"Work and Power" Law of Conservation of Mechanical Energy PPT

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"Work and Power" Law of Conservation of Mechanical Energy PPT

Part 1: Essential knowledge and foundation of literacy

1. Gong

1. Formula: W=Flcosα.

(1)α is the angle between the ________ directions, and l is the displacement of the object ________.

(2)This formula applies to _____ work.

(3) Work is _____ (optional "scalar" or "vector").

(4) Work is a _____ (optional "process" or "state") quantity.

(5)Unit:___________. The symbol is J.

2. Positive work and negative work: Connect the corresponding knowledge with lines.

2. Power

[Thinking] Crane A raises the cargo of 1 ton by 5 m at a constant speed in 60 s, and the other crane B raises the cargo of 1 ton by 5 m at a constant speed in 30 s. Which crane does work faster? How to compare how fast they do work?

Tip: Crane B does the work faster, the same amount of work is done, and the one that takes less time does the work faster.

1. Definition: The _____ between the work W and the time t taken to complete the work is called power.

2. Definition: P=___.

3. Unit: _____ (watt), symbol is __.

4.Power is a ___ quantity.

5. Physical meaning: It is a physical quantity that expresses work _____.

6. Rated power and actual power:

(1) Rated power: _____ power output by the engine during normal operation.

Actual power: The power output by the engine when it is actually working.

(2) In order to ensure the safety of the machinery, try to use _______ when working.

3. Power and speed

[Thinking] When a car goes uphill, why does the driver have to shift to a lower gear to reduce the speed?

Tip: According to the formula P=Fv, when the car goes uphill, the power remains unchanged. In order to obtain greater traction, it is necessary to shift to a lower gear to reduce the speed.

1. Relationship: When a force is on the same straight line as the direction of motion, the power of the force on the object is equal to the product of the force and _____.

2. Relationship: P=Fv.

(1) If v is the average speed of the object, then P = Fv is the ________ within the corresponding time t.

(2) If v is the instantaneous speed, then P represents the _________ at that moment.

3. Application: It can be seen from P=Fv that for vehicles such as cars and trains, when the engine power P is constant, the traction force F and the speed are _____. To increase the traction force, the speed must be reduced.

4. Average power and instantaneous power:

(1)Average power.

① Size: the average power within a certain period of time t.

②Meaning: Indicates the average speed of work done within ________, corresponding to a period of time (or process).

(2) Instantaneous power.

① Size: The instantaneous value of power at a certain moment can accurately describe the speed of work.

②Meaning: Indicates how fast _________ does work, corresponding to a certain moment (or a certain position).

Work and Power PPT, Part 2: Key Competencies·Quality Formation

1. The meaning of work and the judgment of positive and negative

1. There are two conditions for doing work: the force doing the work and the displacement of the object in the direction of the force. Both conditions are indispensable.

2. Factors that determine the size of work: According to the formula of work W=Flcosα, the size of work is determined by the size of the force that does the work and the displacement of the object under force in the direction of the force, as well as the mass of the object and the location of the object. It has nothing to do with factors such as the environment and the state of motion of the object, that is, the work done by force is independent.

【Think·Discussion】

In the following three situations, do people do work on objects? (Physical concepts)

(1) The weightlifter is holding the barbell motionless.

(2) A person carries a pot of flowers on a level ground at a constant speed.

(3) The child pulls the box and moves to the right along the horizontal ground for a certain distance.

Tips: (1) The barbell does not shift, and the force of lifting the barbell does no work on the barbell.

(2) The force of moving the flowerpot is perpendicular to the displacement of the flowerpot, and the person does no work on the flowerpot.

(3) The box undergoes a certain displacement in the direction of the pulling force, and the pulling force does work on the box.

