Placing a floating object in a water bucket

[Siby]

Does insertion of a floating object in a bucket of water increase the whole unit's weight? Please explain.

Eg.
If the bucket of water weighs 10 kgs, then does the insertion of the floating object of weight 2 kgs increase the whole unit's weight to 12 kgms?

[Loosehead]

Yes it does. The object displaces it's weight in water so the level will rise, you are now including the floating object in the overall total. Easy enough to try at home.

[JudgeJ]

Loosehead is, of course, perfectly correct (provided placing the object in the water does not cause it to overflow).

However, when I see questions such as this I�m always interested in people�s thought processes and what they were taught and how they learn. Just what other answer did you expect, Siby?

Just think it through. If, for example, you placed both the bucket of water and the object (not yet in the bucket) on scales and saw that they, together, weighed 12kg, why did you expect they should weigh something different when you floated the object in the bucket? Or, if you placed the bucket with the object already floating in it on the scales, noted the weight, and then removed the object and placed it by the side of the bucket (still on the scales) what did you think would happen? Why should you expect the total weight to change, and if it does, where does the change in weight go to or come from?

I�m not having a dig, Siby. You obviously needed to ask the question. I�m just utterly intrigued!

[mibn2cweus]

Actually a volume of air equivalent to the volume of the object inserted is displaced. However, if the object was originally weighed in air of the same density (and same gravity) this would not alter the outcome. Conversely, if this "experiment" was carried out in a vacuum (as ideally it would) then the displacement of air would not be a consideration.

[Siby]

Thank you all for the answers. I havent tried this yet, but had a small confusion regarding the displacement of water by the body and the subsequent displaqcement of air by the displaced water.

[matt_london]

For a while a could never work out the following: Say a man who weighs 15 stone picks up a boy who weighs 5 stone. His arms are then carrying that 5 stone weight . If someone else came along and lifted up the man then he would be carrying 15 st plus 5 st = 20 st. You then have one man carrying 5 st and another carrying 20 st, which adds up to 25 stone. Where has the extra 5 st come from?

[Tim123]

matt_london,

You counted the weight of the child twice.

I like this one too http://mathforum.org/dr.math/faq/faq.missing.d ollar.html