The question, Scientists shown a fake anti-gravity device to motivate them to build the real thing, reminded me of another short story I read back in highschool (somewhere around the 1990ish timeframe) but likely older. It featured an engineer walking down the street where he sees a street vendor seemingly demonstrating toy saucers that float independent on any other force. He buys one, to figure out how it works, whereupon the salesman reveals the trick, a very thin and nearly invisible thread. The engineer laughs it off and takes it home to demonstrate to his wife. However, he finds that the system only works if he has the toy saucer turned on. Otherwise, the thread snaps under the weight of the saucer.
He starts trying to figure out why that might be the case whereupon the story cuts back to the salesman, who is talking to someone else:
He reveals that the antigravity device is real, but that it's flawed. It can only lift a very small amount of weight, enough to avoid breaking the string. He holds a patent on the technology and he's hoping one of the engineers he's sold the toy to will figure out the problem and try to market it whereupon he can invoke his patent and co-opt their research.
The twist seemed incredibly clever to me at the time. What engineer could pass up the chance to solve such an interesting problem?