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

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"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 method of obtaining the force-changing power

5. Know the concept of power and understand the physical meaning of power

6. Master the definition of power P=W/T and be able to use P=w/t to answer related questions

Work and power PPT, part 2: introduction of new lesson

1. The formulas of work and work

1. Definition of work

When an object is acted upon by a force and a period of ____ occurs in ________, we say that the force has done work on the object.

2. factors that do work

(1) Force; (2) The displacement of an object on ____.

3. formula for work

(1) When force F and displacement l are in the same direction: W=____.

(2) When the force F and the displacement l have an angle α: W = ____, where F, l, and cos α represent ________, ________, and ________ respectively.

(3) The units of each physical quantity: The unit of ____ is N, the unit of ____ is m, and the unit of W is ____, that is, ____.

2. Power

1. definition

The ratio of the ____ a force does on an object to the ____ it takes to complete the work.

2. Definition: P=____.

3. unit

In the International System of Units, the unit of power is ____, referred to as watt, represented by the symbol W.

4. significance

Power is a scalar quantity, which is a physical quantity that represents the work done by an object.

3. Power and speed

1. The relationship between power and speed

P=____(F and v are in the same direction).

2. Derivation

Power definition formula: P=____ Work calculation formula: W=____ Displacement: l=vt �→P=____

3. application

From the power-speed relationship, we know that when the engine power P is constant for vehicles, trains and other transportation vehicles and various lifting machinery, the traction force F and the speed v become ____. To increase the traction force, the ____ speed is needed.

Work and power PPT, the third part: connect with practice and apply what you have learned

1. When a car goes uphill, the driver usually has to "shift gears" to reduce the speed so that the car climbs the hill at a lower speed. Why? Please explain using what you learned today.

When the power of the car engine is constant, according to P=Fv, the traction force is inversely proportional to the speed, so the traction force is increased by reducing the speed so that the car can go uphill smoothly.

2. When the car is going uphill, if the speed is not reduced, what measures should the driver take? Why?

Increase the throttle and increase the output power to obtain greater traction.

Work and Power PPT, Part 4: Frequently Asked Questions

Question Group 1: Understanding of Gong

Question 1 [Multiple choice] Which of the following statements is correct ()

A. Work is a vector, and positive and negative represent directions.

B. Work is a scalar quantity. Positive or negative indicates whether an external force does work on the object or whether the object overcomes the external force and does work.

C. The units of work and energy are both joules, and work and energy can be converted into each other.

D. Work done by force is always completed in a certain process, so work is a process quantity

[Analysis] A is wrong, B is right: work can be divided into positive and negative, but work is a scalar quantity. The positive or negative of work indicates whether power is doing work or resistance is doing work;

Wrong C: One form of energy can be converted into another form of energy by doing work, but it cannot be said that work is converted into energy or energy is converted into work;

Pair D: The work process of force corresponds to time or displacement, so work is a process quantity.

[Basis for solving the problem]

(1) Work is a process quantity. When a force does work, it must correspond to a period of time or a period of displacement.

(2) Work is a scalar quantity. The positive or negative sign of work does not indicate the direction of the work, nor does it indicate the magnitude of the work. Positive work and negative work only represent two different effects of force on objects.

(3) Work is a measure of energy transformation. When a force does positive work on an object, the object's energy increases; when a force does negative work on the object, the object's energy decreases.

Question 2 Regarding the concept of work, which of the following statements is correct ()

A. The force does more work on the object, which means the displacement of the object must be large.

B. The force does less work on the object, which means the force on the object must be small.

C. The force does no work on the object, which means that the object must have no displacement.

D. The amount of work is determined by the magnitude of the force and the magnitude of the displacement of the object in the direction of the force.

Question Group 2: Judgment of whether work is done and whether the work is positive or negative

Question 3 [Multiple Choice] As shown in Figure 7-2-1, an object with mass m is placed on an inclined plane with an inclination angle θ. The kinetic friction factor between the object and the inclined plane is μ. Under the action of external force, the inclined plane accelerates with an acceleration a It makes a uniformly accelerated motion to the left in the horizontal direction. During the motion, the object and the inclined plane are relatively stationary. Regarding objects, which of the following statements is correct ()

A. The supporting force must do positive work B. Gravity must do no work

C. Frictional force may not do any work D. Frictional force must do negative work

[Analysis] Pair A and B: It can be seen from W=Flcos α that the direction of the supporting force is vertically upward and the angle with the displacement direction is less than 90°. The supporting force must do positive work; the angle between the gravity and the displacement direction is equal to 90°. Gravity must do no work.

C is right, D is wrong: The magnitude and direction of the friction force are uncertain. When the acceleration agtan θ, the friction force Ff is upward along the inclined surface. When the acceleration a=gtan θ, the friction force does not exist and no work is done.

Question 4 [2019•Shandong Linyi Senior High School Final Year] As shown in the figure, an object is tied with a light rope and decelerates downward with an acceleration a. Regarding the work done by the force on the object, which of the following statements is correct ()

A. Gravity does positive work, pulling force does negative work, and combined force does positive work.

B. Gravity does positive work, pulling force does negative work, and combined force does negative work.

C. Gravity does positive work, pulling force does positive work, and combined force does positive work.

D. Gravity does negative work, pulling force does negative work, and combined force does negative work.

【Notice】

(1) The formula W=Flcos α is only suitable for calculating the work of constant force.

(2) The quantities W, F, and l in the formula are simultaneous and corresponding. l is the displacement of an object relative to the ground. If a force does work on an object, l is the relative displacement of the object.

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

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Update Time: 2024-07-28

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