Mass and weight, kg and Newtons

  • #1
Femme_physics
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Main Question or Discussion Point

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?
 

Answers and Replies

  • #2
D H
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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.
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.
 
  • #3
Femme_physics
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So weight and mass in kg are the same thing?
 
  • #4
ZapperZ
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So weight and mass in kg are the same thing?
Weight is FORCE. Force has the unit of "Newtons".

Zz.
 
  • #5
Femme_physics
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So when people say "I weigh 60 kg" they mean 60 Newtons? I'm confused.
 
  • #6
ZapperZ
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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.
 
  • #7
D H
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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").
 
  • #8
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
 
  • #9
Femme_physics
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Ah...so it's the laymen who confused me. They must be destroyed.

Thanks ;)
 
  • #10
D H
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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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