A trolley of mass 
is held on a horizontal surface by means of two springs. One spring is attached to a fixed point 
. The other spring is connected to an oscillator, as shown in the figure.
The springs, each having spring constant 
of 
, are always extended. The oscillator is switched off. The trolley is displaced along the line of the springs and then released. The resulting oscillations of the trolley are simple harmonic. The acceleration 
of the trolley is given by the expression
where 
is the displacement of the trolley from its equilibrium position.
The mass of the trolley is 
.
At any given frequency, the amplitude of oscillation of the trolley is constant. Explain how this indicates that there are resistive forces opposing the motion of the trolley.
