[John Gamel]

John Gamel  asked the Naked Scientists:
   
Dear Chris,

Despite the many courses I've taken in physics, chemistry and mathematics, I don't know the answer to this important question:

When someone rollerskates, how do they get the traction that drives them forward? Any force exerted directly backwards can't work, since the wheels on the skates prevent traction. So where does the "push" come from that allows skaters to go so fast?

John Gamel, MD

What do you think?

[lyner]

Hi John
They have to slant their skate so they can exert a force backwards against the ground. They'd never get going (or stop) if both their skates were just pointing fore and aft.
If you watch ice skaters on TV you will see that their back foot is alout at right angles to the front foot as they launch off. They just lean forwards (sort of sideways, usually), push and off they go.
Speed skaters seem to push back and side to side as they're accelerating.
Skiers have the same problem when they want to stop. They jump their skis around so they are across the direction of motion - with the show-off shower of snow.

There's a related question concerning skateboarders. Ever wondered how the boards 'stick' to their feet when they jump? Again, there seems to be a force coming from nowhere.

[RD]

The same "problem" occurs with ice skating,
the solution the skates are angled: they do not run parallel when pushing forward ...
http://www.youtube.com/watch

[lyner]

Why bother angling them? Your feet are on Universal Joints and will get just the correct angle for you. The angle must be pretty small - I had a pair for a while and it was not obvious. They would drag your legs apart if you put both skates down at once. Speed skates are a bit specialist, having a limited application and the angle may be put there on purpose - the blades are about twice as long as normal, too.

[RD]

I meant angled by the skater, as seen in the video I linked to, (slow-mo @ 0:22).
The skates are not parallel when the skater pushes forward.

[LeeE]

Ice skates and in-line roller skates are not flat along the bottom but have a varying degree of camber along the running edge.  For dancing and hockey, where maneuverability is needed, the degree of camber is quite large, whereas for speed skating it is relatively small.  This means that the entire length of the blade, or set of rollers, is only in contact with the ice/ground when it is angled over slightly.  When you push down, the force tends to center the skate against the line of thrust as it's running in an arc.  Conventional roller skates, with a wheel at each corner have a similar steering mechanism whereby angling the skate over turns the pairs of wheels in or out, producing the same arc-following effect.

In both cases, the 'pushing' blade or wheel-set doesn't have to be at right-angles to the direction of movement, and in fact, once you're moving faster than you can push with your leg, it won't work.  You then need to reduce the angle so that you're almost pushing sideways if you want to further increase your speed.

[lyner]

I meant angled by the skater, as seen in the video I linked to, (slow-mo @ 0:22).
The skates are not parallel when the skater pushes forward.

Oh I get it now. You were agreeing with me!!

[lyner]

LeeE
I agree that the blade doesn't need to be at right angles - it just needs a component that isn't parallel. Also, as you say, the faster you go, the shallower the angle needs to be. This is a bit like the action of 'gears' on a bike - small sideways movement of the leg provides a big forward movement. It's, in effect. an 'inclined plane' machine.

[LeeE]

Oops! - the entire length of the blade doesn't make contact if you lean it over...   but once it has cut into the ice a little, or when the wheels on an in-line skate have compressed a bit, so that several are making contact, as the skate is leaned over the contact points are in an arc so you still get the self-centering effect when you push through it.  It's a very small effect but crucial if you want to put a lot of power through the pushing leg without the skate simply slipping backwards or forwards.

[Karsten]

In some ways one might want to talk about what "forward" means. When you roller skate, inline skate, or ice skate you may move forward but if you look at your shoes while this is happening you are going more in a zig zag fashion. As said, the faster you move the less pronounced is the zig zag. You push from the zig-direction to go the zag-direction and since you are pushing a bit sideways you can use the blade/wheels to push. And of course this only works if you lift your foot of the ground when done with pushing (or make a turn).

It gets complicated if you want to try the double push technique.
http://www.nettracing.com/step1.htm

 

[lyner]

It's all resolution of forces and components and things.

[Geezer]

It's all resolution of forces and components and things.

Pretty technical SC!

I recently learned to skate on in-line skates, so I must be an expert on the subject  []

One thing I noticed was that certain muscles in my lower back and thighs were taking a pounding. Not, apparently, the same ones mainly involved in walking.

I think this is because, most of the propulsive force is exerted laterally between the skates. In other words, you are putting a lot of energy into trying to increase the distance between the skates roughly at right angles to the direction of travel.

The skates produce minimal friction with the surface in the straight ahead direction and a large amount of friction normal (at right angles) to the straight ahead direction.

So, you steer the skates slightly outwards relative to each other, and as you try to force them further apart the forces resolve to produce forward motion. If it was not for the significant amount of lateral friction imposed by the skates, you would instead do the splits and probably end up in an emergency room.

Now, obviously, unless you have really long legs, this lateral pushing thing can only go on for a short interval, so when you run out of leg, you lift the rear skate off the ground and glide on the front skate, meanwhile bringing the rear skate up and placing it in front of what was the front skate. The sequence is then repeated.

You can test this by standing still on skates. Create a small outward angle between the blades and try to force them apart. You will move forward as the distance between your feet increases. You can actually propel yourself by alternately pushing outwards and pulling inwards, but it's extremely tiring.

It is possible to go very fast on skates because you have reduced mechanical advantage (think "high gear" on a bicycle) compared to running. When skating, a small amount of lateral motion results in a considerable amount of forward motion. The lateral force you exert only produces a small amount of forward thrust, but because you have very little resistence to forward motion (due to the anti-friction means) you can glide along with (apparently) very little effort. It's only when you stop that you realize you were really generating quite a lot of force.

Least, that's my theory!

(Most of the above works with skis too.)

[lyner]

Sorry. I thought Science was a technical subject.

[Geezer]

LOL!  []

SC, it was a joke! I was referring to your highly technical term "and things"

[lyner]

Fairy Nuff.

[Geezer]

Anyway, what did you think of my "theory"?

Maybe I should re-phrase that! Do you see any major problems with my "theory"?

[lyner]

Yes, it's very much like infinitely variable gears. The finer the slope / angle of the skate, the faster you can go for a given leg speed. It's like squeezing an orange pip between finger and thumb to produce a forwards motion - only in this case you're pushing backwards and outwards - once you get going.
The problem for the untrained skater must be that, as you say, the muscles you use are more side to side drivers than backwards drivers, as in normal running; lots of sideways pressure without a lot of movement - very cramp-inducing, I should think.
There's a parallel with yacht sailing - the keel takes a lot of sideways force for what may be a small force forwards.

[Geezer]

I must admit I flubbed the bit about weight transfer. There's a lot of that going on too, but it's difficult to describe or analyse. An adequate description would probably run into many pages!

The whole business is extemely complicated. The remarkable thing is that we can learn to do it at all.

Yes, there must be good analogies with sailing, but, as I only have a motor boat, I'll stay well away from that one.  []