When we're working problems that involve a pendulum, we almost always IGNORE anything else besides gravity.
Anything else besides gravity that could influence the motion of a pendulum is something that we would eliminate if we could.
Those are air resistance, mass of the string, friction at the top suspension point, and the Coriolis force, due to the Earth's rotation, that tries to change the plane of the pendulum's swing.
This question is probably fishing for the answer "friction".
