Mass and weight, kg and Newtons

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    Mass Newtons Weight
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Discussion Overview

The discussion revolves around the concepts of mass and weight, specifically the distinction between the two in physics and everyday language. Participants explore how mass is measured in kilograms and weight in Newtons, and the implications of these definitions in various contexts.

Discussion Character

  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants assert that to find weight, one must multiply mass by the acceleration due to gravity (9.81 m/s²), resulting in a force measured in Newtons.
  • Others clarify that mass (in kg) and weight (in Newtons) are not the same, emphasizing that weight is a force.
  • There is confusion regarding colloquial usage of "weight" as synonymous with mass, with some participants noting that this can lead to misunderstandings.
  • One participant points out that physicists prefer precise terminology, distinguishing between mass and weight, while acknowledging that lay language often conflates the two.
  • A later reply introduces the idea of "scale weight," which can differ from gravitational weight depending on the context, such as location (Earth vs. Moon vs. space). This adds complexity to the discussion.

Areas of Agreement / Disagreement

Participants express differing views on the terminology and conceptual understanding of weight and mass. While some agree on the definitions in physics, others highlight the confusion arising from everyday language, indicating that the discussion remains unresolved.

Contextual Notes

There are multiple meanings of "weight" that complicate the discussion, including colloquial weight, gravitational weight, and scale weight. The context in which these terms are used can significantly affect understanding.

Femme_physics
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If I know an object has a mass of 5 kg, and I want to find its weight on earth, I multiply it by 9.81 - right? That equals 49.05 kg. Now, if I want to convert it to Newtons, I multiply it by additional 9.81 So it would equal 481.18 [N]?

Is that right?
 
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Femme_physics said:
If I know an object has a mass of 5 kg, and I want to find its weight on earth, I multiply it by 9.81 - right? That equals 49.05 kg.
You do not multiply the mass by 9.81. The mass of the object is 5 kg, period. You multiply that 5 kg by the acceleration due to gravity, 9.81 meters/second2 (F=ma). That yields 49.05 kg·m/s2, or 49.05 Newtons.
 
So weight and mass in kg are the same thing?
 
Femme_physics said:
So weight and mass in kg are the same thing?

Weight is FORCE. Force has the unit of "Newtons".

Zz.
 
So when people say "I weigh 60 kg" they mean 60 Newtons? I'm confused.
 
Femme_physics said:
So when people say "I weigh 60 kg" they mean 60 Newtons? I'm confused.

Would you rather we use pedestrian language, or would you rather we use the exact terminology as used in physics? The CONTEXT of the question here makes a lot of difference, and you will continue to see how the same term used in physics are used differently in everyday language.

So now you have to decide which of these you wish to learn and apply in this question.

Zz.
 
Femme_physics said:
So when people say "I weigh 60 kg" they mean 60 Newtons? I'm confused.
When someone says "I weigh 60 kg" they are using an old colloquial meaning of the word "weight". In that sense, "weight" is a synonym for mass. Physicists don't like that meaning because (a) there is an unambiguous alternative ("My mass is 60 kg") and (b) "weight" is a force to physicists (it would be better to say "I weigh 588 Newtons").
 
the body has mass of 5kg. weight is not measured in kg. it is measured in Newtons. in layman language weight and mass are same but they are different in physics. don't get confused
 
Ah...so it's the laymen who confused me. They must be destroyed.

Thanks ;)
 
  • #10
In this case, the lay meaning of weight as a synonym for mass predates the use by physicists as a synonym for gravitational force.

To make matters worse, there is a third meaning of the term "weight", which is the quantity shown on an ideal spring scale. Call this "scale weight". To illustrate the difference, consider a person whose mass is 60 kg.
  • Person at sea level at the Earth's equator:
    • Colloquial weight: 60 kg
    • Gravitational weight: 588.85 Newtons
    • Scale weight: 586.82 Newtons
  • Person on the surface of the Moon at the Moon's equator:
    • Colloquial weight: 60 kg
    • Gravitational weight: 97.32 Newtons
    • Scale weight: 97.32 Newtons
  • Person in the International Space Station:
    • Colloquial weight: 60 kg
    • Gravitational weight: 528.58 Newtons
    • Scale weight: 0 Newtons
 

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