A physic question

It is directly proportion

The length of elastic string is directly proportional with the number of objects provided that the acceleration of the objects remain unchanged.

This is the experiment about the Newton's second law.
The length of string actually is representing the force(F) used as the length increases when the force increased.
The number of Objects is representing the mass(m) of the object.

F directly proportional to m
F ~ m (proportional relationship)
F = ma (equation)