Why does a pendulum have periodic motion?

a) the pendulum has a spring constant
b) the pendulum obeys hooke's law
c) the spring pulls on the pendulum
d) gravity acts on the pendulum

adding a(n) to an electric circuit would increase the magnetic field around a wire;

b. solenoid

a solenoid is a type of electromagnet whose purpose is to generate a controlled magnetic field.

if the purpose of the solenoid is instead to impede changes in the electric current, a solenoid can be more specifically classified as an inductor rather than an electromagnet.

this is poiselle's equation from fluid mechanic in physics. mech eng