page_title: Weeble
section_title: Design principle
hierarchical_section_title: Weeble / Design principle
caption_attribution_description: English: Illustration of the physics principles by which a "Weeble" roly-poly toy operates. A=position of mechanical equilibrium; B=position of mechanical instability; F=gravitational force; y=vertical axis; m1=low-density mass; m2=high-density mass; C=centroid. Note that between positions A. and B. that C raises slightly and becomes off-center.
context_page_description: Weebles is a range of children's roly-poly toys originating in Hasbro's Playskool division on July 23, 1971. Tipping an egg-shaped Weeble causes a weight located at the bottom-center to be lifted off the ground. Once released, gravity brings the Weeble back into an upright position. Weebles have been designed with a variety of shapes, including some designed to look like people or animals.
  The catchphrase "Weebles wobble, but they don't fall down" was used in advertising during their rise in popularity in the 1970s and during successive relaunches in the early 2000s. The line was coined by advertising executive J. Mitchell Reed, who later went on to lead Tokyo's Grey Daiko advertising firm.
  The Weebles 1971–2011 Price Guide and Index Book lists and shows every Weeble model made over the peceeding 40 years. There are 116 Weebles in total including all egg shaped sizes and variations made during 1971–1983. In 2011 Hasbro started making a new line of larger egg-shaped Weebles and had produced 42 new Weebles as of July 2011.
  A wide range of accessories was available for the Weebles, including vehicles, buildings and furniture. Some sets had themes, such as the Weebles circus set.
context_section_description: Weeble is shaped like an egg—in order for the physics principles to work as intended, the shape must have a bottom which is a more or less smooth (unfaceted) hemisphere (to allow the Weeble to roll) and from the central vertical axis the shape must be nearly cylindrically symmetrical (that is, any plane cut through the vertical axis line must produce close to the same profile).  Next, the shape must be filled with two basic types of unmixed solids, and the volume of the lighter solid must be greater than that of the heavier solid.  Next, the overall shape must have constant positive curvature.  Next, the relationship between the heavy solid and the light solid must be such that any orientation of the object off of the vertical axis line must cause the object's centroid to raise and to become offset.  Lastly, the object must have only one position in which it can achieve stable mechanical equilibrium.
  Combining these characteristics produces a basic Weeble.  In theory, it is not possible to have a Weeble with a centroid that is too low to achieve a stable mechanical equilibrium.
caption: Diagram of the principles of operation of a Weeble. The grey material is of a higher density than the red material, causing the center of mass of the object (labelled C) to be very low. This means that when the Weeble is wobbled, as in position B, the center of mass will be on the other side of the pivot point (where the Weeble is in contact with the ground), and so gravity will cause it to return to an upright position.
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