Competition Reg merchandising has teamed up with Tivola Publishing, the publisher of multimedia CD-ROMs, to give away five copies of their Physicus.

According to the blurb, a meteorite has hit the planet, causing it to stop rotating around its own axis. Things look pretty bad for humanity unless you can get the world rotating again. All you have to do is knock up a quick impulse machine.

Sounds pretty simple, but as we at Vulture Central found out, there's more to it than that. Hours of fun for all the family, and most certainly not just for the kids.

Right that's enough of the hype. If you want the chance to win a copy of Physicus then answer this rock hard question from our in-house Physics expert Lucy Sherriff.

What is the value of the gravitational constant - that's big G, not little g. E-mail your answer here. Closing date is Friday 20th October at 5.00pm. First five out of the hat get the goods. We'll contact the winners to get their postal addresses for despatch. Good luck. ®