In this column Cecil correctly states that milk doesn’t get perceptably heavier when it freezes. But it may have felt heavier if it was more difficult to lift. It’s possible that usually the woman unconsciously took advantage of the momentum of the shifting liquid when lifting and maneuvering the box. She may hve only perceived the lack of slosh as an increased difficulty in lifting, which she interpreted as ‘heavier’.
Also ‘heavier’ is easier to say than whatever more accurate thing she might have said.
Actually the milk is imperceptibly LIGHTER. As the Milk turns to Ice, it is actually less dense and therefore slightly more BOUYENT to the surrounding air. Instead of flexing inward when she grips it, the carton bulges slightly outward. It feels heavier because the grip is slightly weaker as the fingers are more open.
rwj, a given volume of frozen milk weighs less than a given volume of liquid milk. Another way of saying the same thing is that frozen milk is less dense than liquid milk. However, given that the quantity - the mass - of the milk in a gallon jug remains relatively constant as it’s frozen, the weight (which depends only on its mass) is the same. It’s neither lighter nor heavier.
However, frozen milk should be slightly lighter due to its lesser thermal energy. And furthermore, at our house we keep the frozen milk in the basement, which is closer to the earth’s center of gravity. It’s thus subject to greater gravitation and thus weighs more. Whether those two factors balance each other out is hard to say.
I think rwj was saying that it’s (imperceptibly) “lighter” in the sense of “easier to lift”. He mentions the bouyancy of the milk relative to the surrounding air. Just like heavy objects feel lighter when we lift them if they are placed in a medium that makes them more bouyant, even though their mass hasn’t changed.
The distinction between mass and http://dictionary.reference.com/search?q=weight weight]weight can be somewhat confusing. Weight is a measurement of the effect of Earth’s gravity at sea level at Standard Temperature and Pressure. Mass on the other hand, is resistance to acceleration of 1 G in a vacuum.
A balloon may weight .5 grams when the air inside and out is the same temperature. However, if you heat the air inside the balloon, it may float. The floating balloon “weighs” less, even though the mass is the same.
The mass of the milk is constant, the “weight” decreases due to buoyancy.
The increase in buoyancy is greater than the increase in gravity in your basement.