A Feat of Physics Only a Woman Can Win

Good evening, and welcome to tonight’s match! In the blue corner, weighing in at 170 pounds and 5’ 4” tall is Agatha.  In the red corner, weighing in at 205 pounds and 5’ 10” tall is Bruce. It’s going to be a nail-biter folks, as we watch which of these contestants can lift a small chair!

Ok, ok… it lacks the excitement of a boxing match, but it’s definitely more interesting.  The Chair Challenge is a simple experiment you can perform almost anywhere to demonstrate the less obvious physical differences between men and women.  Set up your lab space as follows:

1. Place a small chair next to a wall.
2. Bend down over the chair at a 90 degree angle, with your head touching the wall.
3. Grasp the chair with both hands.
4. Attempt to lift the chair as you return to a standing position.

When a woman performs this Herculean feat, they reach a standing position without trouble.  A man, on the other hand, cannot stand up without falling forward in the process. (This experiment becomes much more fun if you put your head against a swinging door instead of a wall.  Watch men fall right through it!)

What’s happening?

In order to explain this weirdness, we need to learn about an object’s “Center of Mass”.  If we measure the position of each piece of material making up an object, the average of all those positions would be the location of the object’s Center of Mass.  To make this an easier thought exercise, let’s consider the Center of Mass of a construction crane in only one dimension. (Figure 1) The Center of Mass of the crane is just off of the crane’s center point.

When the crane lifts a moderately heavy box, what do you expect to happen?  The Center of Mass shifts further to one side, but not past the wheels and support struts of the crane. (Figure 2)

When the crane lifts a box that’s too heavy, what happens then?  The Center of Mass shifts so far it goes beyond the wheels and support struts of the crane.  That means the crane begins to tip! (Figure 3)

How does this relate to people?

A person’s Center of Mass is just below their belly button, however for women it’s a bit lower still.  When a person bends over the chair with their head touching the wall, their Center of Mass rests directly above their feet. (Figure 4)

When women lift the chair, their Centers of Mass shift as we expect.  However, since their Centers of Mass started closer to their legs to begin with, the weight of the chair isn’t enough to pull their Centers of Mass beyond their feet. (Figure 5)  They can stand up without a problem.

When men lift the chair, their Centers of Mass shift just past their toes.  Like the construction crane discussed earlier, they would tip over without the support of the wall! (Figure 6)

You might be thinking to yourself, “Sure HP, but how does that relate to the real world?”  For me it’s most notable when me or my partner perform household repairs. For instance, if I’m applying sealant where the shower basin meets the wall, I need one hand to support my weight as I bend into the shower.  However, my partner can bend into the shower and use both of her hands to apply the sealant. Ultimately, this experiment shows us that small differences may still result in big consequences. Get a chair and try it yourself!

Another way Center of Mass relates to the real world is in the shape of some birds’ eggs.  If I asked you to think of an egg, you’d visualize the standard oval shape seen in all common eggs.  However, eggs laid by cliff-dwelling birds have the shape of an oval with one end squeezed into a rounded point.  These are sometimes known as pointed eggs, and result in the Center of Mass of the egg to be closer to the fat end.  This shape accomplishes two things – 1) it causes the egg to settle easier and resist tipping, and 2) should the egg tip onto its side and roll, it rolls in a circular motion and stays in one place, rather than rolling in a straight line off the cliff.