Weight has the units of Force (ie Newtons in the Metric system, or Pounds-Force in the Imperial system).

So bathroom scales measure the downward force of an object (while the see-saw variety of scales has a more direct measure of mass than of weight).

If we place an object on a set of bathroom scales in a lift/elevator:

- It is only if the lift/elevator is stationary that the entire Force=Mass x Gravity is borne by the scales, and the scales indicate what we consider the "normal" weight of the object. In the Imperial system, the weight of the object in Pounds-Force equals the mass of the object in Pounds.

- If a lift/elevator accelerating downwards, part of the acceleration due to gravity goes into accelerating the object downwards, and only part of the gravitational acceleration is opposed by the scales, so the object weighs less than "normal".

- Conversely, if a lift/elevator is accelerating upwards, the object weighs more than "normal".

Similarly, if we place an object on bathroom scales at the equator, and increase the rotational speed of the Earth, the object will appear to weigh less. However, we don't normally detect this effect, since the Earth retains a fairly steady rotation, and the oceans and the land of the Earth is already deformed into an oblate spheroid (the

geoid) which takes the rotation of the Earth into account.

For iron weights in air, you can normally ignore this, but air provides an upward buoyancy which reduces the apparent weight of an object below what you expect from the relationship Force = Mass x Gravity. This becomes very significant for light objects, eg a helium balloon can have a negative weight, even though it has a positive mass! Similar comments apply to wood floating in water.

Overall, a more accurate definition of weight would be Weight=Force = (Mass-m)x(Gravity-a), where:

- Mass is the mass of the object

- m is the mass of the displaced medium (which can sometimes be larger than the mass of the object)

- Gravity is the acceleration due to the planet/star's gravity at the relevant altitude

- a is the acceleration of the object due to being placed in a lift, or due to the rotation of the planet (in extreme cases, these can sometimes be larger than the acceleration due to gravity)