Final answer:
In damped oscillations, energy decreases due to work done against frictional force. This non-conservative force converts the system's mechanical energy into other forms such as heat, reducing the amplitude of the oscillations over time. The correct option is C. frictional force.
Step-by-step explanation:
The energy of a particle executing damped oscillations decreases because work is done against the frictional force. Damped oscillations involve a non-conservative damping force which acts to remove mechanical energy from the system, such as elastic potential energy and kinetic energy, and convert it to other forms like thermal energy. As the system oscillates, the energy is not conserved specifically because of the friction or any other dissipative force that does work on the system, against the motion.
In the context of damped oscillations, the damping force is typically a type of friction which can come in the form of air resistance, internal friction within the material, or any other resistive force that consumes the oscillatory energy. Subsequently, the amplitude of such oscillations decreases over time, leading to the eventuality where the oscillating system comes to a stop. This is an example of the energy being dissipated due to the work done by the frictional force in the system.
For a damped harmonic oscillator, the work done by non-conservative forces, such as the damping force, is described as Wnc = A(KE + PE), indicating that the total mechanical energy (kinetic plus potential) is reduced. Therefore, the correct option is C. frictional force, as it is the frictional force that removes energy from the system during damped oscillations.