章节大纲

  • A skateboarder performing a trick, lifting the front end of the skateboard in mid-air.

    This is Sam at Newton’s Skate Park. He just pushed his weight down on the back end of the skateboard to bring the front end up into the air. The force of his weight pressing down on the back of the skateboard overcame the force of gravity holding the front end down.
    ::这是牛顿滑板公园的山姆。 他只是把重量推到滑板的后端,把前端推到空中。 他压在滑板后部的重量力压倒了前端的重力。

    Weight, Mass, and Acceleration
    ::重量、质量和加速度

    Weight is a measure of the force of gravity pulling down on an object. It depends on the object’s mass, which is how much matter the object contains. It also depends on the downward of the object due to gravity, which is the same all over Earth. Weight can be represented by the equation:
    ::重量是测量重力拉下物体的强度的尺度。 它取决于物体的质量, 也就是物体包含多少物质。 它也取决于物体因重力而向下倾斜, 这在地球各地都是一样。 重量可以用方程式表示 :

    F = m × a
    ::F = m = a = m = a

    This is the general equation that relates force to mass and acceleration. When it relates weight to mass and acceleration, the letter F represents the object’s weight in Newtons (N), which is the SI unit for weight. The letter m in the equation represents the object’s mass in kilograms, and the letter a represents the downward . As this equation shows, weight is directly related to mass. As an object’s mass increases, so does its weight. For example, if mass doubles, weight doubles as well. 
    ::这是与质量和加速度相关的一般方程式。当重量与质量和加速度有关时,字母F代表物体的重量,用牛顿(N)表示,牛顿(N)是重量的SI单位。方程式中的字母m以公斤表示物体的重量,字母a表示向下。正如这个方程式所显示的,重量与质量直接相关。当物体的质量增加时,字母F代表物体的重量。例如,如果质量翻倍,重量也翻倍。

    Calculating Weight
    ::计算重量

    Weight is derived from Newton’s Second Law , F=ma, where F is the force of gravity in Newtons, m is the mass of the object in kilograms, and a is the acceleration due to gravity on Earth, 9.8 m/s 2 . 
    ::重量来自牛顿的第二法,F=ma,其中F是牛顿的重力,m是物体的重量(公斤),a是地球重力加速度(9.8米/秒)。


    When the formula is used specifically to solve for the weight of an object, it appears as W=mg. Weight is always measured in force units Newtons, m is the mass of the object in kilograms, and g is the gravitational strength on the planet in N/kg or m/s 2 (g Earth = 9.81 m/s 2 ).
    ::当公式用于具体解决一个物体的重量时,该公式显示为W=mg。 重量总是用强制单位牛顿测量, m 以公斤表示物体的质量, g 以N/kg或m/s2表示行星的引力强度(g Earth=9.81 m/s2)。

    The gravitational force, g, differs throughout the universe. On the Earth, g = 9.81 m/s^ 2 . On the moon, the gravitational force on an object is 1.6 m/s^ 2 . So, the weight of a moon rock on the moon will be one-sixth the weight of the moon rock on the Earth’s surface.
    ::引力 g 在整个宇宙中是不同的。 在地球, g = 9.81 m/s=2。 在月球上,物体的引力为 1.6 m/s=2. 因此,月球岩石在月球上的重量将是地球表面月球岩石重量的六分之一。

    Q : Sam has a mass of 50 kilograms. What is his weight in Newtons?
    ::问题:萨姆的重量为50公斤。他在牛顿的重量是多少?

    A : You can calculate Sam’s weight in Newtons by substituting his mass in kilograms into the weight formula:
    ::甲:您可以将萨姆在牛顿的重量替换成重量公式, 以此来计算萨姆在牛顿的重量:

    F = m × 9.8 m/s 2 = 50 kg × 9.8 m/s 2 = 490 kg • m/s 2 , or 490 N
    ::F=m × 9.8 m/s2 = 50公斤 × 9.8 m/s2 = 490 公斤 = m/s2 或 490 N

    You’re probably more familiar with weight in pounds (lb) than in Newtons. One Newton equals 0.225 pounds. In other words, there are 0.225 pounds per Newton. You can use this ratio to convert Newtons to pounds.
    ::你可能比牛顿更熟悉磅体重(lb),一个牛顿等于0.225磅。 换句话说,每个牛顿有0.225磅。 你可以用这个比例将牛顿转换成磅。

    Q : What is Sam’s weight in pounds?
    ::问题:萨姆的体重是多少?

    A : In pounds, Sam’s weight is 490 N x 0.225 lb/N = 110 lb.
    ::A:按磅计,Sam的重量为490牛顿x0.225磅/牛顿=110磅。

    Summary
    ::摘要

    • Weight is a measure of the force of gravity pulling down on an object. It depends on the object’s mass and the acceleration due to gravity, which is 9.8 m/s 2 on Earth.
      ::重量是测量重力拉下物体的强度的尺度,取决于物体的重量和因重力造成的加速度,地球重力为9.8 m/s2。
    • The formula for calculating weight is F = m × 9.8 m/s 2 , where F is the object’s weight in Newtons (N) and m is the object’s mass in kilograms. The Newton is the SI unit for weight, and 1 Newton equals 0.225 pounds.
      ::计算重量的公式是F = m × 9.8 m/s2,其中F 是物体在牛顿(N)中的重量,而 m 是物体的重量(公斤)。牛顿是重量的SI单位,1牛顿等于0.225磅。

    Review
    ::回顾

    1. What is weight? What is the SI unit for weight?
      ::体重是多少?
    2. Explain how to calculate an object’s weight on Earth from its mass.
      ::解释如何从质量上计算物体在地球上的重量。
    3. Sam’s older sister Nina, who is pictured in the Figure  , has a mass of 52 kilograms. How much does Nina weigh in Newtons? How much does she weigh in pounds?

      A girl with long hair sitting on a skateboard, outdoors near buildings.


      ::萨姆的姐姐尼娜(如图所示)的体重为52公斤。 尼娜在牛顿的体重是多少? 她的体重是多少?
    4. The moon has less gravity than Earth. If you were on the moon, how would your weight be different than it is on Earth?
      ::月亮的引力小于地球。如果你在月球上, 你的重量会和地球的重量有什么不同?