the spring was compressed three times farther and then the block is released, the work done on the block by the spring as it accelerates the block is

The work done on the block by the spring as it accelerates the block is 4kx².

Explanation:

Let initial distance is x.

It was compressed three times farther and then the block is released, new distance is 3x.

The work done in compressing the spring is given by :

So, the work done on the block by the spring as it accelerates the block is 4kx².

if the thermos has a vacuum innner, then heat through conv currents and conduction are reduced = vacuum flask

the entire statement is true and correct.  

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