Push-ups. To strengthen his arm and chest muscles, an 82-kg athlete who is 2.0 m tall is doing push-ups as shown in below Fig. His center of mass is 1.15 m from the bottom of his feet, and the centers of his palms are 30.0 cm from the top of his head. Find the force that the floor exerts on each of his feet and on each hand, assuming that both feet exert the same force and both palms do likewise. Begin with a free-body diagram of the athlete. Figure:

Solution 15E Step 1: From diagram AD = the length of the athlete B = the centre of mass point CD =the length from arms to head BC = the length from centre of mass to the arms. Step 2: The force exerted by the floor to the arms = 2R x Moment on the point C 2 R (x.7) - mg( 1.15 ) = 0 2 R ( 1.7) = mg( 1.15 ) x 2 R = mg( 1.15 ) x 1.7 mg( 1.15 ) Rx 1.7×2 Mass of the athlete, m =81kg Rx 81 ×9.8 ×1.15 =268.49 1.7×2 The force exerted by the earth on a single arm R = 268.49N x