Hypothetical physics

Weight != mass

Weight (Newtons{kilogram/m/s^2) = mass (kilograms) x gravity ( m/s^2) Weight is the force acting on an object due to gravity. This is why your weight on Earth and your weight on the Moon aren't the same. There's even a Magic School Bus episode on this entire concept if you want to know more about it.

So when you say that you can make "your" "weight" changed, what exactly are you talking about? Are you changing your mass? Are they adding/removing neutrons or protons to their molecules? Are they just accumulating extra stuff? Where is that going? Changing molecules has huge impacts, like if you shove too many neutrons into something then it becomes unstable, and that's how you get radioactive decay! Which is not good for most living things.

Or are you changing the force of gravity in an area that just so happens to be the space your body is occupying? Because that has other, far-reaching implications that I am too dumb to do the math for.

 

Well said, Chasejxyz, but I wouldn’t be so quick to dismiss the gravity manipulation angle. Mostly because that’s the only way I can see this working in a way that’s even slightly believable (in a fantasy context, that is).

I think you’d be better off saying “gravity magic makes this 150 pound guy seem to weigh 500 pounds” and leave it at that. I think trying to figure-out the real physics of this is way more trouble than it’s worth. I’d imagine that at the very least, a sudden change of weight like that would just immediately result in the person being crushed under their own weight.

 

Is an explanation really necessary?

That premise could simply be magical realism. Kind of like this story by Stephen King, in which the MC mysteriously loses weight without his appearance changing at all. No explanation is ever given, it just happens.

In reality, apparent weight can change without actual weight changing. If you've ever had a pet or child fall asleep in your lap, you'll have noticed that they felt heavier once they were asleep. But their weight, as measured on a scale, would still have been the same.

The difference between dead weight and live weight is minuscule, but you could expand on that for fantasy purposes. Imagine a greater difference in weight that can be manipulated at will.

 

Adding to all what has been said above, regardless of how that antigravity trick is achieved, you should be very careful of unexpected consequences ShadeZ . I say this because I remember a short story by Isaac Asimov in which one character kills another using an antigravity device in a very unexpected way, and the action was that the object being subjected to the antigravity field was effectively and instantaneously ejected from Earth at light speed while piercing cleanly through everything in its path, including the heart of the unsuspecting victim. The antigravity effect was complete, meaning that the field completely nullified all gravity, and that explained the ludicrous speed of the deadly object (a billiard ball).

So, what I imagine you want to give your species is some sort of "natural antigravity buoyancy", right? Well, you could take a look to animals like whales or sharks, or just fishes in general, and see how they achieve their buoyancy balance under water. Essentially it will be some organ or combination of them that gives them that skill. In your species' case, it could be an organ that requires some "magical" substance to make the antigravity trick possible. Now, if the species have evolved to have such organ, that means they are able to feed themselves with the required substance in their natural environment and that the organ gave them an evolutionary advantage. That, or the species is the creation of a mad scien... wizard.

 

I’ll echo what Slip said:

You’re likely better of just saying “they’re magically heavier” and leaving it there.

Although, I’ll maintain that if their strength is unaffected, an immediate increase in weight would probably just crush them.

 

Perhaps instead of changing their weight or mass, they change their local gravitation. This would have the benefit of allowing them to levitate, jump from great heights, etc. No flight since this would require some type of propulsion.