Two toy cars with a combined momentum of 7 kg m/s collide and stick together. If the velocity of the combined object after collision is 3.5 m/s what is the combined object's mass?

the answer is the first electromagnet (a). this is because it has more coils around the ferromagnetic material. increasing the number of coils increases the surface area in which the metal interacts with the current in the coil. this enables more atoms in the metal to become aligned hence becoming more magnetized.