Thanks. So it looks like it is as I thought.
When you stand on scales, there are 2 forces acting on you, W, your weight downwards and R, the force of the scales pushing up on you.
If the Earth was completely still - no rotation in this case - then you would be at rest on its surface.
If you are at rest then there is zero resultant force on you. (Newton's Law.)
This means that the resultant of those two forces is zero.
Taking
down as positive direction it means W - R = 0
(or W = R)
In other words, the measurement on the scale (R) would be equal to your weight (W).
If the Earth is rotating, however, you are moving in a very large circle around with the Earth's surface at that point.
For you to move in a circle there must be an accelerating force towards the centre of that circle, the so called centripetal force.
As there are only 2 forces acting on you still, W and R, then these 2 forces combined must produce that centripetal force.
This force acts in the downwards direction towards the centre of the circle you are moving in.
In the previous paragraph, I wrote down the equation for the
resultant downwards force when this force is zero. (When you were at rest)
This equation will
always give me the resultant downwards force, but now it isn't zero. It's equal to the centripetal force (F) on you, because you are moving.
It is this that connects with the first bit of the question. As a result of the inclusion of the force F now,
W is not equal to R.
In other words, the reaction of the scales on you pushing up, is not equal to your weight.
It is the reaction force of the scales on you that is actually measured by the scales as your 'weight'. (Through the spring system inside)
The conclusion is that now the scales are not (quite) measuring your weight. (Because R is not equal to W)
The difference is so small that it is insignificant. But it's a useful bit of physics.
BTW. At the north or south pole the scales would read correctly. Do you see why?