What is the difference between mass and weight?
Please read this also, and welcome to the boards.
Gravity pulls on objects. If there is nothing to stop them, those objects will accelerate due to gravity. In our everyday experience on Earth however, objects are not accelerated by gravity because there is something in the way, holding them up. The ground, the floor, tables, stepladders, safety harnesses - on Earth, things are generally on top of other things. That stops everything falling into the Earth’s centre, as they otherwise would do.
The “weight” of an object is the force it exerts, due to gravity, upon whatever is holding it up. Something in a zero gravitational field has no weight. Something that is falling freely, such as an orbiting space station, a trainee astronaut in the Vomit Comet, or a person standing on a freshly opened trap door, has no weight either because there is nothing holding them up. For the guy on the trapdoor, their weightless status is temporary. For a free-falling parachutist, you get a moment of weightlessness when you jump, but then the drag of all that air blasting past you restores the feeling of weight.
Strictly speaking, weight should be measured in units of force, e.g. Newtons, or pounds force, or kilograms force, etc.
The “mass” of an object is a trickier concept, largely because under normal circumstances we never experience mass divorced from weight. Mass is a measure of the inertia of an object, how cumbersome or hefty it is. It is measured in units of mass, e.g. pounds or kilograms.
A person may believe their weight to be 160 pounds. In fact, their weight is 160 pounds force, and their mass is 160 pounds. Take them to the Moon (or anywhere else) and their mass is still 160 pounds, but their weight becomes about 27 pounds force due to the lower gravity.
Now give that person a backpack with a mass of 300 pounds. If they are a very dedicated gym rat they might just about be able to stand up with the thing on, on the surface of the Earth where it has a weight of 300 pounds force.
Take them and their backpack to the Moon, and now the backpack has a weight of 50 pounds force and everything is peachy. Except… the guy strolls at a fair pace down the Moonbase corridor and turns a sharp right at a corner. Then he makes the unwelcome discovery that 300 pounds of mass moving in a straight line doesn’t change direction very easily, even if it only weighs 50 pounds force in the local gravity.
The weight of things on Earth, and their masses, are numerically the same - the 300 pounds mass backpack weighs 300 pounds force in Earth’s gravity. For this reason, and because our experiences of weight and mass are so intrinsically bound together, the concepts are often treated interchangably and people sloppily leave out the “force” or “mass” when they express quantities in pounds. Only the unreasonably pedantic such as myself, or people who have experienced mass divorced from weight or vice versa such as our Moonbase backpacker, seem to care.
thanks, matt! But can you explain(mathematically) something to me?
How do you calculate using mass and not weight?
When I studied engineering , I learned to calculate how thick a steel beam has to be in order to support a heavy weight.
(Think of the arm of a crane, lifting a heavy object on a construction site.)There are simple formulas, but they all use the weight of the object.
Now, in outer space, with no gravity, how do you do that calculation?
I am thinking of the arm on the Space Shuttle, which lifts an object out of the cargo bay. The object was heavy on earth, but in space has zero weight. So how do engineers calculate the thickness needed of the shuttle’s arm to lift the object out? Would an overly “heavy” object cause the arm to sag and break, like on earth?
Nitpick. Units of mass are pounds mass or kilograms (also slugs.) “Pounds” is generally used to denote pounds force. Here on Earth, pounds force and pounds mass are equivalent numerically.
Haj
You calculate just the same way. On Earth the deflections were caused by the weight of the load. Weight is a force, i.e. a vector quantity. The arm in the bay of the shuttle is going to be under load from forces. Except that these forces will only be present when the arm is accelerating the object in question.
You can have very small loads, if you like. I can move a multi-ton object around in space with just a piece of kite string, if I am willing to accept very small accelerations. My WAG is the shuttle arm is fairly lightly constructed. You don’t want the overhead of weight having to be lofted into orbit every time. But as long as the arm moves objects, be they satellites or astronauts, slowly the construction can be correspondingly light.
If you go the moon, you will weigh only a sixth as much, but your mass will not have changed. (Unless you dieted a lot in transit 
)
Mass is measured in grams and kilograms (although you can “misuse” them for weight). A mass scale utilizes counterweights until they balance what is being massed*.
Weight is measured using newtons. A weight scale utilizes a compression or torsion spring which balances the force of gravity to yield the result of what is being weighed.
- Isaac Asimov once pointed out that everyone confuses the two, in part due to the fact that we have no verb for “obtaining the mass of” so that everyone says “weighing” or “weighed” even when they are talking about mass not weight. Asimov also pointed out that the Earth has no weight (it’s in free fall).
AHunter, interestingly, if you went to the moon, your mass would actually increase (admittedly by an unbelievably tiny amount), as you would (net) gain gravitational potential energy.
