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Name: Diane M.
Status: other
Age: 40s
Location: N/A
Country: N/A
Date: 4/9/2003


Question:
Is there a difference between lift and propulsion and if so, what? At a "Science Olympiad" that our school district recently held, one of the projects was to create a hover craft using a 4' square of plywood, vinyl sheeting, and one leaf blower for propulsion.

One of the entries used 2 leaf blowers, one for lift and one for propulsion. They were disqualified because the judges argued that lift and propulsion were the same thing.

I also was under the impression that lift and propulsion were not the same thing, and now my curiosity is killing me. What exactly is lift and what is propulsion?

Incidentally, the winning entry used a leaf blower for lift and a large, industrial fan for propulsion. That seemed to be a direct contradiction of the judges' edict that lift and propulsion are one and the same.


Replies:
Diane,

Lift is a force that does just that. Lift lifts an object, working against gravity. Propulsion propels an object, providing whatever forces are necessary to make the object move from here to there. It is true that lift and propulsion are both forces, but they serve very different purposes.

Dr. Ken Mellendorf
Physics Instructor
Illinois Central College


The difference between lift and propulsion is simply that of direction. Lift and propulsion are terms we apply to forces. So in the sense that they are both forces, they are indeed the same. However, we commonly use the word "lift" to mean a force that acts in the opposite direction of gravity. Gravity is a force that pulls us down, and "lift" is any force that lifts us up. Propulsion is commonly used as a name for a force that moves us in a direction we want to travel, or in other words, propulsion moves us forward. However, propulsion also is used to mean the system that moves a vehicle. A propulsion system on a hovercraft requires components that provide lift and provide forward movement.

I do not think any argument could be won on what exactly is propulsion and what is not when dealing with hovercraft. Propulsion is not really a technical term, so it can pretty much be made to bend any way that is convenient, and with hovercraft especially it does not mean too much.

Not having a copy of the rules, I cannot say for sure if the judges kept to their rules or not. But, it definitely sounds like they did not, and their reasoning definitely does not work out. If the rules simply stated that no more than one leaf blower could be used, then they made the right decisions because the winning entry only had one leaf blower, but the disqualified one had two. This has nothing to do with propulsion vs lift however. If the rules specified that the only means of propulsion could be a single leaf blower, then they would have had to disallow any vehicle with more than one motor. Which they clearly did not do.

If the rules allowed a single leaf blower for propulsion, and other motors were allowed only for lift, then winning entry did not do this--the leaf blower was used for lift, not propulsion, while the disqualified entry did do this. If they rules stated that only those materials mentioned could be used, then both vehicles should have been disqualified because both used additional motors. I hope this helps.

Eric Tolman


Ultimately, this is going to be a definition of terms, because I do not think there is an "official definition" of the two. I would have interpreted the "lift" to mean getting the hover craft off the ground and keeping it there and the term "propulsion" to mean getting the hover craft to move parallel to the ground in a controlled manner. Those terms should have been made clear in the project instructions. To that extent the judges showed poor judgement, in my opinion. Having assisted both in the writing instructions and judging of science fairs, one of the things we gave special attention to was, "Have all the terms, limits, and allowed materials been clearly specified?". Even so, occasionally things were not clear. If there was some "rule" that we had missed we made the decision prior to the event that the contestant was given the benefit of the ambiguity. But again, the decision rests on the definition that the judges make. They are the court of first and last resort usually.

Vince Calder



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