5.7
质量和重量
质量和重量在日常生活中经常互换使用。 例如，医疗记录通常以千克为单位记录我们的体重，而并非使用正确的物理单位——牛顿。 但在物理学中，这两者有着重要的区别。 重量是地球对物体施加的引力，大小取决于距地球中心的距离。 如果我们离开地球表面，重量会发生巨大变化，而质量则不会随位置而变化。 例如在月球上，…
物体的质量是其所含物质量的度量，取决于其中原子的数量。
它的重量是来自附近较大物体（如地球或月球）作用在其上的引力。
由于物质的量不会因所施加的力而改变，物体的质量保持不变。
然而，其重量取决于所处的位置。同一物体在地球上的重量与在火星或月球上的重量不同，但其质量保持不变。重量的变化是由于作用在其上的重力加速度不同所致。
质量永远不会为零，但当重力不作用于物体时，其重量为零。根据这一现象以及牛顿第二定律，一个质量为 m 的物体的重量等于 m 乘以矢量 g。
也就是说，物体的质量越大，其重量就越大。质量是标量，而重量是矢量，方向指向地球中心。
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.