The commutator of a universal motor connects each armature coil to
a. the rotor.
c. the gearbox.
b. the shaft.
d. a brush.

a. the rotor

Explanation:

A commutator is a rotary electric switch that periodically changes the direction of the current between the rotor and the external circuit. In a motor provides energy to the rotor. It consists of a cylinder composed of multiple metal contact segments on the rotor.

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.

we need to know the time interval during which the force acts. impulse of force = momentum change