Stepping on a scale, carpet vs. tile

Why does a scale show a different weight if you step on it on carpet as opposed to tile? And which is the correct weight?

The carpet is flexible and will absorb some of the energy needed to deflect the scale’s mechanism properly. Tile provides the best foundation.

It probably has something to do with the force not being uniformly distributed on the bottom of the scale when its on carpet. A very well designed scale would show very little difference between soft carpet and hard tile. A perfect scale would show no difference.

But that would mean that it would display a lighter weight, wouldn’t it? It actually displayed seven or eight pounds lighter on the tile (so I would much rather the tile reading be right).

And, Crafter, no, it’s not all that great a scale. Cost five bucks or something.

Here we have a significant indicator. Your bodily force (as applied to the scale’s detector) is less accurately measured by a more error prone system, especially on a largely resilient surface. This gives a greater latitude of error in both directions. The mechanical “reverberation” of your carpet may well introduce uncertainty in the positive direction as well.

Do us a favor and perform 20 measurements of your weight upon a carpeted surface versus the tiled one. I’m particularly curious as to the mean deviation. (No, I’m not saying you’re deviant!)

It has nothing to do with “absorbing energy.” For one thing, scales measure force, not energy. And any absorbing of either is over and done with long before the scale produces any reading.

It’s as the previous poster said – different force distribution.

I still want to see the results of a statistical sampling.

Tile, absolutely. Your average digital scale isn’t going to be able to function on a cushy surface.

Also, never step on a scale other than the morningtime, prior to eating. This is vital information if you’re attempting to diet - and do not get on the scales at your gym, as it’s a widely know fact that those scales lie.

markci is correct… energy is absorbed by the carpet only during the short interval when the carpet is being dynamically compressed. This occurs during the first second or so after you step on the scale. If you wait one or two seconds after stepping on the scale, and stay perfectly still (i.e. the “static” condition), the carpet is no longer absorbing energy and the scale should read exactly the same as on a hard floor.

But you will often find a difference. This is because the scale is designed such that it wants to see the force distributed in a certain way on the bottom face of the scale. As an example, a scale may be designed with the assumption that the force will be more-or-less evenly distributed amongst four feet on the bottom of the scale, with the “feet” being nothing more than low profile protrusions. On a hard surface, the center of the scale would not be touching the floor. If you set the scale on a spongy surface, the force will be distributed over the entire bottom surface. This means that the center portion of the scale will be touching the floor and thus slightly compressed, thereby giving a false reading. Again, this is just an example.

I’ve noticed I weight different weights if I step on the scales with my left foot first, then with my right foot first …