It’s has to do with what skateboarders call “pumping in the transition”.
Here’s a very basic demonstration.
While gravity is pulling you through the transition transferring your vertical momentum to horizontal, you are adding to that vertical downward force by pushing against the board. This adds to the eventual horizontal force.
As the rider demonstrates the extra downward push given by the rider is enough to offset and even exceed the forces trying to bring him to a rest.
Good video.
Sorry, I don’t get it yet. “you are adding to that vertical downward force by pushing against the board. This adds to the eventual horizontal force.” I can raise the center of mass by pushing down, but the board is pushing up against me as I push down, equally and oppositely. This should cancel out.
What I can think of is that your body isn’t vertical, and you’re pushing a bit forward, and not all vertically. This would make a net force on you in the forward direction, making unequal forces, as I said. Does that work?
Dr. Strangelove, your explanation made it sound very much like the trick had everything to do with altering the center of mass so it travels through different distances at different times, but I suspect very much it has to do with pushing off the inclined planes available so there is a net force forward. This explains why “Wee Man” Acuna is a more powerful skater than 12-year olds who are taller. The center mass alone doesn’t seem to explain the observable results.
Usually there are multiple ways to explain any given event in Newtonian mechanics (or any other aspect of physics). It’s probably possible to explain some skateboarding maneuvers in terms of pushing off of a slope. But ultimately you will get the same result as tracking CoM.
Friction is constantly bleeding off energy, so to maintain speed the rider has to supply more energy. There’s really only one mechanism here to supply energy, and that’s applying a force over a distance. And the only force they can apply is the acceleration of their own mass. A convenient source of acceleration is gravity, so the only thing left is for the rider to counteract that force through a distance.
Centrifugal acceleration would also work, though it gets more complicated to understand in that case.
How do you increase the amplitude of your swing on a swingset without touching the ground?
By shifting your center off mass off the seat. I wish I understood it better, though. I will point out that standing up on flat ground will create offsetting forces.
My understanding is that being shorter helps a lot with boarding, so searched for why. And I got this article that might shed some light on it. The main physics-based aspect is that shorter people have a lower center of gravity, but it mentions other factors. In particular, apparently taller people are more injury prone, and more likely to have worse injuries. The proposed reason is slower reaction times due to being further from the ground. The idea then is that they have to practice more to get to the same level of skill. And I definitely suspect skill is the ceiling more than physics, as skaters keep getting better and better.
Sit still in the swing. Pull back on the chains. Your butt will scoot forward, and your center of mass will not be vertical from the attachment point of the chains. You will swing forward as your center of mass falls. This motion will carry past the vertical point of the chains. Now reverse the process as you get to the top of that arc. Your backwards swing will go back farther than it would have if you just sat straight up. Continue this over and over. The more you pull the chains, the more effect it has, because your center of mass has farther go to reach the point just under the attachment points.
Do you understand how an ice skater speeds up a pirouette by reducing their angular momentum?
Swings work on the same principle.
If you consider a half pipe as a swing it should be clear. (the half pipe is working as the rope of the swing).
Standing up against the g-forces in the curves provides the work to increase the kinetic energy in the system.
I may be misunderstanding you but I don’t think you have the explanation quite right. Pumping is most effective when pushing your legs down (standing up in your terms) as you descend each ramp, not before. If you just stand up on the moderately level bit at the top you don’t get nearly as much effect.
The key to pumping is to pump on the down and also to pump just prior to each up.
This video has a pretty good explanation I think.
Note also that when skateboarding a big half pipe, you’re standing when your body is nearly horizontal, it doesn’t add potential energy by making your centre of mass higher, it adds kinetic energy by propelling your body horizontally in the direction you want to go.
Edit: And that is why being strong makes a difference. You’re launching yourself forward’s off the down ramp, and up just prior to the up ramp.
There are a couple strategies that work to some extent; I just described one where I thought the physics was pretty clear. You can see some skateboarders in the video pretty much doing as I described when going up ramps (a little crouch as they go up, and then standing fully after getting to the top).
The video does seem to pretty effectively demonstrate that pushing off at the bottom of the curve works better that just crouching. That makes sense. I mentioned earlier that the only way to add energy to the system (aside from friction) is to apply force along a distance. And force requires acceleration. You can use gravity on flat ground (i.e. stand from a crouch), but even better is to wait until you’re at the bottom of a curve and use the centripetal force (plus the force of gravity) to your advantage. If you’re strong enough to push against that with the full vertical distance, you can get more energy out of it (Just ballparking it: if the curve has a ~1 m radius, and you’re going 5 m/s, then you get 25 m/s^2 centripetal acceleration, or 2.5 gees. Add gravity and you get 3.5 That’s a significant boost vs. just 1 gee).
