Kiwiiano wrote:First thought re aircraft takeoff: passengers experience 1G vertically and XG horizontally. Re the acceleration with the Waiau seismograph, the reporter may be getting confused by the logarithmic scale of earthquake energies??
I feel we are getting into that 3% where you aren't right Kiwiano! I don't think logarithmic scales have anything to do with this particular issue.
The article is pretty consistently in using acceleration
for starters, except for the paragraph Bruce already highlighted where it says:
"Readings taken at the North Canterbury town of Waiau during the November event proved a new record for vertical ground acceleration, reaching 3g, or 30 times the force
an airliner passenger feels at take-off."
If it had said "vertical acceleration" instead of "force" then it would have been okay.
Have a look at the diagrams from this geonet article: http://info.geonet.org.nz/display/quake ... Earthquake
Notice that many of the vertical accelerations are shown as less than 1g. This is because they are using 1g to represent the acceleration due to gravity at the earth's surface if you were also in freefall - in other words 1g is talking about an acceleration of just under 10 m/s/s.
Gravity is exerting a downwards force on your body as you accelerate down the runway, and that force is about 9.8 Newtons per kg of mass. We feel and generally refer to this as "weight", as there is usually also the ground or similar creating an equal and opposite force. In this case you have essentially zero vertical acceleration. But when the aircraft rotates and its velocity vector changes direction from parallel to the ground to some reasonable angle up, up and away, then you also accelerate (even if your speed might stay the same). This brief period of vertical acceleration will make you feel a little heavier, and is roughly what I calculated earlier. Presumably this small vertical acceleration is what the reporter was also trying to compare with the approximate 3g vertical acceleration measured at some points during the quake.
So, saying the earthquake's vertical acceleration
was about 30 times the vertical acceleration experienced by passengers leaving the runway during take-off in an airliner would have been acceptable in my book, although perhaps also not a very "useful" way of comparing things. The feeling of weight for a person during take-off (more precisely as the aircraft rotates nose up and starts to climb away from the ground) will briefly be a little higher than when trundling along the runway, but it's not a huge change, or for very long.
The upward force from the floor and the seat on their body will be around 1.1 or 1.2 times what it was just before take off, with the extra coming from the effect of the vertical acceleration. Gravity is still pulling them towards the centre of the earth but now we also have the aircraft accelerating upwards over a few seconds. Your felt weight will increase a little as a result, but this is still only a little more than a third of what their apparent weight would have been if they were at the spot during the earthquake where the 3g vertical acceleration is supposed to have happened.
In other words, for most people a more useful and accurate statement would have been that your body would have felt about three times it's usual weight if you were standing at the spot where the ground underwent maximum vertical acceleration during the earthquake.
You'd also experience that "3g" feeling of tripled weight if you were in an aircraft executing a tight level orbit with an a bank angle of about 65 to 70 degrees.