The amplitude of oscillation of a simple pendulum bob diminishes slowly because

Analyze the question: It asks why the amplitude of a real pendulum gradually gets smaller (damping).

Concept: An ideal simple pendulum would swing forever with constant amplitude because energy is conserved, constantly converting between kinetic and potential energy. In real life, the bob moves through air and the pivot has some friction. These dissipative forces (air resistance and friction at the support) do negative work on the bob, converting mechanical energy into heat. Since the total energy of an oscillator is proportional to the (amplitude)², losing energy means the amplitude decreases. This is called damped oscillation.

Checking each option:

• A. Energy applied by external agencies: This would increase or sustain the amplitude (as in forced oscillation), not reduce it. Wrong.
• B. Frictional forces dissipate the energy of motion: Matches the physics of damping. Correct.
• C. Frequency increases with mass of the bob: The frequency of a simple pendulum is f = (1/2π)√(g/L), which is independent of mass. This statement is false, and it has nothing to do with amplitude loss.
• D. Frequency varies with time: For small amplitudes, frequency stays essentially constant. Wrong.
• E. Period varies with time: Period T = 2π√(L/g) stays essentially constant too. Wrong.

Answer: B.

Exam tip: Whenever you see "amplitude decreases gradually" in oscillation questions, think damping = energy loss to friction or air resistance. Also remember that the pendulum's period depends only on L and g, never on mass or amplitude (for small angles). This eliminates many wrong options quickly.