Wikipedia:Reference desk/Archives/Science/2020 April 30

= April 30 =

Weider weight bench
Are there Weider olympic weight benches available anywhere?99.145.194.98 (talk) 05:28, 30 April 2020 (UTC)


 * Googling ""Weider Pro 395" brings up several companies offering these for sale. --Lambiam 13:57, 30 April 2020 (UTC)
 * This isn't really a science question. --OuroborosCobra (talk) 16:04, 30 April 2020 (UTC)


 * Thanks for the help and kinesiology is science!99.145.194.98 (talk) 22:09, 30 April 2020 (UTC)
 * Your question didn't ask anything about kinesiology. You asked where to buy an olympic weight bench. That'd be like saying asking about where to purchase a tickle-me-elmo was a developmental psychology question. --OuroborosCobra (talk) 15:25, 1 May 2020 (UTC)

Light sail
In episode 2 of Cosmos: Possible Worlds they talk about using a light sail to send interstellar probes. At first they are powered by lasers from Earth, then they carry lasers to fire photons into the sail to keep it accelerating. But wouldn't the backward force of the photons leaving the laser be at least as large as the force on the sail? Bubba73 You talkin' to me? 20:42, 30 April 2020 (UTC)
 * Yes, it's the equivalent of blowing in your own sails on a regular sailing vessel. However, efficient light sails must be made of a reflective material, so if the sails reflect the light from the probe's lasers it has the same effect as just aiming the lasers the other way, so it would still work (but the sails aren't really needed at that stage). - Lindert (talk) 21:21, 30 April 2020 (UTC)
 * By the way, Mythbusters tested "blowing in your own sails on a regular sailing vessel". It works.  I was surprised when I saw the episode (they didn't explain it very well) until I realized that it was equivalent to the thrust reversers on a jet plane: directing the air backward is, again, similar to aiming the fan the other way. --76.71.6.31 (talk) 21:51, 30 April 2020 (UTC)


 * Thanks, that is what I thought about the lasers and light sail. A light sail is supposed to have a very low mass.  Is it feasible to have lasers and (presumably) batteries to provide enough energy to accelerate to relativistic speeds?  Bubba73 You talkin' to me? 22:25, 30 April 2020 (UTC)


 * To accelerate to relativistic speeds on battery power (taking the batteries with you), you need batteries with an energy content that's large compared to the rest mass of the batteries. That won't work with chemical or even nuclear batteries. PiusImpavidus (talk) 08:30, 1 May 2020 (UTC)


 * Ah, yes, that is right. Bubba73 You talkin' to me? 22:20, 2 May 2020 (UTC)