Actually the mass matters more than the weight. You stop and start a car an awful lot more than you go up and down hills, and for stop-start it's the mass that matters. The weight only serves to keep the tyres on the road.
Air resistance is for practical purposes proportional to speed squared (which makes required power speed cubed)
Andy
I know what you mean... but... If I see a sign on a bridge telling me it can carry up to 15kN I have no idea what that means. We've used mass and weight interchangeably for centuries, because we're tied to a planet in a gravity field that is effectively constant. The only reason though is because we're used to it.
When I left school I could tell you the temperature of the air in Fahrenheit and the temperature of a bath in Celsius. I now use metric temperatures exclusively. It took me years.
I still convert my fuel use into MPG. Because I'm used to it.
And that bridge? It can take a tonne and a half. In Earth's gravity. It would be a lot more on the moon. But then, I'm not taking it there. Unlike, say NASA and the Mars probe.
Andy
Don't think I ever recall seeing anything quoted in kgf...
I have an over-active mind, and not enough sleep!
I was just watching television, so naturally mind drifted to matter less de= pressing, and I was idly thinking about the force required to get my car to= whatever speed, and then it occurred to me that I couldn't remember the fo= rmula, so I looked it up and found F =3D ma, and W =3D mg and due to my tir= ed state, I confused myself. It happens.
One misplaced "it's", and they breed. I think we can all learn from this error.
Anyway, after sleeping, I think Ive cracked it.
;-)
Is a unit of force, not of weight.
As I said the common meaning of weight is not the force exerted by a mass due to gravity. A lb of flour is still a lb of flour even in a freely falling lift shaft.
It exerts a force of a pound force when its at rest at the earth's surface. That why we have masses in kilograms and weigh things in kilograms, not kilogram force. That's why atoms have weights that are independent of gravity. We talk of atomic weight, not atomic mass, although we probably shouldn't.
But as you can see its a mess. with people getting confused between how much stuff is in the bag of flour, and what force it exerts when its at rest in 1g. And what to call the separate things.
I side with common usage here.
Not pedantic half baked physics masters who probably don't understand the difference either.
Yeah I know what it is, just don't recall ever seeing it used. pounds force, yes, and newtons, certainly.
[snip]
I side with whichever is appropriate for the context...
If I am working out the work done lifting a mass then I will use weight in its "proper" sense. If on the other hand I am "seeing how much I weigh", I am content with an answer in stone.
Yes but there's no necessity for common parlance to BE right when it makes no practical difference.
Bill
It was drummed into me that we should talk of atomic masses! (And centripetal rather than centrifugal.)
And, colloquially, people with a bit of understanding say things like "A falling bag of flour is weightless". But I am sure they do not think that it then has no mass.
Weight is a *force* = mass x acceleration.
I agree that almost no one in practice uses kg.f any more than experienced chemists use IUPAC names for common chemicals.
It is even worse in the US where aerospace uses foot-poundals and short measure antique British gallons but NASA uses SI and Mars probes smash into the planet and planes fall out of the sky as a result.
But common usage does not clearly distinguish between mass and weight. We are so used to constant Earth gravity that for most practical purposes setting g=1 makes no difference and is very convenient.
Physics does and you would actually find that if you tried to use a pair of bathroom scales or Tescos checkout in freefall the products would all be weightless and a mass balance wouldn't work either.
The products would all be weightless so you would have to weight them by applying a known specific impulse and measuring their change in velocity.
It is good enough to within 0.5% over the entire surface of the Earth. But it is generally weight that is measured these days even in fancy laboratories - in the old days kitchen scales had a set of weights and labs had beautiful chemical balances to measure mass but not any more.
Even mass spectrometers are actually measuring the mass to charge ratio. It has proved very difficult to tie the standard reference mass into the fundamental constants with anything like acceptable precision.
Common usage *and* what is actually being measured.
It is actually rather important to understand that things in freefall are weightless though - that is the foundation of general relativity.
inertia.
weight?
It does. That is what is causing it to free fall(accelerate). If it is not accelerating, some retarding force must be acting to counter its weight.
No. If the surroundings are also in free fall then you cannot tell the difference and are truly weightless. That is how the vomit comet works.
And it is also what happens in orbit. The Earth's gravity is enough to keep the ISS captive but it and everything inside is in free fall.
You can find footage of Stephen Hawking in a weightless jaunt without going into space (he is much too frail for that but would love to go).
Hmm, I thought that was as a result of its *mass*. It gets *weight* by
*not* being in free-fall.
Same thing!
Weight is not exerted by a mass, it is exerted on the mass by the gravitational field. Why do you think that because an object is moving, it has no weight?
There we go again
in 1g. And what to call the separate things.
understand
But you don't have to be inside the free-falling surroundings in order to be weightless. The ISS is still weightless, as you say. The point is that in the former case you can't tell the difference.
inertia.
gravitational field.
without
That is not weighless, it is reactionless. You can only be weighless in zero gravity.
Is the ISS not in free-fall?
What does it mean to be in orbit?
weight is a
inertia.
gravitational
The ISS is not weightless. Its weight is what is providing the centripetal force that keeps it in its orbit.
Sure it's in free fall. But its trajectory is such that it doesn't intersect with the Earth at any point.
Given that the Earth's gravitational field along that of every other piece of matter stretch out to infinity, there is no "zero gravity" anywhere in the universe.
You have weight if you are subject to forces *other* than whatever the gravitational field local to you is applying to you. Such forces can be applied by a rocket, f'rinstance. For us on Earth the force is applied by whatever we are standing on.
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