When you try to move the magnet back and forth between the two coils, the motion of the magnet will be oscillatory and this action will cause current and EMF to induce
According to law of Faraday, current or EMF will be induced when a magnet is moved in the presence of coils
If the magnet continues to move back and forth between the two coils, what might be causing this will be the presence of the induced electromagnetic force between the two coils.
Since we are interested in finding the distance flown by the plane, we would use an instrument measuring length. Since distance flown by the plane is quite significant, a measuring tape is used
