I just came across this on the web. It would be very useful in an emergency situation.
http://www.metacafe.com/watch/746372/how_to_pull_a_van_uphill_with_only_a_rope/
Thoughts?

Drivel, what you gain in purchase you lose more in friction.

Tie the rope to the car and the post , then pull at right angles to the rope. Take up the slack, try again.

A rewoven figure of eight knot is better than a bow-line. Puts less stress on the rope. A poorly tied knot will actually cut the rope.

That's correct, if you're going to pull your car like that every day - some day the rope will break.

Great method for an emergencies!

Regarding the friction. It grows linearly with every bent section of the rope, but each pulley, after the first one (the post), divides the necessary pulling force by 2(so it's exponential). The exponent function grows faster than a linear one. So, the benefit of such a rope system over a single post pulley should be noticeable.

Check these basics of elementary physics involved:
http://www.howstuffworks.com/pulley.htm

When I was younger, my dad taught me to use a much smaller version of this as a version of a trucker's hitch to tie down objects in the back of a load. I've learned the hard way that because of the force you can create, you can easily break whatever load you are tying down. Lol.

Interesting, did you create this video?

I would like to see this with a vehicle that is really stuck, not rolling on pavement. I suspect that either the rope will break, or the person will not have the horsepower to pull the vehicle out of the mud...

I have 100' of 3/8 rope ($12 home depot) in my car for the same reason; but also have 4 small pulleys sized for the 3/8 rope (another few bucks, same aisle in home depot). incorporating the pulleys creates a block and tackle setup and reduces friction dramatically. With 4 pulley's one can transfer 8x the power of tugging a single rope. Add more pulleys = more power (but need more rope to move object the same distance). I've used this method in the woods to drag huge logs around. Pulling the van out of a mud hole would be simple. Of course one could always buy a come-along, but they are not as fun!

Theres a US miltary manual on vehicle extraction with all manner of tricks.

They did a "Dirty Jobs" on this too, it was amazing! They pulled a hummer up a sandy hill, and it was even stuck too.

Crazy what you can do with rope and pulleys.

-Todd

If instead of simply putting the rope through the last loop, he made another pulley-loop and then went back around the tree, would he multiply his force yet again?

That's great... as long as you've got a post or tree nearby.

Sue

Probably, at least in theory. Should really increase the friction too. If you are going to rely on this, I would suggest buying some real pulleys, and a short section of rope/strap to secure one to the tree with...

I agree with having real pulleys, but I'm trying to find out what the possibilities are with just a rope. You may find yourself in a situation with only a rope to pull a car out of the mud.

"...with only a rope..."

In that case, rig this thing up, and good luck...

the army does it like this:

http://www.globalsecurity.org/military/library/policy/army/fm/3-97-61/ch7.htm#fig7-26

WARNING-WARNING-DANGER-DANGER

Under any serious load, the moving part of a nylon rope will very quicky melt/saw right through the stationary loop it is running on. Run the rope over a metal carabiner instead.

He has basically created a 3:1 Z-rig without the use of pulleys. The system will work for short distances, but as already noted, there is loss of mechanical advantage due to the rope on rope friction.

Rope on rope is not ideal, but we utilize this method when creating static back-ties on questionable anchor points such as small trees, poles, etc. We tie the back-tie line (high up, as well as the main or belay line – generally you keep you anchor points low to create sheer force instead of lever force, but when back-tying high, the vector forces will be directed downward) to the tree/pole, place a butterfly knot in the line about ½ back and then run the line around the base of a smooth, but sturdy tree/pole and take the running end back through the butterfly, thus creating a 2:1 mechanical advantage.

Pete