Weight Illusion

Your sense of weight can be misled by your perception of size.

Material

3 cans all the same diameter,

• 1 full and short
• 1 tall and empty (at least twice as tall as the smaller can.)
• 1 tall and full

1 box of cereal the same weight as one of the full cans.(It can be up to 10 % heavier.)
Tape and string

To Do and Notice

Lift the can and then lift the same weight box. Notice which feels heavier.

Tape a string to the can and to the box. Close your eyes and have a helper hand you first one string then the other. Compare the weights. Notice that they feel more equal.

Lift the small full can and the taller full can. Notice how heavy each feels.

Place the empty large can on top of the small full can. Notice that the combination of the two cans feels lighter than the small can alone.

What's Going On?

The apparent size of an object affects our perception of how heavy it feels.

The large box felt lighter than the smaller can even though they were the same weight when you could see the box.

When you did not look at the objects but only felt their weights through the strings they felt more nearly the same weight.

When you placed the empty can on top of the full can you slightly increased its mass and greatly increased its size. The increased size changed your perception of the weight of the small can.

References Illusion in Nature and Art, edited by Richard Gregory, Charles Scribners Sons, NY 1973, p59.

Perception Exploration

 Scientific Explorations with Paul Doherty © 1999 13 June 2000