Why Is an Empty Shampoo Bottle So Easy To Knock Over?

(Inside Science) -- Ever have an almost empty shampoo bottle sitting on the shelf in the shower and it seems to fall at the slightest bump? In a recent article from The Physics Teacher, physicists knocked over shampoo bottles and showcased basic physics concepts such as center of mass and force of impact in a series of simple experiments.

We show and explain how an object with a lower center of mass is typically harder to tip over. This is the case when there is still enough shampoo left to weigh the bottle down. However, when a bottle of shampoo is nearly empty, the mass of the shampoo cap shifts the overall center of mass of the bottle to a higher position, making it less stable.