Scissors, Shears

Scissors are a tool used for cutting thin material which requires little force. They are used for cutting, for example, paper, cardboard, metal foil, thin plastic, food, cloth, rope and wire. They are also used for cutting hair and nails. more...

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Unlike a knife, scissors have two pivoted (or hinged ) blades. Most types of scissors are not particularly sharp; it is primarily the shearing between the two blades which cuts. Children's scissors are even less sharp, and are often protected with plastic.

Although often used interchangeably with "scissors," the term shears is reserved by those in the industry for scissors longer than 15 cm.

Mechanics

Mechanically, scissors are a first-class, double-lever with the pivot/hinge acting as the fulcrum. For cutting thick or heavy material, the mechanical advantage of a lever can be exploited by placing the material to be cut as close to the fulcrum as possible. For example, if the applied force (i.e., the hand) is twice as far away from the fulcrum as the cutting location (e.g., piece of paper), the force at the cutting location is twice that of the applied force at the handles. Specialized scissors, like bolt cutters exploit leverage by having a long handle but placing the material to be cut close to the fulcrum. Scissors cut material by applying a local shear stress at the cutting location which exceeds the material's shear strength.

Kitchen scissors

Kitchen scissors, also known as kitchen shears, are similar to common scissors. The main difference is the location of the fulcrum. Kitchen scissors have the fulcrum located farther from the handles to provide more leverage and thus more cutting power. High quality kitchen scissors can easily cut through the breastbone of a chicken.

Left handed scissors

Most scissors are best suited to use with the right hand, but left handed scissors are designed for use by the left. Left handed scissors obviously have handles which are comfortable to hold in the left hand, but they have other differences, more subtle yet equally important.

Because scissors have overlapping blades, they are not symmetric. This asymmetry is true regardless of the orientation and the shape of the handles: the blade that is on top always forms the same diagonal regardless of orientation. Human hands are also asymmetric and worse, when closing the thumb and fingers do not close vertically, but have a lateral component to the motion. Specifically, the thumb pushes out and fingers pull inwards. For right-handed scissors held in the right hand, the thumb blade is closer to the body so that the natural tendency of the right hand is to force the cutting blades together. Conversely, if right-handed scissors are held in the left hand, the natural tendency of the left hand would be to force the cutting blades laterally apart.

Read more at Wikipedia.org