【英语时差8,16】势能

Potential energy in a crossbow When we think of the concept of energy, we often think of objects in motion: a car rolling down a hill or a jogger running up a hill. This kind of energy-in-motion, called kinetic¹ energy, isn’t the only kind of energy. Another type, which motionless objects can have, is called potential energy. Potential energy is stored energy; that is, energy that can be tapped later. To better understand the concept of potential energy, consider how a crossbow works. If you took a crossbow, pulled its string back, and cocked it so that the arrow remained in place until you decided² to shoot it, you would have created energy, energy that can be released by pulling the trigger. You have increased the potential energy of the crossbow. The amount of potential energy in the cocked crossbow is equal to the amount of work you put in it by pulling it back and cocking it. This potential energy becomes kinetic energy when you pull the trigger. With the release of the trigger, the potential energy of the cocked crossbow converts into kinetic energy as the arrow flies through the air. Except for a bit of energy lost to friction³, the kinetic energy of the flying arrow is equal to the potential energy that was stored in the cocked crossbow. Of course, potential energy doesn’t only exist in crossbows. Other examples include a boulder⁴ on the edge of a cliff, and gasoline in the tank of a car. In effect, potential energy is simply kinetic energy waiting to happen.

 

1 kinetic   

adj.运动的；动力学的

参考例句：

• There exist many sources of energy both potential and kinetic.存在着许多势能和动能的能源。
• The kinetic theory of gases is the best known example.气体动力学理论就是最有名的例子。

2 decided   

adj.决定了的，坚决的；明显的，明确的

参考例句：

• This gave them a decided advantage over their opponents.这使他们比对手具有明显的优势。
• There is a decided difference between British and Chinese way of greeting.英国人和中国人打招呼的方式有很明显的区别。

3 friction   

n.摩擦，摩擦力

参考例句：

• When Joan returned to work,the friction between them increased.琼回来工作后，他们之间的摩擦加剧了。
• Friction acts on moving bodies and brings them to a stop.摩擦力作用于运动着的物体，并使其停止。

4 boulder   

n.巨砾；卵石，圆石

参考例句：

• We all heaved together and removed the boulder.大家一齐用劲，把大石头搬开了。
• He stepped clear of the boulder.他从大石头后面走了出来。