How fast could a horse run on a world with half Earth's gravity?

Say we took an adult racing horse from Earth to a planet identical to Earth, but with half its gravity. Exactly how fast would it be able to run? Would the low gravity have any significant effect on the way it ran?

Note: I'm looking for an answer from someone with a background in physics, preferably with citeable sources.

• @Fayth85 read the first sentence of the post. Sep 8, 2016 at 14:44
• I'd imagine that it's speed wouldn't be affected as much as it's stamina, assuming it could relearn how to run properly, it would be able to perform similarly, just for longer. Sep 8, 2016 at 16:05

The horse could not run at all. The motion would make it flounder around and not accomplish anything useful.

If you could train a horse to run under this condition, the greater loft and longer time between strides would not help the issue that when he does put hoof to ground it needs to move faster with his greater speed. That is, the speed at which the horse can move his legs is the limiting factor. Taking longer between steps does not change that.

So if the running motion worked at all, it would not affect the speed because that’s not the limiting factor. (Caveat: the horse would reach his inherent limit even when carrying a load, when on Earth any load further slows the horse by making strength to lift a limit encountered first.)

• And you know that horses cannot move their legs faster than at earthly top speed because of what? Sep 9, 2016 at 9:15
• Consideration of sprinting vs running for sustained performance. If you could move limbs faster you would (and shorten the duration) because that's what sprinting is. Sep 9, 2016 at 14:34
• Maybe you could move the limbs faster, but then you wouldn't be able to create the same pressure against the ground. Sep 9, 2016 at 15:09
• Somebody will have to do an experiment. Anyone here have a horse and a winch that can be set up over a treadmill? The same pressure gives more loft and time between steps, though. Sep 9, 2016 at 16:01

One thing that you have to keep in mind is that it may not be able to run faster per se, but would definitely need to run differently. A horse working with half gravity would have to move more like a deer. It would have to bound and leap, not necessarily gallop. The same force that would normally push it forward would push it upward considerably.

My guess is that yes, it would move faster, but not by a whole lot. If you look at gazelles, ibexes, and similar deer-like herbivores that spend a lot of time in the air as they run, they get a considerable amount of speed out of pretty small size. They do not run faster flat out than a horse for the most part, but if we assume something the size of a horse moving more like a gazelle, that would probably be pretty fast. Obviously, it would be pretty darn hard for someone to ride it at that point.

It would be hard to say, but it would certainly would move slower.

It's difficult to determinate speed, it may not variate much if the horse learns how to move in the planet, but expect slower movements as gravity also affect how fast you can move.

We have never take a horse to another planet, so we need to focus on other things we took: ourselves.

When we landed on the moon walking or running is much out of the box, because moon didn't have that much gravity (around 1,6 m/s²) and a simple step will throw us into the air. Earth has 9.8 m/s², and a planet with half of it would have around 4.9m/s². Walking may become awkward, but us are aware of the change of planet, so we will get used to it (meaning we will know that walking normally is not going to work). Horses on the other side will lose it! They wouldn't understand what's going on, they'll become scare because they are used to their weight in earth, and also a different acceleration on a planet means that you will have hard time doing other movements.

Also, a little note, be aware that no living creature on this planet can live in a planet which has such a different gravity, our organism will slowly decay and muscles will atrophy, as our whole body is prepare for living on this planet.

• I disagree with that last part. We know there are medical problems caused by the (effective) zero gravity of orbit, but we already know partial workarounds (vigorous exercise) and we don't know how bad it is to live in between zero and one G. That is, is one-half G half as harmful as zero, or less? We don't know yet. So you shouldn't think different gravity is just plain deadly.
– Snow
Sep 9, 2016 at 15:30
• @Snow yes, I know that we may not know how it affects, but as I said, our whole body is prepared to live in this world, it may not be dangerous, but it definitely would make a change on us Sep 12, 2016 at 11:56

I did the math several times, and I think that the horse on Earth would travel faster than the horse on the other planet. In fact, for a given distance that is the same on both planets, the Earth horse traverses it in half the time, and double the speed than the horse on the other planet.

I believe the main reason for this is because any horizontal speed is not affected by gravity at all, it's how hard you can push the ground away and behind you as you run (thanks to friction). But, the more air time you have, the longer it takes to come back down, and therefore you're missing out on hitting the ground. More opportunities hitting the ground and propelling forward = faster horse.

Also, I think that a horse from either planet could adjust to different planets' gravities in a decent amount of time as long as the gravities weren't bone-crushingly "heavy." I assume that if we swapped horses - put the alien horse on Earth and vice versa - that the Earth horse could out-run the alien horse because it doesn't have to work as hard to fight against gravity, but the alien horse is under twice the gravity of what it is used to.