# The Naked Scientists Forum

### Author Topic: What is mass?  (Read 3061 times)

#### Heronumber0

• Guest
##### What is mass?
« on: 01/08/2007 16:41:38 »
I have always, rather glibly, answered: 'it is the amount of a thing' or 'the number of particles in a thing' but there must be a better explanation. Any answers?

#### lightarrow

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##### What is mass?
« Reply #1 on: 01/08/2007 17:33:06 »
I have always, rather glibly, answered: 'it is the amount of a thing' or 'the number of particles in a thing' but there must be a better explanation. Any answers?
The second is wrong because you can change an object's mass without changing the number of its constituent particles.
1. inertial mass of a point body is the rate between force and acceleration  of that body, at low speeds (v<<c).
2. gravitational mass of a point body is the rate between gravitational force and gravitational acceleration of that body, at low speeds.

#### another_someone

• Guest
##### What is mass?
« Reply #2 on: 01/08/2007 17:42:08 »

#### Heronumber0

• Guest
##### What is mass?
« Reply #3 on: 02/08/2007 13:27:00 »
Thanks for the answers.  I am working at a much lower functional level than the post you shared here George.  I just wonder why we use mass as a measurement for amount of apples, potatoes etc... instead of the weight.  This is confusing for a simpleton like me.

#### lightarrow

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##### What is mass?
« Reply #4 on: 02/08/2007 14:58:36 »
Thanks for the answers.  I am working at a much lower functional level than the post you shared here George.  I just wonder why we use mass as a measurement for amount of apples, potatoes etc... instead of the weight.  This is confusing for a simpleton like me.
Infact we do use weight: don't you measure apples, potatoes, gold, chemicals etc... with scales?
In physics, mass is used instead of weight as a measure of the amount of matter, because weight depends also on gravitational acceleration:
P = m*g   P = weight, m = mass, g = gravitational acceleration
so, the same amount of matter weights 6 Newton here on Earth, but 1 N on the Moon and zero N in the outer space, for example.
« Last Edit: 02/08/2007 15:01:15 by lightarrow »

#### Heronumber0

• Guest
##### What is mass?
« Reply #5 on: 02/08/2007 15:59:04 »
Lightarrow, this always confused me and others in school.  Why don't we weigh in newtons? Also, what are the units for weight in Imperial measures?

#### another_someone

• Guest
##### What is mass?
« Reply #6 on: 02/08/2007 17:37:41 »
Lightarrow, this always confused me and others in school.  Why don't we weigh in newtons? Also, what are the units for weight in Imperial measures?

We don't weigh in Newtons because we have been weighing things for a lot longer than we have been discriminating between mass and weight - it is historic, and nobody can be bothered to change it.

Newtons are really a measure of force (e.g. a measure of how much downforce an object has under the influence of gravity).  Since, on Earth, this downforce is proportionate to the mass of the object, and we are used to traditionally using the mass as a way of indicating the downforce, so we continue to do so.

The unit of force in imperial units is a poundal.

#### Heronumber0

• Guest
##### What is mass?
« Reply #7 on: 02/08/2007 21:47:12 »
Thank you George - I remember doing arithmetic with pounds, pennies and ha'pennies in primary school but, for the life of me, I cannnot remember the poundal.  This clears up things for me now.

#### syhprum

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##### What is mass?
« Reply #8 on: 02/08/2007 22:35:28 »
At school from 1941 to 1944 (perhaps in anticipation of a German takeover!) we worked entirely in the metric system but this was pre SI so we had things like cm and ergs.
Despite this I could still remember what a Poundal was but had completely forgoten about Slugs (its inverse)

#### The Naked Scientists Forum

##### What is mass?
« Reply #8 on: 02/08/2007 22:35:28 »