Tightrope walker in the context of Slacklining

A circus is a company of performers who put on diverse entertainment shows that may include clowns, acrobats, trained animals, trapeze acts, musicians, dancers, hoopers, tightrope walkers, jugglers, magicians, ventriloquists, and unicyclists as well as other object manipulation and stunt-oriented artists. The term "circus" also describes the field of performance, training, and community which has followed various formats through its 250-year modern history. Although not the inventor of the medium, Newcastle-under-Lyme born Philip Astley is credited as the father of the modern circus.

In 1768, Astley, a skilled equestrian, began performing exhibitions of trick horse riding in an open field called Ha'penny Hatch on the south side of the Thames River, England. In 1770, he hired acrobats, tightrope walkers, jugglers, and a clown to fill in the pauses between the equestrian demonstrations and thus chanced on the format which was later named a "circus". Performances developed significantly over the next 50 years, with large-scale theatrical battle reenactments becoming a significant feature. The format in which a ringmaster introduces a variety of choreographed acts set to music, often termed "traditional" or "classical" circus, developed in the latter part of the nineteenth century and remained the dominant format until the 1970s.

The moment of inertia, otherwise known as the mass moment of inertia, angular/rotational mass, second moment of mass, or most accurately, rotational inertia, of a rigid body is defined relatively to a rotational axis. It is the ratio between the torque applied and the resulting angular acceleration about that axis. It plays the same role in rotational motion as mass does in linear motion. A body's moment of inertia about a particular axis depends both on the mass and its distribution relative to the axis, increasing with mass and distance from the axis.

It is an extensive (additive) property: for a point mass the moment of inertia is simply the mass times the square of the perpendicular distance to the axis of rotation. The moment of inertia of a rigid composite system is the sum of the moments of inertia of its component subsystems (all taken about the same axis). Its simplest definition is the second moment of mass with respect to distance from an axis.