Q: Obviously, the source of acceleration (without gravity) is transfer of energy from one object to another or change of its own energy structure so that one of its components changes into increasing energy of motion. But gravity occurs with no measurable energy transfer nor is the gravitating mass changing its energy structure in order to produce gravity.
If the cause of acceleration and gravity is not related at all is the Equivalence principle just a coincidence that the resulting effects are physically identical and indistinguishable?
A: By Frank Heile, PhD in Physics from Stanford University
I will try to describe in detail how gravitation works and how acceleration works and why the two different phenomena are really equivalent...
There follows a long and complicated explanation and comparison, but in folksy terms, anybody who has ever swung a bucket of water over his head at the right angle and speed knows this. As far as the water in the bucket is concerned, there is no difference between the gravity and the centripetal force/acceleration* while it is upside down. See also Einstein's famous thought experiment with the man in the accelerating spaceship.
* Acceleration is change in speed OR in direction, see here.
I refer you now to the comment by Nobel Prize nominee Ralph Musgrave:
Assume there’s just one atom in a car cylinder which is at room temperature. The atom bounces up and down between the piston and the top of the cylinder. Atoms at room temperature move around at about 1,000mph. If the piston then moves upwards at 1mph to its “maximum compression” point, the atom will gain a good 100mph in speed.
How is that possible, given the paltry speed of the piston? Answer… During the half second or so during which the piston is moving, the atom collides with it a hundred times or so, and it gains 1mph each time. I’d appreciate nominations for a Nobel Prize for this amazing insight.
So for air at ground level, the effect of gravity is like being driven by a giant piston accelerating at that speed and every time it bounces off the piston (the surface of the earth), it travels back faster than it set off (I'm not clever enough to work out how much faster at this stage and if so, relative to what). Relative to the surface of the earth there is no change in observed speed, just in direction (which as explained above, is acceleration).
Now imagine sweeping up sawdust with long straight strokes of a broom, you will get a bow wave in front of the broom where the sawdust is deeper, the broom is pushing saw dust which is pushing more sawdust etc. The pile is deepest directly in front of the broom and slopes away from it, the sawdust far ahead of the broom is entirely unaffected.
Which is why the atmosphere is thicker at the bottom. Which is another way of explaining why, from the point of view of an individual O or N molecule, in gravity/pressure/temperature* terms, the atmosphere is not static, it is constantly being accelerated from underneath by a giant piston (the surface of the earth), the same as the sawdust in front of the broom is being accelerated by the broom itself or by other bits of sawdust etc.
Like a bow wave, the force is carried ever upwards, so molecules in the upper atmosphere are bounced further out than they would reach under their own devices. So the upper atmosphere is less dense than it 'should' be and hence is cooler than it 'should' be, i.e. cooler than the surface of the Earth in the same way as the lower atmosphere is warmer than it 'should' be etc. To use another analogy, the atmosphere acts like a heat pump or fridge, it transfers heat from some places (cools them) to other places (warms them) but without changing the overall amount of 'heat'.
* I accept that there is minority few that increasing the pressure of a gas does not directly increase its temperature, which would mean that a lot of textbooks are very, very wrong on this one!
Thursday, 22 September 2016
My latest blogpost: Gravity is more or less the same as acceleration.Tweet this! Posted by Mark Wadsworth at 12:06