vA solenoid with 300 turns per centimeter has an iron core with a relative permeability of 100. The solenoid carries a current of 5.0 A. What is the magnetic field at the central axis of the solenoid?

What’s #16 asking me and how do i solve it?

What is the orbital period of a spacecraft in a low orbit near the surface of mars? the radius of mars is 3.4×106m.

Arope has one end tied to a vertical support. you hold the other end so that the rope is horizontal. if you move the end of the rope back and forth with a frequency of 4 hz, the transverse wave you produce has a wavelength of 0.5 m. what is the speed of the wave in the rope?