Grimer wrote: > At 12:26 pm 12/04/2006 -0500, Harry wrote: > >> If you are bicycling fast enough you can cross a >> wooden plank spanning ditch before the plank breaks. >> >> Normally we say this is because it takes time for the >> plank to deform and >break when subjected to a weight. >> >> However, consider for a moment an alternative and >> naive(?) explanation: it is because you weigh less >> when you are moving than when you are stationary. >> The idea is your motion reduces your gravitational >> mass independently of your inertial mass while >> gravitational acceleration remains unchanged. > > > > When I first read this post I was tempted to take > what one might call the Dr Pork approach and dismiss > it without thinking. > > That would have been a mistake because on reflection > I can see that such quosi modo (Introit - not Victor > Hugo) questions
I would say Don Quixote is asking the question. > force one to think about the > fundamentals of mechanics. > > We can start off by imagining the plank has a slight > slope down towards the other side of the ditch. > In this case the bike need not touch the plank at all > and is effectively weightless as it crosses the ditch. > You can take the plank away and the bike will arrive > safely on the other side. I really have in mind a level plank. The plank deflects less when the bike crosses it quickly than when the bike rests on it. Is it only because it takes time for the plank to deflect or could it be that bike also weighs less? http://web.ncf.ca/eo200/dynamics/testing_weight.pdf > Similarly, a cop pursuing a robber can defy the > "Law of Gravity" by flinging himself across the gap > between a high building and a slightly lower one > without dashing himself to pieces in the alley below. > > The problem with Gravity is that most laymen think > of it as a Force. And the problem with the layman's > notion of Force is that it is a static concept which > does not involve time. The gap between the building > is a kingdom in which King Force's rule is absolute. > Enter it, and SPLOSH - you are raspberry jam on the > pavement below. One instant you are at the top of > the building and the next you are mangled on the > ground. It's a discrete change - one might even > describe the attempt to cross the forbidden zone a > quantum leap from life to death. > > It is reminiscent of the way that an electron jumps > from one level to another. Where is the electron in > between? It isn't anywhere (allegedly). What is the > time interval between it leaving one level and > arriving at the other. There isn't any interval > (allegedly). But then quantum physics was always more > the playground of physicists who preferred maths to > engineering. <g> > > If you want to understand the way gravity works you > want to forget about Force altogether. You want to > erase the word from your vocabulary. You want to > think about motion - and motion involves time. > Quod non agit, non existit as Leibniz observed. > What kind of motion? The # raindrops keep falling > on my head # kind. although since gravity raindrops > are rather faster than the watery kind, perhaps a > hail of bullets being fired from the robber's > accomplices in a 'copter provides a more realistic > image. Don Quixotes vs the Windmill? ;-) > If the cop leaps across the gap fast enough he can > evade the bullets save for a flesh wound or two > but if he dallies too long he will finish up a > cheese grater. > > Now the motion which really interests us is not > the motion of the bullets but the motion of the > bike/cop as it/he traverses the gap. This is a > motion nested within a motion - a velocity of a > velocity - and this immediately raises a problem > because we are using the same words at two > different levels which causes confusion in any > hierarchical system. If you call both the > headmaster and his assistant, headmaster, then > you are not going to know when you are talking > to the decision maker. A workable hierarchical > system needs to have very distinctive names for > each level of command otherwise there is chaos. > One system where chaos is anathema is the Army. > In the army you have a plethora of distinctive > names and badges so that everyone knows exactly > who they are talking to and what authority the > person wields. Private, Corporal, Sergeant, > Lieutenant, Captain, Major, Colonel, Brigadier, > General - to name but the single barrelled ranks. > One knows exactly who one is dealing with in the > army. > > In case anyone thinks, "Ah! we have a name for > the second order velocity. We have acceleration." > > Now we haven't! Acceleration is the portmanteau > term which includes both velocities - just as > "squad" might include both > the private ("...a velocity") > and his corporal ("the velocity of..."). > > Likewise, jerk is a portmanteau word equivalent to company, say; jounce is a > portmanteau word equivalent to regiment - and so on. > > The trouble with portmanteaus it that they get increasingly complicated and > difficult to understand. This is because they fail to shift their datum which > is at the level of private in the case of the army and distance/time in the > case of dynamics. > > So, you can see that, in a sense, Harry is right. You do weigh less when you > are moving than when you are stationary. In effect you have been promoted from > Private to Lance-Corporal 8-) > > Cheers, > > Frank > Harry
