The coefficient of restitution (COR) measures elasticity in collisions and is determined through experimentation. It is important in sports equipment, with guidelines for balls, bats, and rackets. A high COR can give an unfair advantage and be dangerous, with regulations in place to prevent injury.
The coefficient of restitution (COR) measures the elasticity of a pair of colliding objects and is the ratio of their final relative velocity to their initial relative velocity. Determined through experimentation, this coefficient essentially measures elasticity. If the coefficient is close to one, the collision is elastic and bouncy, if it is close to zero, the collision is more plastic. Most professional sports have specific guidelines for the coefficient of return of equipment, such as balls, bats and rackets.
Otherwise known as Newton’s Law of Impact, the coefficient of restitution is determined through experimentation. The COR formula for colliding objects is defined as e = (v2-v1)/(u1-u2), where u1, u2 are the pre-collision velocities and v1, v2 are the post-collision velocities of each object respectively . To determine the COR of an object dropped onto a stationary object such as the floor or table the formula is e = √(height of bounce / height of fall). The speeds of objects moving horizontally or obliquely can be difficult to measure.
COR can also be thought of as the proportion of kinetic energy retained by two objects before and after the collision. If the coefficient is close to one, the shock is very elastic, like a ping pong ball dropped on a stainless steel table that has a COR of .92. Most of the kinetic energy present before the collision remains as kinetic energy after the collision. If the coefficient is close to zero, the collision is more plastic like a lump of clay dropped on a wooden table; the clay sticks without bouncing at all. The kinetic energy was converted into sound and other forms of energy, deforming the clay.
The springiness of balls is a very important concept in sports, as materials with more springs could give an athlete an edge. Since 1998, the United States Golfing Association had limited the coefficient of return of golf clubs with standard golf balls to 83. The COR of a National Basketball Association synthetic ball and hard linoleum floor is 79; however, temperature, air pressure, and other factors can affect this number.
In addition to giving players an unfair advantage, sports equipment that has unusually high coefficients of restitution can be dangerous. For example, composite bats are more bouncy than wooden bats when hit by a baseball. Balls hit with composite bats can have unexpectedly high speeds and cause injury to players. In May 2009, the National Collegiate Athletic Association adopted the bat-ball return coefficient (BBCOR) as a means to better test baseball bat performance. All bats must have a BBCOR of less than 0.500.
Protect your devices with Threat Protection by NordVPN
