From: | Adam Getchell <acgetche@****.UCDAVIS.EDU> |
---|---|

Subject: | Re: Knock Back (Warning: Physics) |

Date: | Tue, 11 Apr 1995 22:44:54 -0700 |

> The target doesn't get knocked back because it's only paper :-) If something

> solid and mobile were hit (like a person) they would get a very solid blow

> concentrated in a very small area and acting in a very short time: a very

> high impulse offering a good chance of knocking them back or down. By no

> means guaranteed, though.

No.

As far as knock down is concerned, momentum transfer is the key.

Momentum is mass * velocity. All momentum equations are variations of

m1*v1+m2*v2=mfinal*vfinal (this case is for a perfectly inelastic

collision, where mfinal = m1+m2, such as bullet m1 sticking in person m2.

Yes, there are energy loss terms and so forth but the above is essentially

correct).

What you are using is a modified form of Newton's Third Law. It

is talking about Force in terms of the change in momentum:

F = dp/dt, where p = momentum.

This is really a partial differential equation but only in ususual

cases does mass change, hence dp/dt is approximated by m*dv/dt. In any

case, the modified formula is delta p = F * delta t, which is an

approximation (an inexact differential, technically). The product F *

delta t is usually called the "impulse". Yes, you will produce a large

change in momentum if you have a high force (not pressure, hence the area

of impact is irrelevant) and a *long* amount of time.

But your momentum is still constrained by mass * velocity.

Bullets have low changes of momentums, high Forces and short times,

obeying the version of Newton's law above. Incidentally, that's how they

get their high pressures: they have very small delta t's when they

transfer energy, and small surface areas. This is why kinetic energy

penetrators are dart-shaped (pressure being force divided by area).

By the way, the Third Law of Thermodynamics has to do with all

processes being irreversible ones, hence change in Entropy is always

greater than zero, and never negative (by the 2nd law or Clausius'

Principle).

> Paul J. Adam paul@********.demon.co.uk

========================================================================

Adam Getchell "Invincibility is in oneself,

acgetche@****.engr.ucdavis.edu vulnerability in the opponent."

http://instruction.ucdavis.edu/html/getchell.html