Does Gaining Weight Affect Mass?

[Darrencc]

TL;DR Summary

Teaching eleven year olds about gravity

Hello,
I am teaching eleven year olds about gravity. There is no issue about this subject, however I would like someone to help with a model answer.
As mass never changes. What is the best answer when I am asked that if someone gets 'fatter', do they not put on mass?
Thanks in advance.

 

[tech99]

You seem to be explaining Conservation of Mass. I suggest that for an initial explanation for that age group, if someone becomes fatter, then of course, they have increased their mass. But the mass of food in the cupboard has decreased the same amount.

 

[Ibix]

As mass never changes.

Mass of what never changes?

Put me, some food, and a toilet on a large weighing scale (edit: this needs an airtight container with oxygen supply too, since I'll be breathing). If I eat the food, absorb some of it across my gut wall and store it in my fat cells, then go to the toilet and excrete the molecules I did not absorb, the reading on the scale won't change. But the mass of food changes, my mass changes, and the mass of poo changes.

So all I did was move mass around, taking it from the food and absorbing some and excreting some. So my mass does change, because I added some matter, but the total mass of everything in the system did not change.

This is a trap people fall into all the time. A lot of laws apply to closed systems, where nothing enters or leaves. Mass is constant in a closed system. But in an open system where stuff can come in or out, it manifestly isn't constant, as the example of a person eating shows.

 

[Darrencc]

You seem to be explaining Conservation of Mass. I suggest that for an initial explanation for that age group, if someone becomes fatter, then of course, they have increased their mass. But the mass of food in the cupboard has decreased the same amount.

Excellent. Very helpful, thank you

 

[Darrencc]

Mass of what never changes?

Put me, some food, and a toilet on a large weighing scale (edit: this needs an airtight container with oxygen supply too, since I'll be breathing). If I eat the food, absorb some of it across my gut wall and store it in my fat cells, then go to the toilet and excrete the molecules I did not absorb, the reading on the scale won't change. But the mass of food changes, my mass changes, and the mass of poo changes.

So all I did was move mass around, taking it from the food and absorbing some and excreting some. So my mass does change, because I added some matter, but the total mass of everything in the system did not change.

This is a trap people fall into all the time. A lot of laws apply to closed systems, where nothing enters or leaves. Mass is constant in a closed system. But in an open system where stuff can come in or out, it manifestly isn't constant, as the example of a person eating shows.

Wonderful. Very helpful. Thank you

 

[Mister T]

Place a brick or the like on a scale and show the read out to the class. Next, place another object on top of the brick. The class can see that the mass of something changes when you add mass to it.

The notion that the mass of a system remains constant is valid only for closed systems. In other words, if you don't add or subtract any matter from a system, its mass will stay the same.

 

[Drakkith]

As mass never changes. What is the best answer when I am asked that if someone gets 'fatter', do they not put on mass?

They do. We just use the term 'weight' more commonly in everyday parlance.

 

[Dullard]

This might be a chance to teach the most useful general rule in all of science:

(Some of it) + (The rest of it) = (All of it)

 

[Steve4Physics]

@Darrencc, are you trying to teach the (technical) difference between mass and weight?

Here’s an exercise which I have used with students (slightly older than yours).

You kick a foot-ball shaped stone. It hurts your foot!
You try to lift up the stone but it’s a bit too heavy.

You now travel to moon where the pull of gravity is about 6 times weaker than on earth.
On the moon:
- does it hurt as much when you kick the stone?
- can you lift up the stone?

 

[PeroK]

You now travel to moon where the pull of gravity is about 6 times weaker than on earth.
On the moon:
- does it hurt as much when you kick the stone?
- can you lift up the stone?

How do you get your students to the Moon?

 

[Drakkith]

How do you get your students to the Moon?

You put them on a rocket of course!

 

[PeroK]

You put them on a rocket of course!

Why didn't I think of that?!

 

[Steve4Physics]

How do you get your students to the Moon?

Did I not mention my first name is Elon?

(Not really though, or my username here would have to be elon4phsyics.)

 

[Mister T]

How do you get your students to the Moon?

Sorry to put the thread back on topic. :) But this is a good point. It is much better to come up things that can be demonstrated, or at least a part of the students' common experience.

 

[Mister T]

We just use the term 'weight' more commonly in everyday parlance.

This is something that needs to be explicitly pointed out to the students. The word weight has multiple meanings, it can either refer to mass or to a force. The word weight is actually legally defined as what we refer to as mass. The distinction between force and mass is the important distinction for students to understand. I go so far as to refer to it as the weight force.

 

[Steve4Physics]

Sorry to put the thread back on topic. :) But this is a good point. It is much better to come up things that can be demonstrated, or at least a part of the students' common experience.

Agreed. But as a teaching-exercise, used alongside other material of course, I've found it generates useful discussion and thought.

I'd be interested in any practical demonstration to distinguish between mass and weight if anyone knows of any.

 

[anorlunda]

X

Agreed. But as a teaching-exercise, used alongside other material of course, I've found it generates useful discussion and thought.

I'd be interested in any practical demonstration to distinguish between mass and weight if anyone knows of any.

Students may be able to experience weightlessness for an instant on amusement park rides.

 

[Steve4Physics]

X

Students may be able to experience weightlessness for an instant on amusement park rides.
View attachment 298220

But the problem (explaining the difference between mass and weight) remains. E.g. what do you say to the student who claims they were weightless and therefore also massless?

Also, if defining weight as a gravitational force, freefall is not true weightlessness. Maybe call it apparent weightlessness to avoid conflict with the definition.

 

[PeroK]

I can see no use for the concepts of weight and weightlessness in physics. There are mass, gravity and contact forces. That covers everything.

 

[vanhees71]

There are 4 fundamental interactions (stron, weak, electromagnetic, and gravitational), but I think you can't discuss it in this abstract way with 11-yo students, and to get the issue of the difference between mass and gravitational force $mg$ on Earth right even at higher grades. The idea with the moon seems to be good although I guess one must not underestimate the level of abstraction this thought experiment also needs.

 

[Mister T]

I can see no use for the concepts of weight and weightlessness in physics. There are mass, gravity and contact forces. That covers everything.

Weight, as typically defined in physics, includes the effects of both gravity and Earth's spin.

 

[sysprog]

I'd be interested in any practical demonstration to distinguish between mass and weight if anyone knows of any.

The effort needed to push a car on level ground with transmission disengaged. i.e. to manually overcome its resistance to acceleration, demonstrates its mass. and the effort required to lift it, or to lift part of it with a jack, against gravity, demonstrates its weight.