Re: How many pounds of thrust does it take to levitate a jet ski by 1 foot?

As you probably know weight is the force something feels due to its mass
and gravity.  So your weight changes when you go to the moon because
gravity is smaller there.  

Pounds of thrust is a measurement of force.  When used in this way the unit
"pound" means the force that is produced by 1 pound mass in one standard
earth gravity.  (The SI unit for force is the Newton.)

So the simple answer to your question is that to stop something falling
downwards it needs an equal force upwards to counter its weight.  The
thrust you need is simply the weight of the jetski.  So if the jetski
weighs 200 pounds you would need 200 pounds of thrust directly upwards to
stop it falling downwards.

If you have more than 200 pounds thrust it will accelerate upwards, less it
will accelerate downwards.  The acceleration is given by F=MA.  Remember F
is the resultant force.  So if you have 200 pounds weight and 200 pounds
thrust directly upwards the resultant force is zero.

The second part is how to measure thrust.  It really depends on the
application.  A simple spring scale measures thrust, but it may not be
suitable for your motor idea (then again it may be).  Remember thrust is a
force - that is what you are measuring.

Here's someone's design for a simple model rocket thrust gauge.
 http://members.aol.com/riccnakk/hydlc.html

here's a more industrial one
 http://www.ormondinc.com/Thrust_Systems/Thrust_Stands/thrust_stands.html

Finally this guy measures the thrust of a fan by several different methods.
 http://beale.best.vwh.net/measure/fan/fanspeed.html

Martin Smith