# How big would a planetoid have to be to keep me from jumping off and not coming down? **URL:** \ **Category:** Factual Questions\ **Created:** [August 24, 2004, 10:31am UTC](https://boards.straightdope.com/t/how-big-would-a-planetoid-have-to-be-to-keep-me-from-jumping-off-and-not-coming-down/261243 "2004-08-24T10:31:00Z")\ **Posts on this page:** 7\ **Page:** 2
**Author:** ![BobLibDem](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/boblibdem/32/3149_2.png) [@BobLibDem](https://boards.straightdope.com/u/BobLibDem)\ **Post date:** [August 25, 2004, 12:35pm UTC](https://boards.straightdope.com/t/how-big-would-a-planetoid-have-to-be-to-keep-me-from-jumping-off-and-not-coming-down/261243/21 "2004-08-25T12:35:12Z")
Of course, a planetoid as small as being discussed is not going to have an atmosphere anyway, so the dimples on the ball would be entirely unimportant. ---
**Author:** ![Desmostylus](https://avatars.discourse-cdn.com/v4/letter/d/c57346/32.png) [@Desmostylus](https://boards.straightdope.com/u/Desmostylus)\ **Post date:** [August 25, 2004, 12:52pm UTC](https://boards.straightdope.com/t/how-big-would-a-planetoid-have-to-be-to-keep-me-from-jumping-off-and-not-coming-down/261243/22 "2004-08-25T12:52:13Z")
> [@MartinL](#): > > No, the initial kinetic energy is ALL that drives the ball. It does not gain more energy on its way from nothing. > Rotation may create a lift as you described, but rotation is just another form of kinetic energy. > Now I don’t know whether the kinetic energy is devided into forward movement and rotation right at the beginning, or whether air resistance turns forward movement into rotation later on, but be sure that all the energy is there right from the start. > [@BobLibDem](#): > > Of course, a planetoid as small as being discussed is not going to have an atmosphere anyway, so the dimples on the ball would be entirely unimportant. I don’t think that was the point of **Moe Mentum** ’s post. The point was that **Chronos** ’s method for estimating the initial velocity of the ball doesn’t necessarily work well, because the assumed angle isn’t right, and because the air resistance/lift element could be quite significant. ---
**Author:** ![Desmostylus](https://avatars.discourse-cdn.com/v4/letter/d/c57346/32.png) [@Desmostylus](https://boards.straightdope.com/u/Desmostylus)\ **Post date:** [August 25, 2004, 12:57pm UTC](https://boards.straightdope.com/t/how-big-would-a-planetoid-have-to-be-to-keep-me-from-jumping-off-and-not-coming-down/261243/23 "2004-08-25T12:57:28Z")
Should have thought of this before. > [@](#): > > [Guiness World Records](http://www.guinnessworldrecords.com/content_pages/record.asp?recordid=45183) > > The fastest projectile speed in any moving ball game is approximately 302 km/h (188 mph) in Jai-Alai. This compares with around **270 km/h (170 mph)** for a golf ball driven off a tee. That’s 75 ms[sup]-1[/sup], so r = 43 km. ---
**Author:** ![Desmostylus](https://avatars.discourse-cdn.com/v4/letter/d/c57346/32.png) [@Desmostylus](https://boards.straightdope.com/u/Desmostylus)\ **Post date:** [August 25, 2004, 2:16pm UTC](https://boards.straightdope.com/t/how-big-would-a-planetoid-have-to-be-to-keep-me-from-jumping-off-and-not-coming-down/261243/24 "2004-08-25T14:16:45Z")
(In case it’s not obvious, I should point out that 43 km is not far off **Chronos** ’s initial estimate of 33 km, and inside **Chronos** ’s upper bound of 66 km. So the back-of-the-envelope calculations are working reasonably well so far.) ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [August 25, 2004, 4:03pm UTC](https://boards.straightdope.com/t/how-big-would-a-planetoid-have-to-be-to-keep-me-from-jumping-off-and-not-coming-down/261243/25 "2004-08-25T16:03:40Z")
> [@](#): > > No, the initial kinetic energy is ALL that drives the ball. It does not gain more energy on its way from nothing. Which does not contradict anything said by Mr. **Mentum** (great name, btw!). He did not say that the dimples somehow increased the energy of the ball, merely that they increased the range (which is true). To use an analogous but extreme example, a paper airplane can also (in extreme cases) be thrown hundreds of meters, despite having nowhere near the kinetic energy or momentum of a golf ball. That said, while a dimpled ball in atmosphere will fly further than an undimpled one, and a ball in vacuum will fly further than a smooth ball in atmosphere, I’m not sure how a dimpled ball in atmosphere would compare to a ball in vacuum (that is to say, whether the lift produced by the dimples is enough to make up for the drag). And I was aware that neglecting the atmosphere in my “muzzle velocity” estimation introduced error, and the directly-measured figures for the ball’s initial speed are certainly more accurate, but I prefer to work approximately with figures I have readily available for questions of this sort. ---
**Author:** ![RTFirefly](https://avatars.discourse-cdn.com/v4/letter/r/c77e96/32.png) [@RTFirefly](https://boards.straightdope.com/u/RTFirefly)\ **Post date:** [August 25, 2004, 4:54pm UTC](https://boards.straightdope.com/t/how-big-would-a-planetoid-have-to-be-to-keep-me-from-jumping-off-and-not-coming-down/261243/26 "2004-08-25T16:54:05Z")
Fascinating responses! When I posted the OP, I was thinking in terms of orbital as well as escape velocity, but I’d forgotten that any orbit in this scenario would include the launch point, so the alternatives are escape or fall back to the surface. Thanks, **Mangetout** , for reminding me, and to **Valgard** for the wonderful image. And thanks to **zut** and **Desmostylus** for providing the appropriate formulas and doing the back-of-the-envelope calculations. r=43km to keep the golf ball from leaving, huh? There’s really not a large number of objects in the solar system that are that large: the nine planets, a decent proportion of their moons, and a few of the larger asteroids. The number that can keep a good man down 😉 is somewhat larger, since we’re apparently talking r=3km, plus or minus a bit. **Cronos** - I remember reading _The Rolling Stones_ when I was a kid, oh, about 40 years ago. (I’m not old; I just have a lot of history. :)) I liked it; now I’m going to have to track down a copy and re-read it. ---
**Author:** ![Moe\_Mentum](https://avatars.discourse-cdn.com/v4/letter/m/9d8465/32.png) [@Moe\_Mentum](https://boards.straightdope.com/u/Moe_Mentum)\ **Post date:** [August 26, 2004, 6:13am UTC](https://boards.straightdope.com/t/how-big-would-a-planetoid-have-to-be-to-keep-me-from-jumping-off-and-not-coming-down/261243/27 "2004-08-26T06:13:59Z")
Thanks to **Desmostylus** and **Chronos** for clarifying my point. Sorry for the confusion. [Previous page](https://boards.straightdope.com/t/how-big-would-a-planetoid-have-to-be-to-keep-me-from-jumping-off-and-not-coming-down/261243.md?page=1)