Goal: Reasoning with forces
Source: UMPERG-ctqpe154
A uniform rod of length 4L, mass M, is suspended by two thin strings,
lengths L and 2L as shown. What is the tension in the string at the
left end of the rod?
Goal: Problem solving
Source: UMPERG-ctqpe162
A uniform disk with R=0.2m rolls without slipping on a horizontal
surface. The string is pulled in the horizontal direction with force
15N. The disk’s moment of inertia is 0.4 kg-m2. The friction
force on the disk is:
(4) This problem can be done without the arithmetic complication of
finding the mass from the center-of-mass moment of inertia. This is an
excellent problem for stressing multiple solution methods. This is a
situation where two equations are needed. They can be either the linear
dynamical relation and a rotational dynamical relation, or just two
rotational relationships about different points. Some students may
answer (7) because they are unfamiliar with the expression for moment of
inertia about the CM or because they do not know the Parallel Axis
theorem.
Commentary:
Answer
(2) For many this is straightforward but a few students are confused
about the effect of the unequal lengths of string.