"Law of Conservation of Mechanical Energy" PPT

Description

"Law of Conservation of Mechanical Energy" PPT

Part 1: Essential knowledge and foundation of literacy

1. Mutual conversion of kinetic energy and potential energy

[Thinking] During the upward movement of a trampoline athlete, how do the elastic potential energy of the trampoline, the gravitational potential energy and kinetic energy of the athlete transform into each other?

Tip: Before leaving the trampoline, the elastic potential energy of the trampoline is converted into the athlete's kinetic energy. After leaving the trampoline, the athlete's kinetic energy is converted into the athlete's gravitational potential energy.

1. Forms of mechanical energy: Gravitational potential energy, ________ and kinetic energy are all forms of energy in mechanical motion, collectively called _______.

2. Method of conversion: Mechanical energy can be converted from one form to another through ___________ work.

3. Scalar or vectoriality: Mechanical energy is a state quantity and is _____ (optional "scalar" or "vector").

2. Law of conservation of mechanical energy

[Thinking] How does the mechanical energy change when an object moves from rest to free fall from a certain height?

Tip: During the falling process of an object, the gravitational potential energy decreases and the kinetic energy increases. The gravitational potential energy is converted into kinetic energy, and the total mechanical energy remains unchanged.

1. Content: In an object system where only ___________ does work, kinetic energy and potential energy can be converted into each other, and ____________ remains unchanged.

Law of Conservation of Mechanical Energy PPT, Part 2: Key Abilities·Quality Formation

1. Judgment of mechanical energy conservation conditions

1. Analysis of work conditions: Only gravity and elastic force in the system do work, and other forces do not do work.

2. Energy conversion analysis: Only kinetic energy, gravitational potential energy and elastic potential energy are converted into each other in the system, that is, only the mechanical energy between objects in the system is transferred to each other, then mechanical energy is conserved.

3. Definition and judgment method: When an object moves at a constant speed in the vertical direction or along an inclined plane, the kinetic energy does not change, the potential energy changes, and the mechanical energy is not conserved.

【Think·Discussion】

Situation: As shown in the picture, the roller coaster turns off its engine at a high altitude and rushes down, ignoring all resistance.

discuss:

(1) What force does the roller coaster exert? What work does each person do? (Physical concept)

(2) How does the kinetic energy and potential energy of the roller coaster change? Is the mechanical energy conserved during the descent of the roller coaster? (Model construction)

Tips: (1) The roller coaster is affected by gravity and support force; gravity does positive work, but the support force does no work.

(2) Kinetic energy increases, gravitational potential energy decreases, and mechanical energy is conserved.

[Typical example demonstration]

Among the following situations, the correct statement is ()

A. When an object moves at a uniform speed, mechanical energy must be conserved

B. When the work done by the combined external force on an object is zero, mechanical energy must be conserved

C. When the work done by the combined external force on the object is not zero, mechanical energy may be conserved

D. When an object moves in a curve, mechanical energy must not be conserved

[Analysis] Choose C. If the object moves vertically upward at a constant speed, the kinetic energy remains unchanged and the gravitational potential energy increases, so the mechanical energy is not conserved, so A is wrong; the condition for the conservation of the mechanical energy of the object is not that the work done by the combined external force on the object is zero, but that only gravity or spring force does work. Therefore, B is wrong; if the net external force on the object is gravity, then the mechanical energy is conserved when only gravity does work, so C is correct; when the object moves in a curve, as long as only gravity does work, the mechanical energy is conserved, such as in a flat throw motion, so D is wrong.

【Literacy Training】

1. As shown in the figure, the following statement is correct (excluding friction, air resistance and pulley mass in all cases) ()

A. In Figure A, during the lift-off process of the rocket, if the rocket lifts off at a constant speed, the mechanical energy is conserved; if the rocket lifts off at an accelerated speed, the mechanical energy is not conserved.

B. In picture B, the block slides upward at a constant speed under the action of external force F. The mechanical energy of the block is conserved.

C. In Figure C, when block A compresses the spring at a certain initial speed, the mechanical energy of block A is conserved.

D. In Figure D, when block A accelerates to fall and block B accelerates to rise, the mechanical energy of systems A and B is conserved.

[Analysis] Choose D. In Figure A, whether it is at a constant speed or accelerating, because the thrust does work on the rocket, the mechanical energy of the rocket is not conserved and increases, so A is wrong; in Figure B, the object slides up at a constant speed, the kinetic energy remains unchanged, and the gravitational potential energy increases, then the mechanical energy It must increase, so B is wrong; in Figure C, during the process of block A compressing the spring, the system composed of spring and block A only has gravity and elastic force to do work, and the mechanical energy of the system is conserved. Since the elastic potential energy increases, the mechanical energy of A decreases. , so C is wrong; in Figure D, for the system composed of A and B, air resistance is not considered, only gravity does work, and the mechanical energy of the system composed of A and B is conserved, so D is correct.

2. "Bamboo Dragonfly" is a children's toy. Rubbing the handle with both hands can make the "Bamboo Dragonfly" rise upwards. In a certain experiment, the "Bamboo Dragonfly" rose straight to the highest point after leaving your hands. During this process ()

A. The force of air on the "bamboo dragonfly" is greater than the force of the "bamboo dragonfly" on the air

B. The kinetic energy of "Bamboo Dragonfly" keeps increasing

C. The gravitational potential energy of the "Bamboo Dragonfly" keeps increasing

D. Conservation of mechanical energy of "Bamboo Dragonfly"

【Compensation training】

1. During the motion of the following objects, the mechanical energy can be regarded as conserved ()

A. Falling autumn leaves

B. A person who rises in an elevator at a constant speed

C. A wooden box sliding down a rough slope at a constant speed

D. A shot put that moves in the air after being thrown

[Analysis] Choose D. In addition to gravity, the falling autumn leaves also have resistance to do work, so the mechanical energy is not conserved, so A is wrong; for a person rising in an elevator at a constant speed, the kinetic energy remains unchanged, the gravitational potential energy increases, and the mechanical energy increases, so B is wrong; along a rough slope, the speed is constant The kinetic energy of the sliding wooden box remains unchanged, the gravitational potential energy decreases, and the mechanical energy decreases, so C is wrong; the shot put that moves in the air after being thrown has only gravity doing work during its movement, so the mechanical energy is conserved, so D is correct.

2. (Multiple choice) As shown in the figure, on a smooth and fixed surface, there are two small balls A and B with masses of 1 kg and 2 kg respectively, which can be regarded as particle points. There is a light pole between the two balls. The mass springs are connected, hold A with your hand and place it vertically as shown in the figure. The distance between A and B is L=0.2 m. The ball B has just contacted the curved surface and is at a height h=0.1 m from the horizontal plane. At this time, the elastic potential energy of the spring Ep = 1 J. After free release, the two balls and the spring start to slide from rest to the smooth ground, and then continue to move along the smooth ground. Regardless of the loss of mechanical energy during collision, g is 10 m/s2. Then the correct one of the following statements is ()

A. During the entire process of sliding down, the mechanical energy of the system composed of spring and ball A is conserved.

B. The mechanical energy of the system composed of two balls and springs is conserved during the entire process of sliding.

C. When ball B first reaches the ground, its speed is m/s

D. When the spring is at its original length and the ground is used as the reference plane, the mechanical energy of the two balls moving on the smooth horizontal plane is 6 J.

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Update Time: 2024-07-28

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