Let's call 
the mass of the glider and 
the total mass of the seven washers hanging from the string.
The net force on the system is given by the weight of the hanging washers:
For Newton's second law, this net force is equal to the product between the total mass of the system (which is
) and the acceleration a:
So, if we equalize the two equations, we get
and from this we can find the acceleration:
The net force on the system is given by the weight of the hanging washers:
For Newton's second law, this net force is equal to the product between the total mass of the system (which is
So, if we equalize the two equations, we get
and from this we can find the acceleration: