Is there a difference in the mass of a human body between the states of rest and motion?

Question

is there a difference in the mass of a human body between the states of rest and motion?
If and since there is motion, maybe other factors might influence the mass so instead of comparing mass between the states of rest and motion it might be better to compare mass between states of rest and rest right after motion.
Is there a difference of mass in either of those two comparisons mentioned above?

I searched the web for some time but i could not find an answer. I know it is a stupid question (i am not that smart). If you can explain the answer in simple terms, i would be grateful.

Answer 1

There is a principle called conservation of energy/matter. This means if we have a closed system, like a body, the total energy/matter will not change unless we add or subtract mass to it, OR add or subtract energy. The total energy/matter is conserved, hence conservation of energy/matter.

Adding matter from outside the body would be eating or drinking. Subtracting matter would be peeing or pooping.

Adding energy from outside the body would be like heating someone up. Subtracting energy would be like cooling them down or having them do work. That’s important. By “work”, I am talking about the thermodynamic parameter of force times distance, not work in the normal sense. It is a way of mechanically subtracting energy. The energy goes outside the body into whatever they are working on (pushing on or moving, including the air or the location of their body.) Why would changing the location of their body be energy leaving the body? if they climb a hill that energy of being higher does NOT go into the body from outside the body it goes into the external situation and comes from inside the body). So exercise and motion make energy leave the body.

The other thing is that energy/matter can be in the form of energy or in the form of matter. So the person can burn calories to reduce matter and increase internal energy without either matter or energy going outside the body.

So when someone exercises, they burn matter to convert it to energy, then the energy goes outside the body by way of their activity as work. So they weigh less after that. They have reduced mass by reducing matter. Weight comes from mass (the amount of matter) and gravity. Without gravity, no weight. Without mass, no weight. Takes both.

Answer 2

So this is a bigger question than I think you think you're asking.

Biophysical answer: Vigorous exercise can cause you to lose several hundred grams of water weight. In terms of calorie expenditure, I think a marathon can itself cost a half kilogram of day reserves? So these should be measurable on a scale before and after.

Particle physics answer: of course special relativity introduces the idea of an apparent mass which includes your kinetic energy. This, however, will never show up on a scale for you... scales show weights to ~0.1%, one part in a thousand, you will never go a hundredth of the speed of light, and even if you did that would only change your weight by 0.01%, one part in ten thousand.

Aerospace answer: of course as you approach speeds around 40,270 km/h, you are moving so fast sideways that you're falling at the same rate that the Earth is curving out from underneath you, rather than significantly faster than it, we call this orbit, but it would appear on a scale as a reduction in your weight to 0. This probably goes like $v^2/R$ so to observe 0.1% changes you might need to do 1,000 km/hr? That's a bit faster than a bullet train, sadly.

Rotational physics answer: if you are going in the right direction, which I believe is East, then the Coriolis force on you should point upward. I think a back of the envelope calculation puts this at about 300 km/hr which you would see on some bullet trains?

Bottom line is, there is a notion of weight which does not change much, and that is what we usually mean by “weight”, but many other concerns go into how a scale measures your weight.