机械能守恒定律机械能守恒定律Law of conservation of mechanical eLaw of conservation of mechanical e

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珠海一中　　　　杨林珠海一中　　　　杨林ZhuHai No.1 Middle School YanZhuHai No.1 Middle School Yan

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VocabularyVocabulary

Mechanical energy 机械能Kinetic energy 动能Potential energy 势能Gravitational potential energy 重力势能Elastic potential energy 弹性势能Gravity 重力Free fall motion 自由落体运动Pendular movement 摆动Projectile motion 抛体运动

1.1. 自由落体运动中能量的转化自由落体运动中能量的转化 Energy change the forms in the motion of frEnergy change the forms in the motion of fr

ee fallee fall2.2. 竖直上抛运动中能量的转化竖直上抛运动中能量的转化　　 Energy change the forms in the motion of uEnergy change the forms in the motion of u

pright projectilepright projectile3.3. 实验：实验： experimentexperiment 小球在摆动中机械能守恒小球在摆动中机械能守恒　　 The total mechanical energy remain constaThe total mechanical energy remain consta

nt in the pendular movement of the little ballnt in the pendular movement of the little ball

4.4. 分析自由落体运动分析自由落体运动 : :

Research free fall motion again Research free fall motion again

A

h1

v1

B

h2v2

由动能定理得 :According to the theorem of kinetic energy 12

1212

221 mVmVWG

由功 _ 能原理得 :According to the work _ energy theorem 221 mghmghWG

We can substitute (1) to (2)

The above equation can be rewritten The above equation can be rewritten asas

212

1212

221 mghmghmVmV

12

121

22

221 mghmVmghmV

or

1122 PKPK EEEE

The above equation states that in the free fall motion.the sum of the kinetic energy and the potential energy(or total of mechanical energy) remain constant

Above result not only apply to free Above result not only apply to free fall motion.but also apply to all kinds fall motion.but also apply to all kinds of motion that work is done upon an of motion that work is done upon an object by gravity or spring forces.object by gravity or spring forces.

Then we can get the law of conservation of mechanical energy.

When work is done upon an object by gravity and spring forces.energe change forms _ from kinetic energy to potential energy (or vice versa).yet the total amount of mechanical energy is conserved.

ExerciseExercise

Explain (KE to PE or PE to KE)?Answer (is the mechanical energy remain constant)?1.A bungee chord begins to exert an upward force upon a falling bungee jumper.2.The spring of a dart gun exerts a force on a dart as it is launched from an initial rest position.3.A skier glides from location A to location B across the friction free ice.

4.A baseball is traveling towards a 4.A baseball is traveling towards a man in the bleachers.ignore air man in the bleachers.ignore air friction force.friction force.

5.A weightlifter applies a force to lift a 5.A weightlifter applies a force to lift a barbell above his head at constant barbell above his head at constant speed.speed.