“There’s a great thing about America I’m gonna teach you now, Vic, and you’d better remember this because some people won’t be as nice about it as I am. We gots a thing called freedom of speech here. I can verb any noun I like, and you little freedom-haters don’t got any right to say anything about it. I should pound you for this, but I need a hand here” -the comment received when some (Arab-looking) kid looked at my HS chemistry lab partner funny for a couple of seconds and when he told me “to mass” a watch glass. Sometimes I’m sorry I didn’t dare him to eat the NaOH pellets.
Nitpick. Units of mass are pounds mass or kilograms (also slugs.) “Pounds” is generally used to denote pounds force. Here on Earth, pounds force and pounds mass are equivalent numerically.
Well, I’m a fan of metric myself. I was using “pounds” as pounds mass, with the qualifier of “force” to make the distinction, but if that is not the convention, mea culpa. I’m used to “kilograms” as units of mass with “kilograms-force” occasionally used for force, so I assumed the convention was the same with “pounds” and “pounds force.” Although if you’re going to bring up slugs, I’m going to bring up poundals!
chappachula:
The question of how thick you need to build the space-shuttle arm is an interesting one. The limit isn’t necessarily determined by the mass of the objects that the arm moves around, but how hard the actuator motors in the arm can push.
For example, suppose the arm can shove with a force of 9800 Newtons (1000 kg force, or one tonne force, or 2200 lbs.) The mass of the weightless object that it is trying to push doesn’t make any immediate difference - it will accelerate a 1000kg (mass) satellite at 1g if it pushes at its hardest. A 5000kg satellite will only be accelerated at 0.2g. In both cases, the arm exerts, and is subjected to, the same 9800 Newtons.
As is usual in engineering however, such a simple analysis usually results in things breaking. If you shove something away with the arm pushing with a tonne force for a distance of say, 5 yards, when the arm is fully extended, that still-moving mass will exert rather more than a tonne of tension if the arm then brings it to a halt. The exact value depends upon the distance over which the moving mass is brought to a halt, which depends upon the elasticity in the arm. Like my Moon backpacker with 300 lbs mass on his back, a moving mass can bite you even if it weighs little or nothing.
Maybe I’m misunderstanding what you’re saying here. The arm decelerates the now moving object over 5 meters. How are the forces greater than when it was originally sped up.
Unless you’re referring to changing moments with the configuration of the arm. Maybe I’m being too simplistic…
Weight is what we all understand. If you eat lots of cream cakes, you put on weight, and if you saw your leg off, you lose weight (this is the only known way to lose weight in real life). ‘Weight’ is also the reason why we look better naked when we’re 18 than when we’re 38. That’s why 18 year olds are way more interested in sex than people 38 and over.
Mass is what science geeks talk about. It doesn’t exist except in laboratories and on graph paper, but if the geeks are happy yakking about it, what do the rest of us care? It keeps them busy and away from the cream cakes. This means they don’t gain weight, but as they never have sex or anything, it’s immaterial.
Is mass related to the density of an object? If I was suddenly compacted to 1/10th my size, my weight would remain the same, but would my mass increase in proportion to my increase in density? Or am I way off base?
Mass would not change. Mass is literally how much stuff there is, on an atomic level. If you look at a periodic table you’ll find atomic weights based on the number of neutrons, protons and electrons in an atom. The atomic weight can be used to make good estimates of the number of atoms in a given mass of stuff and is commonly used in chemistry to get correct proportions. Search Avagadro’s number for more on that.
Squeezing all your atoms down does not change the mass… to a point. If you squeeze enough to cause fusion (I didn’t say it would be easy) some of your mass will turn to energy equal to the speed of light squared times the mass that was turned into energy.
Well you’d use an inertia balance. Ok, I admit I’ve never used one but apparently it’s a pan on the end of an arm. The arm vibrates when you pull it off to the side, kind of like a spring. The period is based on the mass of the object in the pan.
Sorry MonkeyMensch, I wasn’t very clear. I was thinking about a situation where the arm starts off “folded”, like the arm of a human shot-putter. It pushs the satellite outwards at maximum force, but fails to let go when the arm is fully extended, so the satellite mass is brought to a halt in a short distance, the arm acting effectively as a tether.
Not that I expect this situation to happen. I was just giving an example of how simplistic calculations generally underestimate the maximum stresses that could be encountered.
ianzin, I suggest the following experiment. Get yourself down to one of the big London train stations, climb down from a platform and stand with your back against one of the line end buffers. Get a helpful loco driver to nudge a nearby, free-standing carriage towards you at some footling speed, say 1 cm a second. Stay where you are. It will shortly become apparent to you that mass does in fact exist, outside of the laboratory.