A cylindrical disc with a mass M and a radius R has a delicate thread coiled around it. One end of the thread is secured while the disc descends vertically from a resting position, uncoiling the thread as it goes. What is thread tension as a function of time?

• A. 1/2 Mg
• B. 1/3 RM
• C. 1/5 gt
• D. 1/9 Rg

Answer: (A)