Why triple point of water is taken standard fixed point in modern thermometry??​

It is because of the fact that triple point of a substance is unique I.e it occurs at one particulars set of a values of pressure and temperature. the melting point of ice and boiling point of water are not unique. the change with change value of pressure or due to the presence of impurities in water

the box does not move because you need to use a force greater than 300 n. you are only using 50 n. it needs to at least be 301 n for the box to move because then it is an unbalanced force and it will move the way that the greater force is pushing it. if you use only 300 n it will be a balanced force and will not move because the force you are exerting on the   box is the same as its force.

hope this !

im sorry if i went too in depth, i am going to include a link to a video on khan academy that will explain this.   hope that you like the video and that it explains more in detail to you.

that is the link.