A woman is moving to a new house and will spend much of her morning lifting boxes

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Question:

A woman is moving to a new house and will spend much of her morning lifting boxes into a truck. She's always heard that she shouldn't "lift with her back" but wants to examine some of the physics at play. While on a break, she decides to compare the force at play in two opposing lifting techniques.

For the "Proper Technique," she keeps her back essentially straight and vertical and uses her legs to lift the box from the ground. For t "Improper Technique," she keeps her legs straight, rotates downward (i.e., her hip is a pivot point) until reaching the box, and then rota back up (as seen in the following GeoGebra applet).

You may alter the "Rotation=" slider to view different moments in the lifting motion. You may also click the "View Back Muscle Arc"

button to zoom in around the motion of the back muscle. Given are the magnitudes of the displayed vectors and the tail-to-tail angle measures curling from the radial vectors to the weight vectors. The forces F1 and F2 are the weight forces of her torso and the carrie box, respectively. The force FB is the force exerted by her back to enact this motion. The r-vectors are the radius vectors between th forces and the pivot point at her hip. The related angle measure moving from rB to FB is always 2. Notice that the magnitude for changes throughout the rotation.