In physics, the coefficient of restitution (COR, also denoted by e), can be thought of as a measure of the elasticity of a collision between two bodies. It is a dimensionless parameter defined as the ratio of the relative velocity of separation after a two-body collision to the relative velocity of approach before collision. In most real-word collisions, the value of e lies somewhere between 0 and 1, where 1 represents a perfectly elastic collision (in which the objects rebound with no loss of speed but in the opposite directions) and 0 a perfectly inelastic collision (in which the objects do not rebound at all, and end up touching). The basic equation, sometimes known as Newton's restitution equation was developed by Sir Isaac Newton in 1687.[1]
The COR is a property of a pair of objects in a collision, not a single object. If a given object collides with two different objects, each collision has its own COR. When a single object is described as having a given coefficient of restitution, as if it were an intrinsic property without reference to a second object, some assumptions have been made – for example that the collision is with another identical object, or with perfectly rigid wall.
In a basic analysis of collisions, e is generally treated as a dimensionless constant, independent of the mass and relative velocities of the two objects, with the collision being treated as effectively instantaneous. An example often used for teaching is the collision of two idealised billiard balls. Real world interactions may be more complicated, for example where the internal structure of the objects needs to be taken into account, or where there are more complex effects happening during the time between initial contact and final separation.
e is usually a positive, real number between 0 and 1:
Values outside that range are in principle possible, though in practice they would not normally be analysed with a basic analysis that takes e to be a constant:
In the case of a one-dimensional collision involving two idealised objects, A and B, the coefficient of restitution is given by: where:
va
vb
ua
ub
This is sometimes known as the restitution equation. For a perfectly elastic collision, e = 1 and the objects rebound with the same relative speed with which they approached. For a perfectly inelastic collision e = 0 and the objects do not rebound at all.
For an object bouncing off a stationary target, e is defined as the ratio of the object's rebound speed after the impact to that prior to impact: where
u
v
In a case where frictional forces can be neglected and the object is dropped from rest onto a horizontal surface, this is equivalent to: where
H
h
The coefficient of restitution can be thought of as a measure of the extent to which energy is conserved when an object bounces off a surface. In the case of an object bouncing off a stationary target, the change in gravitational potential energy, Ep, during the course of the impact is essentially zero; thus, e is a comparison between the kinetic energy, Ek, of the object immediately before impact with that immediately after impact:In a cases where frictional forces can be neglected (nearly every student laboratory on this subject[2]), and the object is dropped from rest onto a horizontal surface, the above is equivalent to a comparison between the Ep of the object at the drop height with that at the bounce height. In this case, the change in Ek is zero (the object is essentially at rest during the course of the impact and is also at rest at the apex of the bounce); thus:
Although e does not vary with the masses of the colliding objects, their final velocities are mass-dependent due to conservation of momentum:andwhere
va
vb
ua
ub
ma
mb
In practical situations, the coefficient of restitution between two bodies may have to be determined experimentally, for example using the Leeb rebound hardness test. This uses a tip of tungsten carbide, one of the hardest substances available, dropped onto test samples from a specific height.
A comprehensive study of coefficients of restitution in dependence on material properties (elastic moduli, rheology), direction of impact, coefficient of friction and adhesive properties of impacting bodies can be found in Willert (2020).[3]
Thin-faced golf club drivers utilize a "trampoline effect" that creates drives of a greater distance as a result of the flexing and subsequent release of stored energy which imparts greater impulse to the ball. The USGA (America's governing golfing body) tests[4] drivers for COR and has placed the upper limit at 0.83. COR is a function of rates of clubhead speeds and diminish as clubhead speed increase.[5] In the report COR ranges from 0.845 for 90 mph to as low as 0.797 at 130 mph. The above-mentioned "trampoline effect" shows this since it reduces the rate of stress of the collision by increasing the time of the collision. According to one article (addressing COR in tennis racquets), "[f]or the Benchmark Conditions, the coefficient of restitution used is 0.85 for all racquets, eliminating the variables of string tension and frame stiffness which could add or subtract from the coefficient of restitution."[6]
The International Table Tennis Federation specifies that the ball shall bounce up 24–26 cm when dropped from a height of 30.5 cm on to a standard steel block,[7] implying a COR of 0.887 to 0.923.
The International Basketball Federation (FIBA) rules require that the ball rebound to a height of between 1035 and 1085 mm when dropped from a height of 1800 mm,[8] implying a COR between 0.758 and 0.776.
Works cited