5.7
質量と重量
質量と重量は、日常会話ではよく同じ意味で使用されます。 たとえば、 医療記録には体重がキログラムで表示されることがよくありますが、正しい単位であるニュートンで表示されることはありません。 しかし、物理学には重要な違いがあります。 重さとは、物体が地球から受ける引力（重力）の大きさです。それは地球の中…
物体の質量は、物体に含まれる物質の量の尺度であり、その中の原子の数に依存します。
その重量は、地球や月など、どちらか最も近い方の大きな物体から作用する重力です。
加えられた力に反応して物質の量が変化しないため、物体の質量は一定のままです。
ただし、その重量は場所によって異なります。地球上の物体は、火星や月にある物体とは重さが異なりますが、その質量は同じままです。重量変化は、重力が作用することによる加速度の違いによるものです。
質量がゼロになることは決してありませんが、重力が作用しないと重量はゼロになります。この観測とニュートンの第二法則を使用すると、質量 m の対象は m x ベクトル g の重さになります。
つまり、オブジェクトの質量が大きいほど、その重量は大きくなります。質量はスカラー量ですが、重量は地球の中心に向けられたベクトルです。
Q1: What is the difference between mass and weight in physics?
Mass measures the amount of matter in an object and remains constant regardless of location. Weight is the gravitational force acting on that mass, calculated as mass times the acceleration due to gravity. While mass is a scalar quantity, weight is a vector directed toward Earth's center. Understanding this distinction is crucial for applying newton s second law correctly.
Q2: Why does an object weigh differently on the Moon than on Earth?
Weight depends on the local acceleration due to gravity, which differs between celestial bodies. The Moon's gravity is only 1.67 m/s² compared to Earth's 9.8 m/s², so a 1.0 kg mass weighs 9.8 N on Earth but only 1.7 N on the Moon. The object's mass remains unchanged; only the gravitational force acting on it varies.
Q3: Can an object have zero weight but still have mass?
Yes. Weight becomes zero when gravitational force does not act on an object, such as in deep space far from any celestial body. However, the object's mass never becomes zero because it always contains matter. This demonstrates that mass and weight are fundamentally different physical properties.
Q4: How is weight calculated using Newton's second law?
According to Newton's second law, weight equals mass multiplied by the acceleration due to gravity, expressed as W = m × g. The greater an object's mass, the greater its weight. This relationship shows that weight is directly proportional to mass when gravity remains constant.
Q5: Why do medical records incorrectly list weight in kilograms instead of newtons?
In everyday conversation, mass and weight are used interchangeably, leading to this common error. Medical records show mass in kilograms when they should display weight in newtons, the correct unit of force. This confusion arises because most daily experiences occur on Earth's surface, where weight variations are minimal.
Q6: Is mass an intrinsic property of an object?
Yes, mass is an intrinsic property representing the quantity of matter in an object. It depends on the number of atoms present and does not change with location or applied force. This makes mass fundamentally different from weight, which varies based on gravitational conditions.
Q7: What happens to weight when an object is dropped?
When dropped, an object accelerates toward Earth's center due to gravitational force, which is its weight. If air resistance is negligible, the net force equals the gravitational force acting on the object. This acceleration demonstrates how weight, as a force, directly causes motion according to Newton's laws.