# Is gaining weight gaining mass?

1. Oct 15, 2004

### jimmy1200

im still confused on this, can someone put the knowledge smack down on me.

2. Oct 15, 2004

### Tom McCurdy

yes

F=ma
since a stays the same
mass must increase
since weight... pounds is a measure of force.

3. Oct 15, 2004

### jimmy1200

i thought mass cannot be created or destroyed

4. Oct 15, 2004

### Phobos

Staff Emeritus
When you eat, you are adding new mass to your body.

Some of it you convert to energy, some of it you use for growth/maintainance (e.g., building new cells), some you store (i.e., fat cells), and some you expel (undigestable stuff).

As Tom said, "weight" is just mass x gravity. You weigh more on the surface of the Earth than you would on the surface of the moon because of the differences in gravity here and there. But your mass is unchanged in those two cases. Your mass is how much stuff you are made of.

You can add mass to your body by eating more food to store more (e.g., fat cells) or build more (e.g., muscle cells). You can lose mass by eating less and/or exercising in which case your body will burn up some fat cells. That mass is lost through conversion to energy, sweating, respiration, etc.

Consider the growth of a baby to an adult. A lot of mass is being added. From where? Food.

5. Oct 15, 2004

### NateTG

No.

Gaining weight is not necessarily gaining mass in Newtonian mechanics. I don't know enough about GR to comment on weight in relativity.

Weight is the amount of force with which gravity acts on an object. Newton indicates that the magnitude of gravity is:
$$\frac{m_1 \times m_2 \times G}{r^2}$$

There are four variables in this formula which suggests that there are four methods for changing weight:
Changing the mass of one object. This is what you suggested as a means of changing weight.
Moving further apart. Clearly, as $$r$$ gets larger the force of gravity on an object - or the weight of that object - gets smaller. An astronaut on the space shuttle weighs less than he does on the surface without an appreciable change in mass. Objects in deep space have almost no weight at all.
Changing the other mass. The mass of the Moon is roughly one sixth of the mass of the Earth. Consequently the force of gravity on an object due to the Moon is approximately one sixth that of that due to the Earth.
Changing the gravitational constant $$G$$. Although physics suggests that it's extremely unlikely, it is concievable that the gravitational constant may chang over time. An objects weight could be changed by the gravitational constant changing.

6. Oct 15, 2004

### bross7

If you are talking about a situation where the surroundings are constant and you gain mass, then mass and weight would increase/decrease together.

7. Oct 18, 2004

### doxigywlz

i thought that weight=mass x gravity. so, i'd think that in order to keep the two equal, mass would have to increase becase gravity is constant. Weight/gravity=mass..... is that wrong? that's what i was taught, at least.

8. Oct 20, 2004

### dav2008

First off, the correct statement is "energy cannot be created nor destroyed".

Like the previous posters said, weight is just a measurement of the force exerted on you by gravity and this force is proportional to your mass.

9. Oct 20, 2004

### dav2008

Ok, so the proposition is that if you go to a planet where gravity is smaller, your weight will be smaller. Also, in order to lower your weight you have to either lower your mass and/or lower the acceleration due to gravity.

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