fulcrum, and two hooks are still hung at point A. The pulling force on the lever is FA. Using a zero-calibrated spring dynamometer is correct. Measure the pulling force FB at point B to balance the lever horizontally, then FB·lOB_______(optional ">", "=" or "<")FA·lOA.

[Analysis] (1) Suppose the graduation value of the lever is l and the weight of a hook code is G. According to the lever balance condition F1l1=F2l2, it can be obtained, 2G×2l=nG×4l, then n=1; (2) " "Power × distance from the fulcrum to the point of action of the power = resistance × distance from the fulcrum to the point of action of the resistance" is obtained when the lever is balanced in a horizontal position and the directions of power and resistance are both vertically downward, which is the experimental process. There is no change in the direction of power or resistance, item C is correct; (3) In question picture B, the center of gravity of the lever is not on the fulcrum, and the gravity of the lever itself affects the rotation of the lever, causing the magnitude ratio of the pulling force FB to be determined by the lever balance condition The calculated value is too large, FB·lOB>FA·lOA.

【Answer】(1)1　(2)C　(3)>

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Update Time: 2024-09-08

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