Impact force

From Wikipedia, the free encyclopedia

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

1 Theory
2 Applications
3 Example
4 External links

[edit] 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't 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.

[edit] 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 greater 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.

[edit] Example

Since

$f=ma\;$

for a mass m accelerating at a, then assuming an ideal system, we can set the impact force as,

$f=m\frac{dv}{dt}$

for a time interval dt.

For example, a train that weighs 1 kg moving at 500 m/s and that hits a 'perfect' steel wall where it uniformly decelerates from 500 m/s to 0 m/s in .02 seconds, has an approximate impact force of 25000 N. Thus, a body which decelerates more quickly has a greater effective impact than one which decelerates more slowly.