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

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

Part One: Learning Objectives

1. Master the formula of work W=Flcos α and the applicable range of the formula, and be able to use the formula of work to calculate.

2. Understand the concepts of positive and negative work, and be able to calculate the total work done by multiple forces according to formulas.

3. Understand the concept of power and be able to use the definition of power P= and the relationship between power and speed P=Fv to conduct relevant analysis and calculations.

Work and Power PPT, Part 2: 01 Sort out the teaching materials and lay a solid foundation

1. Gong

1. The formula of work: W=________, where F, l, and α are ________, the size of the displacement, and ____________ respectively.

2. Work is a ___ (fill in "vector" or "standard") quantity. In the International System of Units, the unit of work is ________ and the symbol is _____.

2. Positive work and negative work

1. Conditions for a force to do positive or negative work on an object

It can be known from W=Flcos α

(1) When α=π2, W=__, force F does no work on the object.

(2) When 0≤α<π2, W 0, force F does __ work on the object.

(3) When π2<α≤π, W 0, force F does __ work on the object.

2. Calculation of total work

When an object undergoes a displacement under the combined action of several forces, the total work done by these forces on the object is equal to:

(1) The ________ work done by each component force on the object.

(2) The work done by ______ of several forces on an object.

3. Power

1. Meaning: Power is a physical quantity that represents the ______ of an object.

2. Definition: The ratio of work W to the ______ used to complete the work.

3. Definition: P=____. Unit: ____, abbreviated as ____, symbol ____.

4. The relationship between power and speed: P=Fv (F and v are in the same direction).

Application: From the relationship between power and speed, it is known that vehicles, trains and other transportation vehicles and various lifting machinery, when the power P of the engine is constant, the traction force F and the speed v are ______ (fill in "positive" or "inverse") ratio, it is necessary to To increase traction, you need ______ speed.

5. Power is ____ (fill in "standard" or "vector") quantity.

Ready to use

1. Decide whether the following statements are true or false.

(1) l in the formula W=Flcos α is the distance the object moves. ()

(2) As long as an object receives a force and moves, the force must do work. ()

(3) Work can be divided into positive and negative, so work is a vector. ()

(4) According to the formula P=, the more work done, the greater the power. ()

(5) The faster the force does work on the object, the greater the power of the force must be. ()

(6) When a car climbs a hill, it is often necessary to shift to a high gear. ()

2. As shown in Figure 1, an object with a mass of 1 kg rests on a smooth horizontal surface. Now give the object a pulling force F of 10 N and an angle of 60° to the horizontal direction. The object is under the action of the pulling force F. It moves along the horizontal plane for 2 s, then within these 2 s, the work done by the pulling force F is ____ J and the power is ____W.

Work and Power PPT, Part 2 Content: 02 Exploring Key Points to Improve Literacy

1. Understanding of gong

guided inquiry

1. Observe Figure 2 and analyze which person in the figure has done work on the object?

2. As shown in Figure 3, the object advances l along the horizontal plane under the action of a force F that makes an angle α with the horizontal direction. How much work does force F do on the object?

knowledge deepening

Understanding of the formula W=Flcos α

1. The work done by a certain constant force F on an object is only related to l and α, and has nothing to do with the motion state of the object and whether the object is also subject to other forces.

2. Work is a scalar quantity, it has no direction, but it has positive and negative values.

3. The formula W=Flcos α is suitable for calculating work done by constant force. If it is a variable force, this formula is no longer applicable.

Example 1 As shown in Figure 4, the total mass of a person sitting on a sled and the sled is m. Under the action of a constant pulling force F that forms an angle θ with the horizontal plane, the person moves a distance l along the horizontal ground to the right. It is known that the sled and The kinetic friction coefficient between the ground is μ and the acceleration due to gravity is g. Then the force exerted on the sled is

A. The work done by the supporting force is mgl

B. The work done by gravity is mgl

C. The work done by the pulling force is Flcos θ

D. The work done by sliding friction is -μmgl

2. Understanding of positive and negative work and calculation of work

guided inquiry

The displacement of an object moving horizontally to the right under the action of force F is l. The directions of the pulling force are shown in Figure 5A and B respectively. Find the work done by the pulling force on the object in the two cases.

