Answer:
gravity
Step-by-step explanation:
ok what i understand from your query is that a spring balance with a metal piece attached to it is weighing it down, a spring balance is measured in newtons (f=ma), the acceleration part is gravity (-9.81 m/s^2). The metal piece is attached to the string.
hope this answers your question and please take my answer with a grain of salt and refer to some webpages or videos