9.4 速度

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  速度速度

  

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  Did you ever play fast- pitch softball? If you did, then you probably have some idea of how fast the pitcher throws the ball. For an athlete like the one in the opening image, the ball may reach a speed of 120 km/h (about 75 mi/h). A fast-pitch pitcher uses a “windmill” motion to throw the ball. This is a different technique than other softball pitches, and it explains why the ball travels so fast.
  ::你玩过快速投球吗?如果你玩过快速投球吗?如果你玩过,那么你可能就会知道投手投球的速度有多快。对于像开球图像中的运动员来说,投球速度可能达到120公里/小时(约75毫/小时)的速度。快速投球投手使用“风车”运动来投球。这与其他投球的投球方法不同,它解释了投球速度如此快的原因。

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  Introducing Speed
  ::引入速度

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  How fast or slow something moves is its speed . Speed determines how far something travels in a given amount of time. The SI unit for speed is meters per second (m/s). Speed may be constant, but often it varies from moment to moment.
  ::移动的速度或速度是它的速度。 速度决定着某事物在一定时间内行走的距离。 速度的 SI 单位是每秒的米。 速度可能是不变的, 但往往会随时间而变化 。

   

   

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  Average Speed
  ::平均平均速度

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  Even if speed varies during the course of a trip, the average speed can be calculated using this formula:
  ::即使旅途中速度不尽相同,也可以使用这一公式计算平均速度:

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  $$\begin{array}{r}\text{speed}=\frac{\text{distance}}{\text{time}}\end{array}$$

  
  ::速度= 距离时间

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  For example, assume you go on a car trip with your family. The total you travel is 120 miles, and it takes 3 hours to travel that far. The average speed for the trip is:
  ::比如,假设你和家人一起出车旅行。总旅行时间是120英里,要3小时才能走那么远。平均旅行速度是:

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  $$\begin{array}{rl}\text{speed}& =\frac{120\text{mi}}{3\text{h}}\\ & =40\text{mi/h}\end{array}$$

  
  ::速度=120米3 h=40米/小时

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  Q: Terri rode her bike very slowly to the top of a big hill. Then she coasted back down the hill at a much faster speed. The distance from the bottom to the top of the hill is 3 kilometers. It took Terri ¼ hour to make the round trip. What was her average speed for the entire trip? ( Hint : The round-trip distance is 6 km.)
  ::问:Terri骑着她的自行车非常缓慢地骑到一个大山顶。 然后她以更快的速度向山顶靠岸。 从山底到山顶的距离是3公里。 Terri花了1/4小时才往返。 她整个行程的平均速度是多少? (提示: 往返距离是6公里 ) 。

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  A: Terri’s speed can be calculated as follows:
  ::A:Terri的速度可计算如下:

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  $$\begin{array}{rl}\text{speed}& =\frac{6\text{km}}{0.25\text{h}}\\ & =24\text{km/h}\end{array}$$

  
  ::速度=6公里0.25小时=24公里/小时

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  Instantaneous Speed
  ::即时速度

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  When you travel by car, you usually don’t move at a constant speed. Instead you go faster or slower depending on speed limits, traffic lights, the number of vehicles on the road, and other factors. For example, you might travel 65 miles per hour on a highway but only 20 miles per hour on a city street (see the pictures in the Figure .) You might come to a complete stop at traffic lights, slow down as you turn corners, and speed up to pass other cars. Therefore, your speed at any given instant, or your instantaneous speed, may be very different than your speed at other times. Instantaneous speed is much more difficult to calculate than average speed. 
  ::当您乘坐汽车旅行时,通常不会以固定的速度移动。相反,您的速度会加快或放慢,取决于速度限制、交通灯、路上车辆数量以及其他因素。例如,您可能每小时在高速公路上行驶65英里,但在城市街道上行驶时速只有20英里(见图中的图片 ) 。您可能完全停留在交通灯下,当您转弯时会放慢速度,并加速通过其他汽车。因此,您的速度或瞬时速度可能与您其他时间的速度非常不同。 快速速度比平均速度要难计算得多。

  

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  Calculating Distance or Time from Speed
  ::计算速度的距离或时间

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  If you know the average speed of a moving object, you can calculate the distance it will travel in a given period of time or the time it will take to travel a given distance. To calculate distance from speed and time, use this version of the average speed formula given above:
  ::如果您知道移动对象的平均速度,可以计算该对象在给定时间段中行走的距离或行走给定时间段所需的时间。要计算速度和时间的距离,请使用上面给定的平均速度公式的这一版本:

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  distance = speed × time
  ::距离 = 速度 × 时间

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  For example, if a car travels at an average speed of 60 km/h for 5 hours, then the distance it travels is:
  ::例如,如果一辆汽车以每小时60公里的平均速度行驶5小时,其行驶距离如下:

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  distance = 60 km/h × 5 h = 300 km
  ::距离=60公里/小时×5小时=300公里

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  To calculate time from speed and distance, use this version of the formula:
  ::要从速度和距离计算时间, 请使用此版本的公式 :

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  $$\begin{array}{r}\text{time}=\frac{\text{distance}}{\text{speed}}\end{array}$$

  
  ::时间=远距速度

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  Q: If you walk 6 km at an average speed of 3 km/h, how much time does it take?
  ::问题:如果以平均3公里/小时的速度行走6公里,需要多少时间?

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  A: Use the formula for time as follows:
  ::A: 使用以下时间公式:

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  $$\begin{array}{rl}\text{time}& =\frac{\text{distance}}{\text{speed}}\\ & =\frac{6\text{km}}{3\text{km/h}}\\ & =2\text{h}\end{array}$$

  
  ::时间=远距速度=6平方公里/小时=2小时

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  Summary
  ::摘要

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  • How fast or slow something moves is its speed. The SI unit for speed is meters per second (m/s).
    ::移动的速度有多快或多慢是它的速度。 SIS 速度单位是每秒(m/s)米。
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  • Average speed is calculated with this formula:
    $$\begin{array}{r}\text{speed}=\frac{\text{distance}}{\text{time}}\end{array}$$
    
    ::平均速度使用此公式计算: 速度= 距离时间
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  • Speed may be constant, but often it varies from moment to moment. Speed at any given instant is called instantaneous speed. It is much more difficult to calculate than average speed.
    ::速度可能是恒定的, 但往往会随时间而变化。 任何给定的瞬间速度都称为瞬时速度。 计算比平均速度要困难得多 。
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  • Distance or time can be calculated by solving the average speed formula for distance or time.
    ::距离或时间可以通过解析距离或时间的平均速度公式来计算。

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  Review
  ::回顾

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  1. What is speed?
     ::什么是速度?
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  2. If you walk 3 kilometers in 30 minutes, what is your average speed in kilometers per hour?
     ::如果你在30分钟内行走3公里, 你的平均速度是每小时3公里?
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  3. Compare and contrast instantaneous and average speed.
     ::比较和对比瞬时速度和平均速度。
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  4. What distance will a truck travel in 3 hours at an average speed of 50 miles per hour? 
     ::一辆卡车在3小时以每小时50英里的平均速度行驶到什么距离?