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Let's begin by determining the equilibrium position of a seesaw pivot. You and a friend play on a seesaw. Your mass is 90 kg, and your friend's mass is 60 kg. The seesaw board is 3.0 m long and has negligible mass. Where should the pivot be placed so that the seesaw will balance when you sit on the left end and your lriend sits on the right end? Figure Clo kg 3.0 m 60kg 1012 > I Review l Constants SOLUTION SET UP First we sketch the physical situation which is shown in (gure 1), and then we make a tree-body diagram as shown in (59%), placing the pivot at a distance a: from the left end. Note that each weight is the corresponding mass multiplied by g, so g will divide out of the linal result Remember that counterclockwise torques are positive and clockwise torques are negative. For the seesaw to be in equilibrium, the sum of the torques about any axis must be zero. We choose to take torques about the pivot axis; then the normal force acting at the pivot doesn't enter into the analysis. The moment arms are :1: tor you and (3.0 m 7 z) for your friend. SOLVE Your torque is Tye\" : +(90 kg)ga:. Your friend is 3.0 m 7 a: tram the pivot, and the corresponding torque is Tmend : 7(60 kg) >