Astudent pushes a 12.0 kg box with a horizontal force of 20 n. the friction force on the box is 9.0 n. which of the following is the best approximation of the box’s acceleration?

The acceleration of the box its 0,91 m/s²

Explanation:

According to Newton first law, the force applyed to some object its equal to the mass of the object plus the acceleration of the same object.

   F=m.a ; clearing the acceleration of the equation ⇒ a=F/m

  Now in the problem its apeared the friction force and according to de phsysics definition its a force thats its against of movement.

  So the force its going to be equal to F= 20N - 9N.

 Replacing in the equation a=(20N - 9N)/12kg.

 Solving ⇒ a ≅0, 91 m/s²

electromagnetic shielding is the practice of reducing the electromagnetic field in a space by blocking the field with barriers made of conductive or magnetic materials. shielding is typically applied to enclosures to isolate electrical devices from their surroundings, and to cables to isolate wires from the environment through which the cable runs. electromagnetic shielding that blocks radio frequency electromagnetic radiation is also known as rf shielding.

the shielding can reduce the coupling of radio waves, electromagnetic fields and electrostatic fields. a conductive enclosure used to block electrostatic fields is also known as a faraday cage. the amount of reduction depends very much upon the material used, its thickness, the size of the shielded volume and the frequency of the fields of interest and the size, shape and orientation of apertures in a shield to an incident electromagnetic

from wikipedia