State two conditions necessary for a mass to be undergoing simple harmonic motion.
A trolley of mass 
is held on a horizontal surface by means of two springs attached to fixed points P and Q, as shown in the figure below.
The springs, each having a spring constant 
of 
, are always extended. The trolley is displaced along the line of the springs and then released. The variation with time 
of the displacement 
of the trolley is shown in the graph below.
The acceleration 
of the trolley of mass 
may be assumed to be given by the expression
Calculate the angular frequency 
of the oscillations of the trolley.
Determine the time 
shown on the graph above.
.
