A2L Item 185
- Description: Identify the torque about one end of a uniform rod suspended by two strings of unequal length.
- Goal: Reasoning and hone the concept of torque.
- Source: UMPERG-ctqpe152
- Keywords: Mechanics, Tension, Torque
The question for students:
A
uniform rod of length 4L, mass M, is suspended by two thin strings,
lengths L and 2L as shown. What is net torque about the left end of the
rod?
- 0
- 4MgL
- 8MgL
- 12MgL
- None of the above
Commentary for teachers:
Answer
(1) Since the rod does not rotate the total torque must be zero about any point. Many students overworry this problem not realizing that, independent of the angle of the rod, the other string is twice as far as the center of mass of the rod.