"Integration of this Chapter" Simple Machinery PPT

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"Integration of this Chapter" Simple Machinery PPT

High School Entrance Examination Focus Experience

1. (2018·Zhejiang Jinhua High School Entrance Examination) As shown in the figure, a homogeneous lever with even scales is used to conduct an experiment on lever balance conditions (each hook weight is 0.5 N). The following statement is correct ()

It can be seen from the picture in the question that the right end of the lever is tilted up. To balance the lever in a horizontal position, the balance nut should move to the right end, so A is wrong.

Suppose the weight of a hook code is G and the length of one grid is l. It turns out: 3G×2l=2G×3l, and the lever is balanced.

Add a hook code of equal mass to each hook code on both sides of the lever. Now: 4G×2l<3G×3l, so the lever is no longer balanced and the right end of the lever sinks, so B is wrong.

When the spring dynamometer moves from position a to position b, the moment arm of the pulling force F becomes shorter. According to the balance condition of the lever, the pulling force becomes larger, that is, the indication of the dynamometer becomes larger, so C is correct.

If the gravity of the lever is ignored, F'×2l=3G×4l can be obtained according to the balance condition of the lever, and the indication of the dynamometer is F'=6G=6×0.5 N=3 N. Use the device as shown in the title picture to conduct research. Since the center of gravity of the lever is at the midpoint of the rod, that is, the center of gravity of the lever is not on the fulcrum, the influence of the gravity of the lever must be considered. The gravity of the lever and the gravity of the hook will cause the lever to rotate counterclockwise, so the indication of the spring dynamometer should be greater than 3 N, so D is wrong.

2. (2018·Zhejiang Ningbo High School Entrance Examination) As shown in the figure, F1=4 N, F2=3 N, the spring dynamometer indication remains unchanged. At this time, object A is stationary relative to the ground, and object B moves at 0.1 m/s The speed of the object A makes a uniform linear motion to the left on the surface of object A (not counting the spring dynamometer, the self-weight of the pulley and the rope, and the friction between the pulley and the rope). Which of the following statements is wrong ()

A.The power of F2 is 0.6 W

B. The spring force gauge reads 9 N

C. There is friction between object A and the ground

D. If you increase F2, object A may move to the left

From the picture in the question, the pulley block is used horizontally, n=2, the moving speed of the pulling end v=2v object=2×0.1 m/s=0.2 m/s, the pulling power P2=F2v=3 N×0.2 m/s=0.6 W, so A is correct.

Excluding the self-weight of the spring dynamometer, the pulley and the rope, and the friction between the pulley and the rope, the spring dynamometer indicates F=3F2=3×3 N=9 N, so B is correct.

The friction force of object A on B is 2×3 N=6 N, and the pulling force on object A is 4 N. Therefore, object A has a tendency to move to the left relative to the ground. There is friction between object A and the ground, so C is correct.

If F2 is increased, the pressure of B on A and the roughness of the contact surface remain unchanged, the friction between B and A remains unchanged, the force on A remains unchanged, and it is still in a stationary state, so D is wrong.

3. (2018 Chongqing High School Entrance Examination Paper B) When building new houses, some farmers often use simple pulleys as shown in the picture to lift building materials. In a process of lifting building materials, construction workers used a pulling force of 400 N to lift a building material weighing 600 N by 3 m at a constant speed in 10 s. Then the following judgment is correct ()

A. The useful work is 1 200 J

B. The gravity on the pulley is 100 N

C. The mechanical efficiency of the pulley η = 75%

D. The speed at which the free end of the rope moves is 0.3 m/s

4. (2017 Shandong Dezhou High School Entrance Examination) In the pulley block shown in the figure, the gravity of the movable pulley is 1 N. Xiaoqiang uses a pulling force F of 6 N to pull an object with a gravity of 10 N through the pulley block at a constant speed. The object rises 0.4 in the vertical direction. m. In this process, the additional work and mechanical efficiency are ()

A.0.4 J　83.3%B.0.8 J　91.7%

C.0.8 J　83.3%D.0.4 J　91.7%

5. (2017·Zhejiang Hangzhou High School Entrance Examination)

Xiao Jin placed a 0.6 m long wooden stick with negligible mass on his shoulder. A 40 N object was hung on the back end A of the stick. The fulcrum O on the shoulder was 0.2 m away from the back end A. He pressed the front end B with his hand. Keep the wooden stick horizontally balanced. As shown in the figure, the mass of Xiao Jin is 50 kg. At this time, the pressure of the hand pressing the wooden stick is ________N, the supporting force of the shoulder on the wooden stick is ________N, and the pressure of the person on the ground The size is _________N. (g is taken as 10 N/kg)

6. (2017·Jiangsu Zhenjiang High School Entrance Examination)

As shown in the figure, a pulling force of F=20 N is used to lift an object with a gravity of G=30 N by 2 m at a constant speed. Ignoring the friction between the wheel axles and the weight of the rope, the movable pulley G moves =_________N. The useful work W in the above process is =_________ J, the mechanical efficiency of the moving pulley η = _________.

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Update Time: 2024-09-29

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