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
The 100 lbs is a “force” caused by one “G”, gravity. The concept is you have pound forces and a pound “mass” inherent in every object. Hence 100 lb-mass times one “G” = 100 lb- force. What your 100 lbs weighs on Earth. 100 lbs-force. If “G” is zero in space your 100 lbs-mass x0 = 0 weight (force). This concept applies no matter what G you have. Like acceleration forces, etc. 100 lbm at 2G = 200 lbs-force
It is more difficult to measure mass in space for sure! The main thing mass gives you in a weightless environment is inertia. So how much force do you have to apply to something to get it moving.
For example, a paperclip has very little mass, so you could gently push it and send it sailing across the space station. The new toilet, I presume, is pretty massive, so it would take a lot of force to start (or stop!) It moving.
Weight I believe is directional. Gravity is pulling us down by an amount scaled by our mass. In space, you’re not pulled in any specific direction. A 500 lb object is weightless, but imagine trying to push it. It wouldn’t be easy.
One way to work out the mass of an object is to use springs. Mass makes it difficult to change an object’s *velocity* – the speed and direction it is moving in. The amount of force required is directly proportional to it’s mass. So if something is twice as massive, it takes twice the force to accelerate it. By pushing and pulling objects via springs, we can see how much the spring stretches or compresses, and use that to calculate mass.
On Earth, we are used to the additional gravitational force. In fact, weight is literally the force of attraction between an object and Earth because of gravity. In the imperial system, pounds can be a unit of mass *or* of force. This means that an object that *weighs* 100lbs has a mass of 100lbs, and also that the gravitational force on that object at Earth’s surface is 100lbs. But if you took that object to the moon, the mass would be the same but the weight would change. It could have two different amounts of pounds depending on what you meant!
In metric/SI, we use kilograms for mass and Newtons for force. This avoids that issue. An object has a certain mass in kg, which is fixed. The weight in N can change depending on gravity.
Weight is to mass as interest is to money.
Imagine for a moment that for the past ten thousand years there was only one rate of interest- 10% a year. Sure you could imagine the rate would be different but come on, it never actually is. So when someone says interest is a hundred bucks a year you know the money is 1000. It’s just… Known…
Well that’s the weight thing. Weight is mass x acceleration and acceleration is always always always gravity. So you start to think of these things as the same thing. But they are not.
When acceleration increases by more than gravity weight goes up but mass stays the same.
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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