What is the magnitude of the kinetic frictional force?

The magnitude of the kinetic frictional force is 25 N.

Explanation:

Kinetic frictional force :

A force which is act between moving surface it is called kinetic frictional force.

When an object moves on the surface then it experienced a force in the opposite direction of its movement.  

The friction force always acts opposite of the actual force.

The kinetic friction force is defined in mathematically terms

Where, N = normal force

=  friction coefficient

According to figure,

Hence, The magnitude of the kinetic frictional force is 25 N.

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

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.