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. (1/3)Mg
• B. (1/3)RM
• C. (1/5)gt
• D. (1/9)Rg

Answer: (A)