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Why is it that adding weight increases time? Is it only because of increased traction? I believe someone wrote that an increase weight would increase speed coming off the ramp; to my knowledge, this is not true, because:

kinetic energy at bottom = potential energy at top
.5(mass)(velocity^2) = (mass)(gravity)(height)
velocity = radical (2*gravity*height)

therefore more mass does not equal a greater velocity at the bottom of the ramp (Unless my physics is incorrect, which is quite possible).

twototwenty wrote:Why is it that adding weight increases time? Is it only because of increased traction? I believe someone wrote that an increase weight would increase speed coming off the ramp; to my knowledge, this is not true, because:

kinetic energy at bottom = potential energy at top
.5(mass)(velocity^2) = (mass)(gravity)(height)
velocity = radical (2*gravity*height)

therefore more mass does not equal a greater velocity at the bottom of the ramp (Unless my physics is incorrect, which is quite possible).

You're right that increased mass does not result in increased velocity at the bottom.
What increased mass gets you is greater momentum (mass x velocity). It is the momentum that friction and rolling resistance "eat into/gobble up." With....not-squishy wheels, that rate of momentum loss is pretty comparable with lower or higher mass, so more momentum gets you to target distance faster (the rate at which it slows down is slower). In the case NinjaChicken raises, its not increased traction, its increased wheel deformation (and the energy/momentum that absorbs) that's slowing things down/increasing time.

The center of mass is the important part; higher CM= more GPE transferring to KE

lucwilder42 wrote:The center of mass is the important part; higher CM= more GPE transferring to KE

Just to elaborate/clarify a bit.
Higher, as in starting its fall higher above the ground. Its the total height/distance the CM falls that determines CE. So, that means not only do you want to start the CM as high up as possible, you also want to have it fall as far down as possible- as in end up low/close to the floor. Obviously, there are tradeoffs....
Getting the CM as high as possible would involve a light chassis with a big chunk of mass way back at the rear of the vehicle. That, however, when it gets to the floor, means a heavy load on the rear wheels, and very little load on the fronts, which means real problems getting it to run in a straight line. If your weight is distributed evenly between the axles, though, then your CM is starting 1/2 the length of your wheelbase (+ 1/2 the diameter of your rear wheels) down the ramp - below the 1m ceiling by-10, 20cm or more.
How to optimize weight distribution is another one of the many interesting facets of this event.

As we found in our case though, adding 2.5 kg in order to raise the CoM as high as possible isn't always best. The friction and energy loss was too much, and it negatively impacted our time. Our current CoM is ~90cm high from the floor.

How big is your car that its CoM is 10 cm lower than the top of the hieght limit?

twototwenty wrote:How big is your car that its CoM is 10 cm lower than the top of the hieght limit?

Well, the top of our ramp is at 96cm, and the center of mass in relation to the vehicle is about 4 or 5 cm from the rear end of the vehicle (the weight that we use is placed directly over the rear axle).

Those of you who have been to competitions, have you seen anything really impressive or unique?

Yesterdays Invitational at Wright State University had an unusual interpretation of the rules, specifically the track layout. The center point of the start line and the target point were clearly not perpendicularly aligned.

ohiostar wrote:Yesterdays Invitational at Wright State University had an unusual interpretation of the rules, specifically the track layout. The center point of the start line and the target point were clearly not perpendicularly aligned.

Interesting. Did you talk to the supervisor about this? I stopped by Gravity Vehicle briefly and didn't notice anything particularly out of whack, but also wasn't looking that closely. How far off do you think it was?