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"Pulley" Simple Machine PPT (Fixed Pulley and Moving Pulley in Lesson 1)
Part One: Knowledge Management
1. fixed pulley
Definition: A pulley whose shaft is fixed during operation is called _____.
Essence: It is an equal-arm lever (i.e. power arm = resistance arm).
Features: It can neither save effort nor distance, but it can change the _____ of force.
Rule: When a fixed pulley is used to lift a heavy object, the distance the power point moves is equal to the height of the object being lifted.
2. moving pulley
Definition: A pulley whose shaft moves with the object during work is called a moving pulley.
Essence: It is a labor-saving lever whose power arm is the resistance arm _____.
Features: Ideally, half the force can be saved, but the distance will be doubled, and the direction of the force can be changed _____.
Rule: When using a movable pulley to lift a heavy object, the power is 1/2 of the weight of the object, but the distance the power action point moves (i.e. the distance the free end of the rope moves) is twice the height of the object being lifted (i.e. save half the force and move more twice the distance).
Difference: The difference between a fixed pulley and a movable pulley mainly depends on whether the axis of the pulley moves with the object when the free end of the rope moves. If the axis of the pulley is fixed, it is a fixed pulley; if the axis of the pulley moves together, it is a movable pulley.
Pulley PPT, part 2: classification and exploration
Type 1: Characteristics of fixed pulleys
(2018·Shanghai Jing'an District) As shown in the figure, object A is lifted at a uniform speed using force F1, F2, and F3 respectively. Regardless of friction, the relationship between the three forces is ( )
A． F1＜F2＜F3 B. F1＝F2＝F3
C. F1>F2>F3 D. F2>F1＝F3
[Analysis] Because the fixed pulley is equivalent to an equal-arm lever, it can only change the direction of the force without saving effort. Therefore, the fixed pulley pulls the same object A along three different directions with equal pulling forces, that is, F1, F2, and F3. are equal to the gravity of the object. Therefore, B conforms to the meaning of the question. So choose B.
[Enlightenment] This question mainly tests the understanding and mastery of the working characteristics of the fixed pulley. Know the characteristics of the fixed pulley: it does not save effort but can change the direction of the force.
Type 2 Characteristics of moving pulleys
(2018•Tengzhou, Shandong) As shown in the figure, G = 200 N, the pulley weight G′ = 40 N, if the object can rise 2 m at a constant speed, the following statement is correct (excluding friction and rope weight) ( )
A． F = 440 N, moving up 4 m
B． F = 440 N, moving up 1 m
C. F = 240 N, moving up 4 m
D. F=240 N, moving up 1 m
[Analysis] This is a special use method of the movable pulley. It can be regarded as a deformation of the lever. The fulcrum is O, OA is equivalent to the power arm, and OB is equivalent to the resistance arm. Its characteristic is that it takes twice as much effort but saves half the distance, as shown in the figure.
Therefore, the applied force F=2G+G′=2×200 N+40 N=440 N, and the distance moved by the pulling force application point is s=12h=12×2 m=1 m. So choose B.
[Enlightenment] This question tests the special use of moving pulleys. This question can be answered very well simply by using what we have learned about moving pulleys.
[Summary of rules] This question is about a special use method of moving pulleys. In normal use, the object is hung on the axle and one end of the rope is pulled; while in special use, the opposite is done, where the object is hung on the end of the rope and the axle is pulled directly. The special method of using the movable pulley uses twice the force, but can save half the distance.
Pulley PPT, the third part: evaluation in class
1. (Corresponding Example 1) (2018·Liuzhou, Guangxi) The top of the school flagpole is equipped with a fixed pulley. Do this ( )
A． It saves effort and can also change the direction of force application.
