I found a .25 kg tennis ball off a brick wall at 15 m/s and a 50° angle with the wall and it rebound elastically at the same reflected angle. If the ball fell 150 N in the collision how long did it last?

0.11 m/s and 0.28 m/s should be correct

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.