When picking up an object from the ground, it is generally recommended that you "lift with your legs" - that is, raise and lower yourself by bending your knees but keeping your upper body upright. If instead, you "lift with your back" by bending at the hip so that your upper body is angled, then you put a great deal more stress on your spine and back muscles. Consider a person who is "lifting with their back" such that they are bent at the hip with their upper body is parallel to the ground (i.e., their spine is oriented horizontally). Let us calculate the tension in the back muscles and the compression on the spine in this situation. We will model the spine and upper body as a horizontal rigid rod or uniform density with a length of 50.0 cm and a mass of 40.0 kg. Assume that the person attempts to lift an object with their arms, which we will model as attached at the far end of the rod. Support of the back in this position is provided primarily by the erector spinalis muscle which we will model as being attached at one end to the spine at a point 33.0 cm from the hip at an angle of 10 degrees; the other end of the muscle is attached to the lower body below the hip.

A 2×12 board of airdried Southern Yellow Pine (~12% moisture) is about 2.18 kg per linear foot. A physicist and his wife carry a 16-foot length of Southern Yellow Pine lumber, the physicist gallantly carries the board from one end while his wife carries the board 4 feet from the other end. How much weight is each spouse carrying? This is an equilibrium problem. Assume the board is perfectly uniform.

A meter stick is supported by a knife-edge at the 50-cm mark. You hang masses of 0.40 kg and 0.60 kg from the 20-cm and 80-cm marks, respectively. Where should you hang a third mass of 0.30 kg to keep the stick balanced?

You must lean quite far forward as you rise from a chair (try it!). Explain why.