# Rotational Inertia of a skateboarding wheels?

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1. Aug 30, 2016

### skoande

• Member advised to use the homework template for posts in the homework sections of PF.
I've gathered data of a skateboard going up an include and rolling back with a force plate.
- Vertical force
- Velocity

The vertical force graph looks like this:

The first bump is when the skateboard rider hits the incline.

I'm doing an investigation and I don't know how this force and velocity can relate.
Can someone more experienced lay out related concepts and things I can look into? What things I can compare?

2. Aug 30, 2016

### Staff: Mentor

How did you measure the force of what?

What does this have to do with the tiny rotational inertia of the wheels?

Do you know a relation between force and something that is connected to speed?

3. Aug 30, 2016

### skoande

I put a force plate on the skateboard. It reads 680N just by standing on it (I weigh 68kg; 1kg = 10N so 86*10=680N).
When I enter the slope, the reading of the plate goes up, followed by going down.

As for the rotational inertia, I was thinking of the formula of energy conversion of a ball rolling down on an incline. PE converts to rotational KE and transitional KE.
I have found this formula: mgh = (1/2)mv2 + (1/2)Iw2

If it were to just slip down (no rotational energy) we would just have mgh = (1/2)mv2
But a skateboard has 4 wheels...

My initial idea was how an added force (through "pushing" down with my legs at the incline) affects the exiting velocity.
Does anyone know if there's some kind of theoretical formula that I could compare with my data?

4. Aug 30, 2016

### Staff: Mentor

Compare the mass of those wheels to your mass. I don't know you and your skateboard, but I would expect your mass to be several hundred times the mass of the wheels.
It will affect the exit velocity, indeed.

As your force plate tilts, the force it measures changes its direction, which can make the analysis difficult, but if the incline is not too steep this probably doesn't have a large effect.

5. Aug 30, 2016

### haruspex

In trying to understand what contributes to the measured force, the role of your legs is crucial. Think about what the force has to achieve. You and the skateboard have to change direction quite quickly. That's a change in velocity, so implies an acceleration. The greater the speed, the greater the change in velocity, and the faster it has to happen.
Unfortunately, or fortunately, you and your legs are not a rigid structure. You naturally absorb shock with your legs. That spreads the acceleration of your body out over a longer time. The more you allow your legs to do that, the lower the registered peak force. (But the extra force will persist longer. What do you think might be constant and why?)

What you wrote above suggests that you are not just using your legs as shock absorbers, but actively using your muscles. That will change things yet again, though probably to a lesser extent.