Q15. To be in stable equilibrium your center of mass must be over the middle of your feet. Carrying a heavy load in your arms moves the center of mass of the person-load system forward which makes you feel unstable. You correct this feeling by leaning backwards to move the center of mass back over the middle of your feet.
Q17. Your center of mass (CM) is near your navel when you are lying flat. When you sit on the floor, your CM is somewhere between the CM of your upper body (somewhere in your chest) and the CM of your lower body (near your knees). This point is not on your body but in the space in front of your body.
Q2. The bungee jumper is at rest at his lowest point but is not in equilibrium. The bungee cord is stretched and exerts a force on the jumper. This force is larger than the weight which is why the jumper is pulled back upward and there is a net upward force on the body. The jumper therefore has an acceleration at the zero velocity lowest point and is not in equilibrium.
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