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  • 1.12速度

    Lesson Objectives
    ::经验教训目标

    • Outline how to calculate the speed of a moving object.
      ::大纲 如何计算移动对象的速度 。

    Lesson Vocabulary
    ::词汇表课程

    • speed
      ::速度速度

    Introduction
    ::导言

    Did you ever play softball? You may have found it easy to hit the ball as it was tossed toward you. Well, that's only one kind of softball. Fast-pitch softball is a whole new game. The ball is not pitched slowly. Instead, it speeds in nearly as fast as a baseball. 
    ::你玩过垒球吗?你可能发现打球很容易,因为球是投到你身上的。那是唯一的一种垒球。快球是全新的游戏。球没有缓慢投球。相反,球的速度快得跟棒球一样快。

    Female athletes can throw the ball at speeds of around 120 km/h. That's about 75 mi/h. For a male athlete, the ball may travel even faster. The speed of the ball makes it hard to hit. If the ball changes course, the batter may not have time to adjust the swing to meet the ball.
    ::女性运动员可以以大约120公里/小时的速度投球。 大约75米/小时。 对于男性运动员来说,球的飞行速度可能更快。 球的速度使球很难击中。 如果球的航程发生变化, 球手可能没有时间调整秋千以迎击球。

    lesson content
     
    In fast-pitch softball, the pitcher uses a "windmill" motion to throw the ball. This is a different technique than other softball pitches. It explains why the ball travels so fast.

    Speed
    ::速度速度

    Speed  is an important aspect of motion. It is a measure of how fast or slow something moves. To determine speed you must know two things. First, you must know how far something travels. Second, you need to know how long it takes to travel that far. Speed can be calculated using this formula:
    ::速度是运动的一个重要方面。 它是一个测量速度或速度速度的尺度。 要确定速度, 您必须知道两件事。 首先, 您必须知道速度有多远。 第二, 您需要知道要多久才能走那么远。 速度可以用这个公式来计算 :

    speed=distancetime

    A familiar example is the speed of a car. In the U.S., this is usually expressed in miles per hour (see  Figure  ). Think about a trip you and your family made in the car. Maybe the trip covered 120 miles and it took 3 hours. What was the car's speed?
    ::一个熟悉的例子就是汽车的速度。在美国,通常每小时以英里表示(见图 ) 。 想想你和你的家人在车里的旅行。 也许行程长达120英里,花了3小时。汽车的速度是多少?

    speed=120 mi3 h=40 mi/h

    The speed of a car may also be expressed in kilometers per hour (km/h). The SI unit for speed is meters per second (m/s). This is the unit of measure a scientist would use. 
    ::汽车的速度也可以以每小时公里(公里/小时)表示,速度的SI单位为每秒(米/秒)米。这是科学家使用的测量单位。

    lesson content
     
    Speed limit signs like this one warn drivers to reduce their speed on dangerous roads.

    Instantaneous vs. Average Speed
    ::与平均速度相比的即时速度

    When you travel by car your speed may vary. In other words, you usually don't move at a constant speed. You may go faster or slower. It may  depend on the speed limits, traffic, and traffic lights. For example, you might travel 65 miles per hour on a highway. When going through a city you may only be going 20 miles per hour. You might even have to stop at traffic lights. You slow down as you turn corners. You speed up to pass other cars. It's easy to figure out the average speed, but what about your speed at any moment?
    ::当您乘车行驶时,速度可能会不同。换句话说,您通常不会以固定速度移动。您可以走得更快或更慢。这取决于速度限制、交通和交通灯光。例如,您可能每小时在高速公路上行驶65英里。在穿越城市时,您可能只每小时行驶20英里。您甚至可能不得不在交通灯下停车。您在转弯时会放慢速度。您会加快速度以通过其他汽车。很容易找到平均速度,但您的速度如何呢?

    The speed you are going at any given instant is called instantaneous speed. It may vary from moment to moment, and is hard to calculate. Fortunately, we have speedometers to tell us how fast we are going in a car at any moment. You may even have a speedometer on your bike.
    ::您在任何特定时刻的行驶速度被称为瞬时速度。 它可能随时间而变化, 并且很难计算。 幸运的是, 我们有速度计可以告诉我们我们乘车的速度有多快。 您的自行车上甚至可能有一个速度计 。

    lesson content
     
    Cars race by in a blur of motion on an open highway but crawl at a snail's pace when they hit city traffic.

    It's easy to figure out the average speed. The average speed is the total distance traveled divided by the total time it took to travel that distance. To calculate the average speed, you can use the general formula for speed that was given above. What if you took a 75-mile car trip with your family? Your instantaneous speed would vary throughout the trip. If the trip took a total of 1.5 hours, your average speed would be:
    ::很容易找到平均速度 。 平均速度是旅行的总距离除以旅行该距离所需的总时间。 要计算平均速度, 您可以使用上面给出的一般公式来计算速度 。 如果您与家人一起旅行75 英里的汽车, 您的瞬时速度会在整个行程中变化。 如果行程总共花费1.5小时, 您的平均速度是 :

    average speed=75 mi1.5 h=50 mi/h

    You Try It!
    ::你试试看!

    Problem:  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 15 minutes to make the round trip. What was her average speed for the entire trip?
    ::问题: Terri骑着她的自行车 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15分钟才往返。 她整个行程的平均速度是多少?

    Lesson Summary
    ::经验教训摘要

    • Speed is a measure of how fast or slow something moves. It depends on the distance traveled and how long it takes to travel that distance. The average speed of an object is calculated as the change in distance divided by the change in time.
      ::速度是衡量某物移动速度或速度的尺度。它取决于所走的距离和所走的距离。一个对象的平均速度是用时间变化所除的距离变化计算的。

    Lesson Review Questions
    ::经验回顾问题

    Recall
    ::回顾

    1. What is speed? How is it calculated?
      ::什么是速度?它是如何计算出来的?

    Apply Concepts
    ::应用概念

    1. Sam ran a 2000-meter race. He started at 9:00 AM and finished at 9:15 AM. He started out fast but slowed down toward the end. Calculate Sam's average speed during the race.
      ::Sam跑了一个2000米的赛跑。 他从上午9点开始, 早上9点15分结束。 他开的很快,但向最后放慢了速度。 计算Sam在比赛中的平均速度 。

    Think Critically
    ::仔细仔细思考

    1. Explain how a distance-time graph represents speed.
      ::解释一个远时图如何代表速度 。

    Points to Consider
    ::需要考虑的要点

    In this chapter, you read that the speed of a moving object equals the distance traveled divided by the time it takes to travel that distance. Speed may vary from moment to moment as an object speeds up or slows down. In the next lesson, "Acceleration," you will learn how to measure changes in speed over time.
    ::在本章中, 您读到移动对象的速度等于行走该距离所需时间所分隔的距离。 当一个对象加速或减慢时, 速度会随时间而变化。 在下一个课“ 加速” 中, 您将学习如何测量时间速度的变化 。

    • Do you know what a change in speed or direction is called?
      ::你知道速度或方向的改变叫什么吗?
    • Why might measuring changes in speed or direction be important?
      ::为什么衡量速度或方向的变化很重要?