Skate board physics

Factual Questions
1

I’m currently working on a TV commercial, were I have to animate a 3D character doing some skateboarding tricks, perusing through many reference videos I noticed that, contrary to what I had thought (or rather never given any thought), skaters doing their thing in ramps and bowls actually can gain speed going from side to side without kicking against the ground… pardon my complete lack of lingo, skateboarding wasn’t my thing as a kid. :o

That got me thinking by what process they increase their speed, just from watching carefully it would seem that they’d crouch slightly at the bottom of a ramp and then straighten up a little just as they enter the upward curve; but I haven’t quite deciphered how that would add more momentum.

Any ideas?

2

If you crouch when on the way down, before you’re going horizontal, you add a little more of the potential energy in your body to kinetic energy downwards, adding to your speed. If you straighten up just as you enter the upwards curve you’re pushing down on the ramp adding vertical speed to your body just as the ramp turns your horizontal speed into vertical. Both add to your total velocity.

Or in simpler terms, you add a little fall at the downward side and sorta jump a little up at the upwards side.

3

It’s basically the same process as “pumping” on a swing, if that helps.

4

I posed a similar question in this thread:“Explain the kinematics of a playground swing.”

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:

  1. click on the hand button to set the height adjustment to manual mode.
  2. click and drag the incenser to start it swinging gently.
  3. 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.

Same deal with the skateboarder on the halfpipe: crouch while at the highest point, and as you cross the trough, stand up. You did mechanical work raising your center of mass (or preventing it from going lower while your feet went lower), increasing your gravitational potential energy, and this will enable you to travel farther up the opposite slope.

5

I see, I had a half backed thought of it being similar to a swing scenario, but I hadn’t piece it together yet.