Problem 87P

Two metal disks, one with radius R1 = 2.50 cm and mass M1 = 0.80 kg and the other with radius R2 = 5.00 cm and mass M2 = 1.60 kg, are welded together and mounted on a frictionless axis through their common center (Fig. P9.77). (a) What is the total moment of inertia of the two disks? (b) A light string is wrapped around the edge of the smaller disk, and a 1.50-kg block is suspended from the free end of the string. If the block is released from rest at a distance of 2.00 m above the floor, what is its speed just before it strikes the floor? (c) Repeat part (b), this time with the string wrapped around the edge of the larger disk. In which case is the final speed of the block greater? Explain.

Solution 87P

Problem (a)

Step 1:

Radius of disk 1 R1 = 2.50 cm or 2.5 x 10-2 m

Radius of disk 2 R2 = 5.00 cm or 5.0 x 10-2 m

Mass of disk 1 M1 = 0.8 kg

Mass of disk 2 M2 = 1.6 kg