Problem 2B-02: A seesaw problem

Mary weights 30 kg. One day she was sitting on one end of a seesaw. A big boy pushed down the other end of the seesaw and placed a weight slightly heavier than 30 kg to hold Mary high in the sky as shown in Fig. 1. The boy then ran away. How would you advice Mary to get down to the ground without jumping down from the seesaw outright?






Scroll down for solution













Solution:

Mary's body has a center of mass point. According to Eq. (6) of Topic 2B-02, we can consider a particle of mass 30 kg is located at the center of mass point and is generating a torque to rotate the seesaw clockwise around point O as shown in Fig. 1. On the other hand the weight is also generating a torque to rotate the seesaw counter clockwise around point O. Since the torque generated by the weight is slightly larger than the torque generated by Mary's body, the seesaw does not move and Mary cannot get down. If Mary bend her body backwards as shown in Fig. 2, the center of mass point of Mary's body shifts farther away from point O. Thus the torque generated by Mary's body will increase and will overcome the torque generated by the weight. The end of the seesaw that Mary sits will come down to the ground.