效率效率效率

播放段落

This man is pushing heavy boxes on a dolly. If he were to push the boxes across the ground without the dolly, it would take a lot more force because he would have to overcome the sliding between the boxes and the ground. Using a dolly reduces the friction because rolling friction between the wheels and the ground is much less than sliding friction.
::这个男人把重箱子推到一个圆圈上。如果他把盒子推到没有圆圈的地上,那么要用更多的力,因为他必须克服盒子和地面之间的滑动。 使用圆圈可以减少摩擦,因为轮子和地面之间的滚动摩擦远比滑动摩擦要少得多。

播放段落

Machines and Friction
::机器和摩擦

播放段落

A dolly is a because it changes a force to make work easier. What is work? In physics , work is defined as the use of force to move an object over a . It is represented by the equation:
::Dolly 是因为它改变了一种使工作更加轻松的力量。 什么是工作? 在物理学中,工作被定义为用武力将物体移动到一个物体上。 它由方程式表示 :

播放段落

Work = Force x Distance
::工作=强制x距离

播放段落

All machines make work easier, but they don’t increase the amount of work that is done. You can never get more work out of a machine than you put into it. In fact, a machine always does less work on an object than the user does on the machine. That’s because a machine must use some of the work put into it to overcome friction. Friction is the force that opposes motion between any surfaces that are touching. All machines involve motion, so they all have friction. How much work is needed to overcome friction in a machine depends on the machine’s efficiency.
::所有机器都使工作更加容易,但是它们并没有增加完成的工作量。 你永远无法从机器里得到比机器里更多的工作。 事实上,机器对一个物体的工作总是比对机器的使用者少。 这是因为机器必须使用其中的一些工作来克服摩擦。 摩擦是所有机器在触动的表面之间反对运动的力量。 所有机器都涉及运动,因此都有摩擦。 要克服机器中的摩擦需要多少工作取决于机器的效率。

播放段落

Machine Efficiency
::机器效率

播放段落

Efficiency is the percent of work put into a machine by the user (input work) that becomes work done by the machine (output work). The output work is always less than the input work because some of the input work is used to overcome friction. Therefore, efficiency is always less than 100 percent. The closer to 100 percent a machine’s efficiency is, the better it is at reducing friction. Look at the ramp in the Figure . A ramp is a type of simple machine called an . It is easier to push the heavy piece of furniture up the ramp to the truck than to lift it straight up off the ground, but pushing the furniture over the surface of the ramp creates a lot of friction. Some of the force applied to moving the furniture must be used to overcome the friction with the ramp.
::效率是用户( 投入工作) 投入机器完成的工作( 输出工作) 效率的百分率。 输出工作总是比输入工作少, 因为有些输入工作被用来克服摩擦。 因此, 效率总是低于100%。 机器的效率越接近100%, 减少摩擦的效果就越好。 看看图中的坡道。 坡道是一种简单的机器, 叫做“ ” 。 将重型家具拖上坡道, 而不是将它直推到卡车上, 但将家具推到坡道表面会产生很多摩擦。 用于移动家具的一些力必须用来克服坡道的摩擦。

播放段落

Q : Why would it be more efficient to use a dolly to roll the furniture up the ramp?
::问:为什么用圆圈把家具卷上坡道会更有效率?

播放段落

A : There would be less friction to overcome if you used a dolly because of the wheels. So the efficiency of the ramp would be greater with the dolly.
::答:如果你因为轮子而使用圆圈,那么要克服的摩擦就会减少。这样,斜坡的效率会随着圆圈而提高。

播放段落

Calculating Efficiency
::计算效率

播放段落

Efficiency can be calculated with the equation:
::效率可以用下列等式计算:

播放段落

$\begin{array}{r}\text{Efficiency}=\frac{\text{Output work}}{\text{Input work}}\times 100\mathrm{\%}\end{array}$
::效率=产出工作 投入工作x100%

播放段落

Consider a machine that puts out 6000 joules of work. To produce that much work from the machine requires the user to put in 8000 joules of work. To find the efficiency of the machine, substitute these values into the equation for efficiency:
::考虑出6000焦耳的机器。 要从机器中生产这么多的工程, 需要用户输入8000焦耳的工程。 要找到机器的效率, 请用这些值来代替效率的方程式 :

播放段落

$\begin{array}{r}\frac{6000\text{J}}{8000\text{J}}\times 100\mathrm{\%}=75\mathrm{\%}\end{array}$
::60000 J80000 Jx100 75%

播放段落

Q : Rani puts 7500 joules of work into pushing a box up a ramp, but only 6700 joules of work actually go into moving the box. The rest of the work overcomes friction between the box and the ramp. What is the efficiency of the ramp?
::问题:Rani将7500焦耳的工作投入到把盒子推上坡道上,但实际上只有6700焦耳的工作投入到移动盒子中。其余的工作克服了盒子和坡道之间的摩擦。坡道的效率是多少?

播放段落

A : The efficiency of the ramp is:
::A:坡道的效率是:

播放段落

$\begin{array}{r}\frac{6700\text{J}}{7500\text{J}}\times 100\mathrm{\%}=90\mathrm{\%}\end{array}$
::6700 J7500 Jx100 90%

播放段落

As demonstrated above, the efficiency of a machine is simply the ratio of power used to the power supplied (power output divided by power input). Use the simulation below to determine how to maximize the efficiency of a wind turbine:
::如上文所示,机器的效率仅是所用功率与所提供功率之比(电输出除以电输入)。

播放段落

Summary
::摘要

播放段落
• A machine always does less work on an object than the user does on the machine, because the machine must use some of the work to overcome friction.
  ::机器对物体的操作总是比对机器的用户少,因为机器必须使用某些工作来克服摩擦。
播放段落
• Efficiency is the percent of work put into a machine by the user (input work) that becomes work done by the machine (output work). It is a measure of how well a machine reduces friction.
  ::效率是用户(投入工作)投入机器的工作(投入工作)的百分率,它成为机器完成的工作(产出工作)的百分率。 这是衡量机器如何减少摩擦的尺度。
播放段落
• You can calculate the efficiency of a machine with this equation: $\begin{array}{r}\mathrm{E}\mathrm{f}\mathrm{f}\mathrm{i}\mathrm{c}\mathrm{i}\mathrm{e}\mathrm{n}\mathrm{c}\mathrm{y}=\frac{\mathrm{O}\mathrm{u}\mathrm{t}\mathrm{p}\mathrm{u}\mathrm{t}\mathrm{w}\mathrm{o}\mathrm{r}\mathrm{k}}{\mathrm{I}\mathrm{n}\mathrm{p}\mathrm{u}\mathrm{t}\mathrm{w}\mathrm{o}\mathrm{r}\mathrm{k}}\times 100\mathrm{\%}\end{array}$
  ::您可以用此方程式计算机器的效率 : 效率=产出 投入x100%

播放段落

Review
::回顾

播放段落
1. Why is the output work of a machine always less than the input work done on the machine?
   ::为什么机器的输出工作总是少于机器的输入工作?
播放段落
2. What is the efficiency of a machine?
   ::机器的效率是多少?
播放段落
3. If a machine requires an input of 5000 joules of work to put out 4000 joules of work, what is the machine’s efficiency?
   ::如果一台机器需要5000焦耳的工程投入 才能投入4000焦耳的工程, 那么机器的效率是多少?