This phenomenon is called Nuclear Magnetic Resonance.
When I met my future wife, she was working in the medical research building next door to the communications building where I worked. (We shared a parking lot.) MRI was not a thing yet, and she was doing research in Nuclear Magnetic Resonance. I learned a lot about it when I walked next door to visit her in her lab. Strange as it may seem, several years earlier, her older brother was involved in the invention of the CAT scan. When we got married, I figured that our kids had at least a 50% chance of inheriting some brains. So we had some, and they've done OK.
Answer:
a) see attached, a = g sin θ
b)
c) v = √(2gL (1-cos θ))
Explanation:
In the attached we can see the forces on the sphere, which are the attention of the bar that is perpendicular to the movement and the weight of the sphere that is vertical at all times. To solve this problem, a reference system is created with one axis parallel to the bar and the other perpendicular to the rod, the weight of decomposing in this reference system and the linear acceleration is given by
Wₓ = m a
W sin θ = m a
a = g sin θ
b) The diagram is the same, the only thing that changes is the angle that is less
θ' = 9/2 θ
c) At this point the weight and the force of the bar are in the same line of action, so that at linear acceleration it is zero, even when the pendulum has velocity v, so it follows its path.
The easiest way to find linear speed is to use conservation of energy
Highest point
Em₀ = mg h = mg L (1-cos tea)
Lowest point
Emf = K = ½ m v²
Em₀ = Emf
g L (1-cos θ) = v² / 2
v = √(2gL (1-cos θ))