B) It’s material moves due to convection currents.
Answer:



Explanation:
<u>Simple Pendulum</u>
It's a simple device constructed with a mass (bob) tied to the end of an inextensible rope of length L and let swing back and forth at small angles. The movement is referred to as Simple Harmonic Motion (SHM).
(a) The angular frequency of the motion is computed as

We have the length of the pendulum is L=0.81 meters, then we have


(b) The total mechanical energy is computed as the sum of the kinetic energy K and the potential energy U. At its highest point, the kinetic energy is zero, so the mechanical energy is pure potential energy, which is computed as

where h is measured to the reference level (the lowest point). Please check the figure below, to see the desired height is denoted as Y. We know that

And

Solving for Y



The potential energy is


The mechanical energy is, then


(c) The maximum speed is achieved when it passes through the lowest point (the reference for h=0), so the mechanical energy becomes all kinetic energy (K). We know

Equating to the mechanical energy of the system (M)

Solving for v


The magnitude of the electrostatic force between two charges is given by:

where
ke is the Coulomb's constant
q1 and q2 are the two charges
r is the separation between the two charges
We can see that the magnitude of the force is directly proportional to the charges. This means that when one of the charges is doubled, the magnitude of the electrostatic force will double as well, so the correct answer is
A) <span>The magnitude of the electrostatic force doubles</span>
The frequency of the pendulum is independent of the mass on the end. (c)
This means that it doesn't matter if you hang a piece of spaghetti or a school bus from the bottom end. If there is no air resistance, and no friction at the top end, and the string has no mass, then the time it takes the pendulum to swing from one side to the other <u><em>only</em></u> depends on the <u><em>length</em></u> of the string.