Our recent CHRISTMAS LECTURES were all about human space travel.
So obviously we had to talk about zero-g and weightlessness, which are some of the most
fundamental experiences of being in space.
This led to some discussion in the office about what weight is.
I thought it'd be good to unpick in a video some of the issues that came up,
because actually I think there's a lot of subtlety, dare I say it some confusion, in
the way people talk about the physics of weight.
One of the biggest misunderstandings is around the use of the term 'weightlessness', which
lots of people assume means that in space you don't have any weight.
But this isn't true.
This misunderstanding isn't because the definition of weight is confusing at all.
It's actually really simple, and exactly as you learn it at school.
Weight is just the force on an object due to gravity.
Or if you want to be more precise, it's the size of the gravitational attraction between
me and the earth,
or the ball and the earth, or this cup, this chicken, and the earth.
The problem I think is that the way we learn about weight at school
doesn't quite capture the way that people think about weight and experience it in real
life.
As I sit here, I can say that I can feel my weight bearing on my feet and bum, even inside
my body.
But is really my weight that I'm feeling?
Remember what we just said: my weight is just the force of attraction between me and the
Earth.
But there's clearly a lot of stuff between me and the centre of the earth:
the floor, the ground, a load of magma...
As the force of gravity pulls me down, repulsive forces are set up within all that stuff,
and at the interfaces of all those objects, and these are intermolecular and inter-atomic
forces accumulating all the way to the centre of the earth.
The end result of that is that I just sit here, not falling.
Those forces oppose and counteract the effect of gravity.
So the really important point here is that you don't really feel your weight either.
It's all of these other forces that support you that you experience as weight.
This is where the confusion comes in.
When we use the term 'weightlessness' in the context of space and astronauts,
we aren't talking about an absence of weight itself, but an absence of what you experience
as weight on earth.
In other words, the absence of the opposing forces that support you.
Confusion around this had led to another misconception, that there is no gravity in space.
But gravity is everywhere. The universe is filled with gravitational fields, you can't
escape them.
So the idea of being weightless due to the absence of gravity makes no sense at all.
Interestingly, the effect of the Earth's gravity doesn't change that much when you go into
space anyway,
at least into low earth orbit, like the astronauts on the International Space Station.
The altitude of the ISS is about 400km, which sounds a lot,
but is only actually about 6% of the radius of the earth,
so gravity there is almost as strong as it is on the surface.
What's really happening in space is that there's nothing to counteract the effect of gravity.
There are no opposing forces to hold you up,
so instead you just fall freely.
With the ISS, which looks exactly like this, this is exactly what's going on.
It's not floating or hovering, it's just falling like anything else towards the centre of the
earth.
It's just going so fast, that it overshoots and overshoots and overshoots,
until it eventually finds itself back where it started, and goes round again.
So weightlessness is just a state of unopposed motion under gravity,
all you need to do to create weightlessness, is drop something.
When you do that, you remove all of those opposing forces, and the object falls.
This is how we can create temporary weightless environments, like in aeroplanes like the
'Vomit Comet',
which deliberately fly along paths to simulate freefall.
So I think the difficulty reconsiling this formal definition of weight, with ideas like
weightlessness
comes from this conflation of these two slightly different ideas:
on the one hand, you have the weight you learn about in school, as a gravitational attraction,
on the other hand you have the slightly subtler idea of weight as something you experience
and feel.
In the CHRISTMAS LECTURES we really wanted to get into this equivalence between freefall
and weightlessness.
I think physicists would normally say weightlessness is a consequence of freefall,
but maybe with this view of weight, it'd be better to say that freefall is a consequence
of weightlessness.
As soon as you stop exerting other forces on an object, it has no choice,
like Wile E Coyote, but to fall under gravity.