[Catalist] Gravity

[Chris Creagh]

Hi Warren

I am happy for you to take the floor with the relativity stuff but I would like to comment on the bus going around the corner.

If the bus is turning left then the friction between tyres and road is what allows the truck to turn the corner and therefore the frictional force is what is supplying the centripetal force i.e. arrow should point to the left not the right. As long as the static frictional force (coefficient of friction x normal force) is greater than the required centripetal force (mv^2/r) the bus will stay on the road. If the frictional force cannot meet its obligations, the bus will not turn the corner and will skid in the direction of its velocity at the moment the frictional force failed to do its thing.

The bus tipping over is a story for another day.

All the best
Chris

From: Catalist [mailto:catalist-bounces at lists.stawa.net] On Behalf Of Warren Stannard
Sent: Wednesday, 17 August 2016 11:35 AM
To: catalist at lists.stawa.net
Subject: Re: [Catalist] Gravity

Hi Alan
You make some excellent points. Here is my two bob’s worth… (document attached in case my pictures don’t appear on your mail program)
A suitable analogy is a thing often called “centrifugal force”. If you consider a bus taking a fast corner there is a real possibility that the bus will tip over. If we sketch a free body diagram for the bus we have to invent a force to make the bus tip over.
[cid:image001.png at 01D1F87E.A846E040]
The problem arises of course because the bus is in an accelerating reference frame.

Similarly, from our point of view being an observer on the surface of the Earth a falling object accelerates downward. So we invent a force to describe this (gravity) – and this works very well most of the time.
Here is another way to describe gravity:
[cid:image002.png at 01D1F87E.A846E040]
According to general relativity time is warped near a large mass – time proceeds at a slower rate the closer we are to the centre of this mass. An object takes the shortest path in ‘spacetime’ (called a geodesic). From the diagram the shortest path crosses the time lines at right angles, the object’s height decreases and from our perspective it appears to accelerate toward the Earth.

Hope that helps

Warren


Dr Warren Stannard PhD, MAppSc, BSc, DipEd
The Einstein-First Project
School of Physics
University of Western Australia
email: warren.stannard at uwa.edu.au<mailto:warren.stannard at uwa.edu.au>
Phone: 61 8 6488 1826 (office)
           0406 544031 (mobile)




From: Catalist [mailto:catalist-bounces at lists.stawa.net] On Behalf Of Alan Gent
Sent: Tuesday, 16 August 2016 3:38 PM
To: Catalist at lists.stawa.net<mailto:Catalist at lists.stawa.net>
Subject: [Catalist] Gravity

Can somebody please explain whether gravity is in fact a force? My understanding is that it Einstein showed that gravity is an effect of the curvature of space-time - and that was way back in the early 1900s, yet notables such as Professor Brian Cox consistently refer to the 'force of gravity'. There was also a recent program on ABC I believe, concerning 'anti-gravity' and in the program there was absolutely no mention of Einsteinian physics, and constant referral to 'gravitational force'. I actually thought it was rather a childish program.
Can we now assume that Newtonian physics has won the battle and we can really forget about Einstein, unless it suits us not to?
I realise not many teachers promote Einsteinian physics, it being in the 'too hard basket', but is there somebody out there that can enlighten me?
If gravity is actually a force, then there's no point in even mentioning theories of relativity, is there?

Alan Gent.
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