[Typical example demonstration]

There is a car moving to the left on a level road. There is a fixed incline in the car. A person is sitting on the incline. The car and the person are always relatively stationary. Which of the following statements is correct ()

A. When the car is traveling at a constant speed, the supporting force of the slope facing the person does no work.

B. When the car slows down, the slope does negative work on the supporting force of the person.

C. When the car slows down, the friction force exerted by the slope on the person must do negative work.

D. When the car accelerates, the friction force exerted by the slope on the person must do positive work.

[Analysis] Choose C. A person is affected by three forces: gravity, vertical slope upward supporting force, and frictional force. No matter whether the car is accelerating, uniform, or decelerating, the angle between the supporting force and the direction of the speed is less than 90°, that is, the supporting force does positive work. Therefore, A and B are wrong; when the car moves at a constant speed, the total external force on the person is equal to 0, that is, the resultant force of the support force and friction force on the person is in the same direction as the person's gravity. Then when the car decelerates to the left, Only when the resultant force of the supporting force and the frictional force is upward and to the right can the resultant force of the person be horizontally directed to the right. It can be seen that the component of the frictional force in the horizontal direction must be greater than the component of the supporting force in the horizontal direction, so the angle between the frictional force and the speed It must be greater than 90°, that is, the frictional force does negative work, so C is correct; similarly, when the car accelerates to the left, the resultant force of the supporting force and the frictional force is upward to the left, so that the resultant force of the person can be horizontally directed to the left, then It can be seen that there are three situations of friction. In the first situation, the component force of the friction force in the horizontal direction along the slope must be smaller than the component force of the supporting force in the horizontal direction. The friction force does negative work. In the second situation, the friction force is zero. The friction force No work is done. In the third case, the friction force does positive work, so D is wrong.

[Rule method] How to judge whether positive or negative work is done

(1) Judgment based on the angle α between the force F and the object's displacement direction l - often used in situations where constant force does work.

(2) Judgment based on the angle θ between the force and the instantaneous velocity direction of the object - often used in curved motion situations.

【Motif question】

1. (Multiple choice) In the [Typical Example] situation, if the inclined plane in the car is changed to a horizontal plane, and people sit on the horizontal plane, the car and the person will always be relatively stationary. Then the following statement is correct ()

A. When the car is traveling at a constant speed, the horizontal surface does not do any work to the supporting force of the person.

B. When the car slows down, the horizontal surface does negative work on the supporting force of the person.

C. When the car slows down, the friction force on the horizontal surface of the person must do negative work.

D. When the car accelerates, the friction force on the horizontal surface of the person must do positive work

[Analysis] Choose A, C, or D. When the car is traveling at a constant speed, the person is affected by two forces: gravity and the vertical and horizontal supporting force. No matter whether the car is accelerating, uniform or decelerating, the angle between the supporting force and the speed direction is equal to 90°, that is, the supporting force is not Work is done, A is correct, B is wrong; when the car slows down, the person is affected by three forces: gravity, vertical and horizontal upward support force, and horizontal friction force to the right. That is, the direction of the friction force on the person is opposite to the direction of the speed. The frictional force does negative work, C is correct; when the car accelerates, the person experiences friction on the horizontal plane moving horizontally to the left, in the same direction as the speed, the frictional force does positive work, D is correct.

2. (Multiple choice) The person in [Typical Example Demonstration] got off the car and entered the elevator to go upstairs. It is known that the process from the 1st floor to the 20th floor experienced a motion process of first accelerating, then uniform, and then decelerating. Then the elevator The work situation of human support force is ()

A. Positive work is done when accelerating, no work is done when the speed is constant, and negative work is done when decelerating.

B. Positive work is done when accelerating, and negative work is done when the speed is constant and decelerating.

C. Positive work is done when accelerating and at a constant speed, and negative work is done when decelerating.

D.Always do good work

[Analysis] Choose D. In the process of acceleration, uniform speed, and deceleration, the supporting force and the person's displacement direction are always the same, so the supporting force always does positive work on the person, so D is correct.

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For more information about the PPT courseware "Work and Power Law of Conservation of Mechanical Energy", please click the Work and Power ppt Law of Conservation of Mechanical Energy ppt tag.

"Work and Power" Law of Conservation of Mechanical Energy PPT courseware:

"Work and Power" Law of Conservation of Mechanical Energy PPT Courseware Part One: Learning Objectives 1. Understand the concept of work 2. Master the applicable conditions of W=Flcos a and be able to use it for calculations. 3. Understand the meaning of positive work and negative work, and be able to find the total work of multiple forces. 4. Master the power of change..

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Update Time: 2024-07-28

This template belongs to Physics courseware People's Education Press High School Physics Compulsory Course II industry PPT template

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