The double walls of the shiny inner wall and the outer cover prevent heat from escaping or entering. The thermos flask’s design includes a vacuum between the double walls and a shiny inner surface to minimize heat transfer by conduction, ...
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A thermos flask prevents heat loss through all three mechanisms by using a vacuum to stop conduction and convection and a reflective coating on the inner surfaces to minimize radiation, thereby maintaining the temperature of the contents.
The scientific definition of work requires a force causing a displacement, whereas the everyday understanding of work involves any physical effort or energy expenditure, regardless of whether there is displacement.
No work is done on the load when you stand still with it on your head because there is no displacement of the load in the direction of the force you are applying.
Standing still for a few minutes with a heavy load on your head results in tiredness and expenditure of energy, but no work is done on the load according to the scientific definition.