1. Statics November 4, 2005

2. 60 1m 1kg 5 0 0 A 1kg ball is hung at the end of a rod 1m long. If the system balances at a point on the rod 0.25m from the end holding the mass, what is the mass of the rod? • ¼ kg • ½ kg • 1 kg • 2 kg • 4 kg

3. 90 5 0 0 A (static) mobile hangs as shown below. The rods are massless and have lengths as indicated. The mass of the ball at the bottom right is 1kg. What is the total mass of the mobile ? • 5 kg • 6 kg • 7 kg • 8 kg • 9 kg ? 1m 2m ? 1kg 1m 3m

4. Answers • 1 kg. The rod is uniform, with a center-of-mass at 50 cm or halfway down the rod or 0.25 m from the pivot point. The net TORQUE must be zero, so the mass at 0.25 m must be equal at the same distance from the pivot. • 6 kg.Use torques in two steps: (1) find the big mass on the bottom left (lower rod only) (2) use the entire lower rod assembly (with two masses) to find the mass on top right Finally add up all the masses.