What could slow the earth’s spin in a sufficiantly short ammount of time (say, hours or days) for people to witness the rapid changes.
Obviously there would be disaster, but what kind?
Would things weigh less? Would the sea rise?
More importantly, how quickly would it have to happen to reverse the flow of time?
Sorry. Leaving. 
A rapid accretion of mass or being squashed into a disc would, by conservation of angular momentum, yield a slower rate of rotation.
Assuming that the second is pretty fantastic, the first might arise from the impact of some enormously massive body. Were that the case, it would most certainly be the conservation of linear momentum (pushing the planet out of orbit) and the fact that the equivalent of several billion H-bombs has just detonated which would be of immediate concern.
The only way I could see of slowing the spin such that only the changes due to that change alone would be observed is perhaps by a truly massive alien ship built of eg. neutronium landing on Earth, or super-engineers in the future hollowing out Earth and building continent-sized “ears” on the planet, for a laugh.
There is a slight variation in g due to centrepetal forces, and so the variation in this variation would be tiny indeed (numerical example left for homework). The major change would be the difference in day/night length and the time between tides, on which many important climatic and biological processes depend.
What if a very dense astrological body was ‘captured’ - orbited against the spin. Would it’s gravitational influence slow the earth?
Well, no more night and day, or at least substantially longer nights and days. A lot of biological clocks being messed up. Substantial weather changes. I’m not really qualified to go into detail, but I don’t think it would be the catastrophe you imagine. Not in the short term, anyway.
If anything, things would weigh more, since the centrifugal force would cease (yes, I know it’s an imaginary force). If anything, the sea level would decrease, for the same reason.
Yes; tidal forces can slow down the rotation, but in order for this to happen in such a way as to slow the Earth perceptibly, the object would have to be incredibly massive, the tides would be catastrophic (they would probably affect more than just the water on the planet) and there might even be a problem with the gravitational gradient (the side of Earth facing the body would be closer to the other side and thus subjected to greater forces) - the Earth’s crust might actually be broken up by it and the planet might assume an ellipsoidal shape.
I’d get an extra hour in bed every morning. This surely outweighs the risk.
Mangetout - this is more like it.
Let’s say this captured body was superdense (but not so much we’re pulled out of orbit)- could the seas rise on the facing side and sink on the other?
How would humans fare with lengthened days - how much heavier would everything be without centrifugal ‘force’? - wouldn’t things be lighter on the facing side?
No, because the Earth would be pulled into an ellipsoidal shape, the sea would be deeper on both the facing and opposite sides, just like it does now with normal tides.
If every person on the earth lined up around the equator and started walking east, the reaction force that is applied by their feet would slow the earths rotation.
I think this is the most do-able solution so far.
When they stopped walking, the effect would stop and things would return to normal.
You can alter the spin of planets using heavy orbiting objects- a neutron star or black hole would work fine, especially one under acceleration;
there aren’t any neutron stars or black holes in this solar system, so some people hae suggested using a planet in a grazing orbit (don’t knock them together, though; we already have one moon, thank you);
Paul Birch has recommended using a high density mass particle stream looped around the planet and the Sun; this is part of his design for terraforming Venus, which I admire for it’s tenacity, but would be very expensive.
alternatively you could wrap the Earth in a giant circular electromagnet, and turn it into an engine, pushing against the Solar magnetic field or against a similar torus on the Moon.
The energy required is phenomenal, on the same order as the energy required to rip the crust into orbit. So be careful.
Oh, yes, it can be done without impacts-
The Earth’s weather would be destabilised, hurricanes, tides, ice melting, certainly not recommended.
why do you you want to know?
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Chronos will be along you smack you about for that remark. In fact the centrifugal force is as real as any, but it depends on your frame of reference. From an inertial frame of reference it does not exist; but for an accelerating frame of reference, it is quite real.
The moon is already slowing Earth’s spin, and eventually the moon (at a much more distant orbit, since it’s also receding) will adopt a geosynchronous orbit and seemingly hover over one part of Earth at all times. Earth’s day will be longer than a current month.
Of course, long before then, the Sun will go into its red-giant stage and the whole thing will be moot.
Yep, and that’s why Jupiter’s day is so short, because its moons don’t have enough mass to significantly slow down its rotation.
Would we see the affects of rapid decelleration? Considering we are rotating at a rotational velocity of approximately 1000 mph at the equator. How fast of a slow down are we talking about here? Would we all go shooting east at hundreds of miles per hour?
Oddly, Venus has no moons at all and its day is ridiculously long. I’m not sure a hard-and-fast rule can be written, here.
That might have something to do with its proximity to the sun.
Tyler, That’s the sort of thing I’m thinking about - If rapid deceleration happened would everything not-tied-down slide in one direction?
My OP was based on musing over what disaster movie scenarios havn’t been covered yet.
The Bad Astronomer discusses this in his review of The Core. WARNING: SPOILERS!