knowledge deepening

1. Understanding and judging positive and negative work

Example 2 (Multiple choice) An object with mass m is stationary on an inclined plane with an inclination angle θ. The inclined plane moves a distance l uniformly to the right in the horizontal direction, as shown in Figure 6. The object is always stationary relative to the inclined plane, then the following statement is correct yes

A. Gravity does positive work on the object

B. The work done by the net force on the object is zero

C. Friction does negative work on the object

D. The supporting force does positive work on the object

2. Calculation of total work

When an object undergoes a displacement under the joint action of multiple forces, the work done by the resultant force on the object is equal to the algebraic sum of the work done by each component force on the object. Therefore, there are two methods for calculating the work of the resultant force:

(1) First calculate the work done by each force on the object W1, W2, W3... based on W=Flcos α, and then find the algebraic sum of the work done by all forces, that is, W = W1+W2+W3+....

(2) First find the resultant force Fsum from the combination of forces or according to Newton's second law, and then calculate the total work from Wsum = Fsum lcos α, where α is the angle between the direction of Fsum and the direction of l.

Note: When the displacements exerted by several forces are different in a process, only method (1) can be used.

Example 3 (Mid-term of the first semester of high school in Jiaxing Seven Schools, 2019) As shown in Figure 7, an object with a mass m = 2 kg is acted upon by an oblique upward force F = 10 N at an angle of 37° to the horizontal direction. The distance the ground moves to the right from rest is l=2 m. It is known that the kinetic friction factor between the object and the ground is 0.3, and g is 10 m/s2. Find the total work done by the external force on the object. (cos 37°= 0.8, sin 37°＝0.6)

Work and power PPT, the third part: 03 in-class drill, implementation point by point

1. (Judgment of positive and negative work) (2018·Shandong Summer Academic Proficiency Test) In order to cooperate with the students on duty in cleaning the floor, a student lifted the stool up and then slowly put it back to its original place. During this process, the student did work on the stool. The situation is

A. Always do positive work B. Do positive work first, then negative work

C. Always do negative work D. Do negative work first, then positive work

2. (Calculation of work) (End of the first semester of Minxian No. 1 Middle School and Senior High School in 2018) Use a horizontal constant force F to act on an object with mass m, so that it moves a distance l in the direction of the force on a smooth horizontal surface. The constant force F The work done by the constant force F is W1; then the constant force is used to act on an object with a mass of 2m, so that it moves the same distance l in the direction of the force on a rough horizontal surface. The work done by the constant force F is W2. Then the relationship between the work done by the two constant forces is yes

A.W1>W2 B.W1

C.W1=W2 D.Unable to determine

3. (Calculation of total work) As shown in Figure 10, a skier with mass m = 50 kg slides down from the top of a slope with a height h = 30 m from stationary. The inclination angle of the slope is θ = 37°. The distance between the ski and the snow surface is The dynamic friction coefficient μ = 0.1. Then when the athlete slides to the bottom of the slope: (g is 10 m/s2, sin 37° = 0.6, cos 37° = 0.8, the quality of the equipment is not included)

(1) How much work does the gravity on the skier do to him?

(2) What is the total work done by each force on the athlete?

4. (Understanding of power) Regarding power, which of the following statements is correct?

A. It can be seen from P= that the power at any time can be calculated as long as the values ​​​​of W and t are known.

B. From P=Fv, it can be seen that the power of the car must be proportional to its speed.

C. It can be seen from P=Fv that the traction force must be inversely proportional to the speed

D. When the power of the car is constant, the traction force must be inversely proportional to the speed.

5. (Instantaneous power analysis) (2018·Shangqiu City High School Final Semester Nine Schools Joint Entrance Examination) When the pilot is conducting quality training, the process of grabbing the swing bar and starting to swing from the horizontal state to the vertical state is shown in Figure 11. The instantaneous power change of the pilot under gravity is

A. Keep increasing

B. keeps decreasing

C. Increase first and then decrease

D. First decrease and then increase

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