Impact force

An impact force is a high force or shock applied over a short time period. Such a force or acceleration can sometimes have a greater effect than a lower force applied over a proportionally longer time period.

Theory

At normal speeds, during a perfectly inelastic collision, an object struck by a projectile will deform, and this deformation will absorb most, or even all, of the force of the collision. Viewed from the conservation of energy perspective, the kinetic energy of the projectile is changed into heat and sound energy, as a result of the deformations and vibrations induced in the struck object. However, these deformations and vibrations can not occur instantaneously. A high velocity collision (an impact) does not provide sufficient time for these deformations and vibrations to occur. Thus, the struck material behaves as if it were more brittle than it is, and the majority of the applied force goes into fracturing the material. Or, another way to look at it is that materials actually are more brittle on short time scales than on long time scales.

Applications

* A nail is normally pounded with a series of impacts, each being a single hammer blow. These high velocity impacts prevent friction with the wood on the sides of the nail from retarding the forward motion of the nail.

* A pile driver does the same thing, on a much larger scale.

* An impact wrench is an analogous device designed to impart torque impacts to bolts to tighten or loosen them. At normal speeds, the forces applied to the bolt would be dispersed, via friction, to the mating threads. However, at impact speeds, the forces act on the bolt to move it before they can be dispersed.

* In ballistics, bullets utilize impact forces to puncture surfaces that could otherwise resist substantial forces. A rubber sheet, for example, behaves more like wood at typical bullet speeds. That is, it ruptures, and does not stretch or vibrate.

See also

* Impulse
* Charpy impact test
* Cushioning
* Izod impact strength test
* Shock