In park or vert skateboarding, the riders are able to go as long as they like without putting down a foot. The flatland/street people have to push the board around, but these people are imparting unequal forces, overcoming the energy losses, and maintaining good speed through a run. I can see the body sort of pumping at points, but I don’t understand how they’re getting net force forward, and so often. Who knows this?
I don’t know much about skateboard parks, but suppose you’re on a skateboard and going along an undulating track, like a sine wave. If you crouch as you’re traveling up a hill, then your center of mass will travel less than the vertical distance of the hill. When you reach the top, you can stand, and then rolling down again your center of mass goes the full distance down.
Since you’re effectively travelling downward more than upward, it’s like rolling downhill on average. Of course there’s no free lunch–you put energy into the system when you stood up. It’s not all that different from a swingset, where you get going by moving your center of mass.
Do a search on Pump Track. It’s mostly used by cyclists but the same principles apply.
Pumping. Bikes are the same. You push down when the bike / skateboard is on a downhill and lift / absorb going up. It’s not a free-ride, the energy comes from your legs.
I don’t think that really works. Your centre of mass can’t keep going down. When you reach the top and stand your centre of mass is being lifted back up.
So you don’t stand right at the top. You stand after you’re starting to go back down again. That pushes the board downward, which translates to forward.
Right, that’s how the energy comes into the system. At that point, pushing against the ground doesn’t affect the motion of the skateboard (since it’s basically flat). But crouching as you go uphill does affect your motion (reducing the velocity loss).
Taken to an extreme, we can imagine a sawtooth profile. The skater jumps a 1’ vertical, then rolls down the ramp. Repeat as necessary. However, how this is accomplished makes a difference. If the skater kinda crouches in place to clear the vertical, and then gradually stands as they roll down the ramp, they aren’t going to accomplish much. They need to “reset” their center of mass at a certain point to put energy into the system. I think it works best if the standing happens at a relatively flat location, but I need to think about it a bit.
Ok.
Now do vert ramp.
Like a half-pipe, say? Same basic idea. Crouch as you’re travelling up a wall. On the flat bottom, stand. Actually, I think it’s even easier because you can crouch at any time travelling vertically, even downward. As long as you aren’t exerting much force on the rolling surface, it doesn’t matter when you do it.
To make it a bit more concrete–suppose your center-of-mass is 3’ above ground while standing and 2’ while crouching. You’re going at a speed such that your center will hit 12’ vertical. Starting from the bottom while standing, then, your CoM travels 9’ up. At some point though you crouch, and you travel 10’ down. You pick up more speed than you started with, assuming friction isn’t too bad. You’re back at the flat bottom of the half-pipe again, and if you stand you raise your CoM back up to 3’ (with some energy input by you) and you can repeat.
My problem with this is that trick amplitude doesn’t seem to correspond with rider height. Shawn White isn’t very tall, but got huge air. Trick height does seem to correspond to gender, and in a way that isn’t explained by height of the person. This greatly implies there’s something related to power in the ability to get above the ramp.
The description used here says that there’s nothing a person can do to increase the airs besides stand up all the way, but some people go way higher than others.
If there were no friction, there would be no limit to the amount of air any given rider could achieve. Each pass adds to the velocity. Taller riders may have some advantage in that they can crouch further, but there might be other things that counteract this, like greater friction or losses when they hit the ground. So while it’s easy to say how they gain velocity, it’s not so easy to determine how any given rider will compare with another in practice.
So you’re saying you don’t know? I’m not being sarcastic. One of my measurements of being a decent person is being able to just admit you don’t know, which I have to do pretty often at work.
What I’m saying is that ultimately it’s a matter of the rider controlling the position of their center of mass at various points on the ride. How this translates to Shawn White getting more or less air than some other rider, I have no idea, and without a detailed analysis, neither does anyone else.
Well, detailed analysis can include personal experience. I have a feeling that this board is possibly too nerdy and middle-aged to have that. Maybe I should ask on a skateboarding subreddit.
Every old person was young once, and skateboarding has been around for a long time. So there may be people here with experience, but it’s not me.
Your OP is phrased as a basic physics question, and there are quite a few people here that can answer those. It sounds like you’re interested in a more practical explanation, though. I don’t think there’s going to be huge overlap between the two since so much of sport is essentially intuition. Riders aren’t doing physics calculations as they ride, but they learn what works and what doesn’t.
But the “physics” explanations doesn’t explain the observable results. Some riders are able to manipulate unequal forces more than others, even if they’re the same heights, which means there’s more to it than just standing up, it seems.
Also, the physics explanation is intrinsically tied to the practical application. There can’t be one without the other.
Plenty of us middle aged nerds have ridden pump tracks. It’s hard to describe the specific actions you take in order to work your body and the physics. Try it yourself, it’s pretty safe.
A couple of things:
- I gave a simplified set of actions to make the underlying physics clearer. In practice, riders aren’t going to have such a clean division of actions, like going from a full crouch to a full stand precisely at the bottom of the half-pipe. Riders are going to blend their motions together, plus they have to account for balance and their own physical limits.
- The physics just tells you the upper bound of what’s possible. Since riders aren’t shooting into space, we can conclude that a few other things are coming into play.