How? giving reasons
IN terms of Kinetic energy of molecules , heat and thermodynamic
IN terms of Kinetic energy of molecules , heat and thermodynamic
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By shaking, the milk's molecules will obtain extra "kicks" as they bounce off a moving wall. This will increase their average kinetic energy. Since average kinetic energy is proportional to temperature, this increase the temperature of the milk. Note that the thermos prevents this added heat through shaking from escaping to the outside world (thermally isolated system).
Some people get confused on the kicking story, because they say: but hey, the molecules that approach a side of the wall that is receding from them bounce off slower! That's true, but since kinetic energy is proportional to velocity squared we have, when the molecule's velocity is v and the wall's velocity is u, that:
For the half of molecules that bounce off the approaching wall kinetic energy will change from 1/2 m v^2 to 1/2 m (v+u)^2. For the half of molecules that bounce off the receding wall kinetic energy will change from 1/2 m v^2 to 1/2 m (v-u)^2.
The former is 1/2 m v^2 + m v u + 1/2 m u^2
The latter is 1/2 m v^2 -m v u + 1/2 m u^2
If you add this (50/50) then the + and - cancel (this is what people sometimes erroneously see as the only term) but what is added is 1/2 m u^2. Fortunately this is independent on the sign of u (direction of shaking) as it should!
Some people get confused on the kicking story, because they say: but hey, the molecules that approach a side of the wall that is receding from them bounce off slower! That's true, but since kinetic energy is proportional to velocity squared we have, when the molecule's velocity is v and the wall's velocity is u, that:
For the half of molecules that bounce off the approaching wall kinetic energy will change from 1/2 m v^2 to 1/2 m (v+u)^2. For the half of molecules that bounce off the receding wall kinetic energy will change from 1/2 m v^2 to 1/2 m (v-u)^2.
The former is 1/2 m v^2 + m v u + 1/2 m u^2
The latter is 1/2 m v^2 -m v u + 1/2 m u^2
If you add this (50/50) then the + and - cancel (this is what people sometimes erroneously see as the only term) but what is added is 1/2 m u^2. Fortunately this is independent on the sign of u (direction of shaking) as it should!
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As you shake the bottle the milk will move back and forth on the inside walls of thermos, This work which is primarily obtained from the person who is shaking will dissipate as internal energy of the walls of the thermos and the milk raising their combined temperature.
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Maybe - maybe not , If the flask is full there is no useful work done on the milk : milk as a system?
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Yes it does.