Like I said, this isn’t the only way of looking at the problem; you can always break out the free-body diagrams as well. But looking at the way force is applied to the CoM gives a pretty clear picture, IMO. Note that the change in curvature at the transition is pretty crucial; if you were just going down an infinitely long ramp, pushing off would just cause you to bail.
All that said, I did underestimate the contribution of centripetal forces, and I think this may help the OP with the answer. Anyone can stand from a crouch on flat ground. But not everyone can push against 3 gees, or perhaps even more at high speeds. A really strong person, even if on the short side, is going to do better there.
Ramps are not the crux, curves are. “Pumping” is about manipulating angular momentum: gaining angular momentum by reducing the radius (standing up), transferring that angular momentum to forward momentum. (at the end of the curve) Resetting your body (crouching) when you are not moving along a curve and … rinse and repeat.
Like this.
Not true.
When I was a kid I figured out how to get a skateboard moving from a dead stop, on flat ground, without putting a foot on the ground - and keep it moving after that. Here’s my shot at an explanation:
-
Start with one foot on the rear of the board (so you’re able to kick up the front end), and the other somewhere forward on the board. Get your center of mass near your rear foot.
-
Lean forward (your-forward, not board-forward), so you start to fall face-first. You are accelerating your center of mass sideways relative to the board, building up some velocity perpendicular to the direction the board is oriented.
-
Now kick up the front end of the board and spin it 90 degrees to get it back under you so you stop falling. You and your board are now moving stably in a direction 90 degrees from the board’s original orientation.
This process can be repeated, falling alternately face-first and back-first, to build up speed. As you build up speed, the angle to which you re-orient the board each time gets less.
This is analogous to what a skater does. From a dead stop, they have to push off perpendicular to their rear blade. As they get going faster and faster, the kickoff angle of each individual pump action becomes smaller.
Actually, in that playground swing discussion I just linked to, there was a link to this physics simulator of a swinging incenser:
http://www.sciences.univ-nantes.fr/sites/genevieve_tulloue/Meca/Oscillateurs/botafumeiro.html
In a whole separate thread on skateboard physics, I had posted the following:
In the end, this physics simulator was helpful. It was for explaining a swing, but it serves equally well for explaining how energy can be delivered to a skateboarder on a half-pipe. In the simulator, do the following:
- click on the hand button to set the height adjustment to manual mode.
- click and drag the incenser to start it swinging gently.
- Now grab the other end of the rope and move it as follows:
-at the lowest point of its arc, pull on the rope to raise the incenser; this delivers gravitational potential energy, allowing it to swing higher on the following arc.
-at the highest point of its arc, pay out rope so the incenser has a longer swing radius.If you do this right, in fairly short order you can have the incenser swinging beyond 180 degrees of arc.
I think the physics simulator is broken now, because the hand operation of the height adjustment doesn’t seem to work. But I wrote the above when it was working, and step 3 matches what you said upthread.
As to why some skaters can do it better than others, obviously there’s going to be an optimum way to do it (optimum timing, optimum amount to move, and so on, as well as keeping your balance sufficiently that you don’t wipe out). And hitting those optima is going to be a matter of skill. As with anything else, some people are just more skilled at it than others.
This is what I meant. I was pointing out that standing on the flat part doesn’t help, and you have to push up on the inclined plane where you’re leaning into your direction of travel. This also explains why children don’t do as well as adults: they don’t push with as much power and don’t generate as much unequal forces. They don’t get as much net force in the direction of travel.
Yes, I’ve done that. If you push off in another direction, to the side, you can get a net force in that direction, with a component of that vector in your intended direction of travel.
- I forgot about that.
- Flatland competition skaters don’t do that much because it’s not as effective as pushing with a foot on the ground.
- Hi, Opal (super deep cut, there)
Aye; excellent video! Glad to see it was already posted in this thread!
Physics Girl did a cool show with Rodney Mullen to talk about this same kind of thing:
One of the guys in Richard_Pearse’s video demos that technique, more or less (at 0:58). It does depend on the lateral friction of the wheels, though, whereas the various pumping techniques would work even with omnidirectional wheels. You don’t get much acceleration because the only “push” you get is against gravity trying to tip you over. At shallow angles, that’s not much.
There are also snakeboards, with extra hinges compared to skateboards. Same basic idea but they don’t require kicking up the board since the trucks can be independently steered.
This video has a pretty good explanation I think.Note also that when skateboarding a big half pipe, you’re standing when your body is nearly horizontal, it doesn’t add potential energy by making your centre of mass higher, it adds kinetic energy by propelling your body horizontally in the direction you want to go.
Edit: And that is why being strong makes a difference. You’re launching yourself forward’s off the down ramp, and up just prior to the up ramp
Thank you!
This is what I was talking about, and it disproves this bit about the center of mass, and standing up after the ramp, and angular momentum (as far as I can tell). It’s what I said: pushing off an inclined plane gives net forces that have a component/vector in the wanted direction of travel. I don’t know where that other stuff came from.