B． Save effort, but cannot change the direction of force application
C. It neither saves effort nor changes the direction of force application.
D. It doesn’t save effort, but it can change the direction of force application.
2. (Corresponding Example 2) (2018·Chenzhou, Hunan) As shown in the figure, excluding the dead weight of the pulley and the friction between the rope and the pulley, the correct relationship between the tensions FA, FB, and FC of the three spring dynamometers is ( )
A． FA:FB:FC=3:2:1
B． FA:FB:FC=1:2:3
C. FA:FB:FC=6:3:2
D. FA:FB:FC=2:3:6
[Analysis] Based on the force borne by each section of rope, analyze the relationship between the tension forces FA, FB, and FC of the three spring dynamometers, as shown below: Assume that each section of rope bears the force of F, and FC bears the tension of the three rope sections. , FC=3F; FB bears the tensile force of two sections of rope, FB=2F; FA bears the tensile force of one section of rope, FA=F. Therefore FA:FB:FC=F:2F:3F=1:2:3, so B is correct and A, C and D are wrong. So choose B.
3. (Corresponding Example 2) (Ningbo, Zhejiang) As shown in the figure, the same movable pulley is used to lift the same object successively, so that the object rises to the same height at the same speed and uniform speed. The forces used are F A and F B respectively. The power of the pulling force is They are P A and P B respectively. If friction, the weight of the movable pulley and the weight of the rope are not taken into account, the relationship between F A and F B, P A and P B is ( )
A． F A < F B, P A = P B
B． F A>F B, P A>P B
C. F A>F B, P A=P B
D. F A＜F B, P A＜P B
Pulley PPT, part 4: layered operations
1. (Corresponding Examples 1 and 2) (2018·Guigang, Guangxi) As shown in the figure, two devices A and B with the same pulley are used to lift two objects A and B with a weight of 10 N at a constant speed. It is known that the pulley weighs 1 N and the rope The friction between the weight and the pulley is ignored. Then the following statement is correct ( )
A． Hand pulling force F B = 10 N
B． The net force on object A is equal to the net force on object B.
C. If both objects A and B are lifted by 0.5 m, more useful work will be done by using device B.
D. If both objects A and B are lifted by 0.5 m, then the work done by F A due to the pulling force of the hand is more than the work done by F B.
2. (Corresponding Examples 1 and 2) (Zaozhuang, Shandong) As shown in the figure, when the same object is moved at a constant speed along the same level ground with a pulley, the pulling forces are F A, F B, F C, and F D respectively. Compare their sizes ( Ignoring the weight of the pulley and the friction between the pulley and the string), the incorrect one is ( )
A． F A = F D B. F B＞F A
C. F B=F C D. F D＜F C
[Analysis] It can be seen from the figure that the first and fourth pulleys move with the object and are movable pulleys; regardless of the weight of the pulley and the friction between the pulley and the string, half the force can be saved when pulling the object horizontally, while the fourth pulley Because the pulling force direction of the pulley is not horizontal (the power arm is slightly smaller), the force will be slightly larger, that is, F D > F A = 12f, so A is wrong. The second and third pulleys are fixed pulleys. Using this pulley cannot save effort, but it can change the direction of the force; when pulling an object horizontally or obliquely, the power arm is equal to the radius of the pulley (that is, the power arm remains unchanged), so the pulling force The size remains unchanged, that is, F B = F C = f; that is, F B = F C > F D > F A. Choose A.
3. (Corresponding to Examples 1 and 2) In order to explore the characteristics of movable pulleys and fixed pulleys, the following two methods are designed to lift heavy objects. The following methods and understandings of the exploration are correct ( )
A． Use the movable pulley to lift the weight to height h, and the dynamometer also rises to height h.
B． If the same object is lifted to the same height, the pulling force of the movable pulley is smaller and the work done is less.
C. When the weight is constant, reducing the mass of the movable pulley can reduce the pulling force
D. If a fixed pulley is used to pull a heavy object, it will save the most effort when the pulling force is vertically downward.
4. (Corresponding to Examples 1 and 2) (2018·Tangshan, Hebei) As shown in the figure, the same pulley is used to pull the same object in two different ways to make uniform linear motion on the horizontal plane. The pulling forces used are F1 and F2 respectively, excluding the rope. If there is friction between the weight and the pulley shaft, then ( )
A． F1>F2 B. F1＜F2
C. F1＝F2 D. Unable to judge
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