4) a) Two children are sat on a see-saw. Shelia is 35kg and is sat at the end, 2.0 m from the pivot. Michael is 45kg and sits on the opposite side of the pivot. Calculate at what distance (x) must he sit to balance the see-saw.

(11) A tapered log 3.0 m long weighs 210 N and has its center of gravity 1.0 m from the thick end. It is being used as a bridge across a ditch and is supported at its ends by the edges of the ditch. A 600 N man stands on the log 1.0 m from the thin end. How much support is required at each end of the log? 3m FN=mant eug+ 1.om

(I) A tower crane (Fig. 9–48a) must always be carefully balanced so that there is no net torque tending to tip it. A particular crane at a building site is about to lift a 2800-kg air-conditioning unit. The crane's dimensions are shown in Fig. 9-48b. (a) Where must the crane's 9500-kg counterweight be placed when the load is lifted from the ground? (The counterweight is usually moved auto- matically via sensors and motors to precisely compensate for the load.) (b) Determine the maximum load that can be lifted with this counterweight when it is placed at its full extent. Ignore the mass of the beam. (a) Counterweight M = 9500 kg +3.4 m- 7.7 m m = 2800 kg FIGURE 9-48 (b) Problem 3.