Gravity Vehicle C

[Balsa Man]

An important misunderstanding on "low"-
Low down to the ground- as in close to the ground -when you come off the ramp- to maximize the height the center of mass has fallen (and hence, the amount of energy), not low as in low mass/less mass/low weight.

To maximize momentum you want max mass (as in weight). How good your bearings are largely determines how fast your momentum goes away, so better bearings = better speed score.

[questionguy]

I am making a curved ramp out of plywood, does anyone have any tips/advice about buying the wood, such as thickness?

[illusionist]

I think this is a given, but try to get it as thin as possible. I doubt that you would need too much strength for the ramp surface (just remember not to make it out of balsa though).

[fishman100]

I think this is a given, but try to get it as thin as possible. I doubt that you would need too much strength for the ramp surface (just remember not to make it out of balsa though).

Why would you want to make it as thin as possible? Wouldn't it be better to have thicker wood so the structure is stronger?

[illusionist]

I think this is a given, but try to get it as thin as possible. I doubt that you would need too much strength for the ramp surface (just remember not to make it out of balsa though).

Why would you want to make it as thin as possible? Wouldn't it be better to have thicker wood so the structure is stronger?

Sorry, I should have worded that differently. I was talking about only the surface of the ramp. The part that the vehicle will be running on. The side walls and supports need to be stronger.
Thinner wood gives you much more flexibility. If you have ramp with a very steep curve, then you will need more flexibility. The rest of the structure should be strong and sturdy, but the top surface does not need to. (In my opinion)

[questionguy]

I think this is a given, but try to get it as thin as possible. I doubt that you would need too much strength for the ramp surface (just remember not to make it out of balsa though).

Why would you want to make it as thin as possible? Wouldn't it be better to have thicker wood so the structure is stronger?

Sorry, I should have worded that differently. I was talking about only the surface of the ramp. The part that the vehicle will be running on. The side walls and supports need to be stronger.
Thinner wood gives you much more flexibility. If you have ramp with a very steep curve, then you will need more flexibility. The rest of the structure should be strong and sturdy, but the top surface does not need to. (In my opinion)

That's what I was thinking, but I wasn't sure if the wood would be too thin, that over time, it would weaken and possibly crack. Any thoughts?

[Balsa Man]

Just brace it well from the underside and you should have no problems. 1/8th inch thick mahogany ply, or panneling will probably work; all depends on how tight a curve you need to pull.

[fishman100]

I see, thanks for clarifying.

[questionguy]

Just brace it well from the underside and you should have no problems. 1/8th inch thick mahogany ply, or panneling will probably work; all depends on how tight a curve you need to pull.

Gracias! Moving to the the other aspect of the event, is the ideal way to build the vehicle to make it as close to the 2.5 kg limit as possible? If this were the case, would constructing the vehicle out of metal be more efficient than using wood?

[Balsa Man]

Gracias! Moving to the the other aspect of the event, is the ideal way to build the vehicle to make it as close to the 2.5 kg limit as possible? If this were the case, would constructing the vehicle out of metal be more efficient than using wood?

Thanks

To your first question, basic physics says, yes, you want it as heavy as you can. Its about momentum. Without getting into all the details/fine points, two vehicles, one heavy, one light, ignoring bearing and rolling friction, and rotational inertia of the wheels, will come off the ramp at the same velocity. The one with more mass will have a lot more momentum. Momentum is directly proportionate to mass – at the same velocity, twice as much mass = twice the momentum. As they roll along, friction, from the axle bearings, and rolling resistance, will at some rate, “eat into”/absorb that momentum, and as that happens, the vehicle will gradually slow down. The heavier one, because its putting more load on the bearings, will have somewhat higher friction. But, with decent bearings, the difference in friction (the rate at which momentum is “consumed”) will be pretty small – on the order of a few percent. So, let’s imagine running two vehicles together off the same ramp; say the heavier one is twice as heavy as the light one (has 2x the momentum coming off the ramp). If we ignore the difference in friction for a second, and assume the rate at which friction absorbs momentum is the same, what’s going to happen? The lighter one is going to fall farther and farther behind the heavy one; its going to slow down at a faster rate. With these assumptions, the heavier one is going to go twice as far. Ok, so we factor in the greater friction in the bearings of the heavier one; a few percent difference. The heavier one is not going to pull away from the lighter one quite as fast; its going to go somewhat less than twice the distance. But in terms of time to travel a given distance, the heavy one is faster.

Second question, not so much a matter of efficiency as.....practicality. Rolling down a 1m high ramp is going to have the vehicle moving at....what 2-3 (maybe a bit more) meters per second; significant forces on the chassis as it transitions to horizontal movement at the bottom; significant forces on the wheels and axles. You don’t want things – like axle/bearing alignment - moving around (absorbs energy and cuts down the precision of staying on the run line). What the chassis needs to be is strong – stiff. Metal will do that. It can also be done with wood. In some ways, wood is easier to work with than metal; depends on the tools you have access to.