Eli5: Weight vs mass in space

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Explain like I’m 5. Ok I’m confused. If I “weigh” 100 lbs on earth it is due to the pull of the earths gravity on my body. But in space I’d be weightless. So how do you calculate mass of something in space ? All this thought because of the new toilet going up to the space station !!!

In: Physics

6 Answers

Anonymous 0 Comments

Getting hit by a tennisball probably won’t hurt. Getting hit by a bowlingball that’s traveling at the same speed definitely will.

The principle behind this is *inertia*: an object’s resistance to changes in their motion. Heavier objects require more force to stop them. That’s where the concept of *mass* really comes from: it’s a measure of inertia.

*Weight* on the other hand is related to *gravity*. Officially it’s “the gravitational force something exerts on its support”. When you stand on a scale you exert a force on it. Since the scale can’t move through the floor, it compresses a little and that’s where you get the reading from. When you try that when you’re freefalling, the scale is falling at the same rate you are so the net force is zero. That’s what weightlessness really means: you’re falling as fast as your surroundings.

For technical details, see [https://en.wikipedia.org/wiki/Mass_versus_weight](https://en.wikipedia.org/wiki/Mass_versus_weight)

Since gravity (and therefore weight) is proportional to mass, the easiest way to measure mass is with a scale. But you can still use inertia if gravity is unavailable. This will usually involve collisions or rotations compared to reference masses.

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