## Gravity Vehicle C

Flavorflav
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### Re: Gravity Vehicle C

fishman100 wrote:1) Final velocity = sqrt(2*height*gravitational acceleration)
but in another post earlier in the year final velocity was said to have been the sqrt(height * (gravitational acceleration)).

Don't know where the 2 in the first equation came from.

2) Time it takes for a falling mass to fall 1 meter = sqrt((2*distance)/9.8)), so at a 1m distance it takes about 0.452 seconds to drop
3) Velocity = same formula as final velocity; v = 4.427 m/s^2 --> 2.10 m/s
4) One of Balsa Man's runs at regionals: 2.61 seconds at 6.5 m --> 2.49 m/s
If you compare #3 and 4 they don't match up, and what puzzles me is that #3 theoretically the best possible time...

5) To calculate the force of gravity (gravitational acceleration) multiply 9.8 by the mass of the object. So for last year, that would be 9.8*2.5 = 24.5 N
http://www.wikihow.com/Calculate-Force-of-Gravity
Plug that into #1 and you get 7 m/s^2 --> 2.64 m/s, but that doesn't match with #3 or 4. Close but it's also theoretical, however it differs from the other theoretical equation (#3) by 0.54, which is a long time for something that should have the same results...
I don't know what you are doing, either. It works the same way whether you use kinematics or energy.
1: You calculated the time correctly, but the velocity incorrectly. Vf=at, so with t=0.452 Vf would be 4.43 m/s
2: mgh=1/2mv^2, so v^2=2gh and, given h=1, v=(2g)^1/2, which is 4.43 m/s.

Balsa Man, when you said the sliding mass produced faster times, you were comparing it with a fixed-in-the-center mass, right? It should not improve your time over a low fixed-in-the-rear mass, so the benefit you are getting over the latter is really in accuracy rather than speed, is it not?

illusionist
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### Re: Gravity Vehicle C

Flavorflav wrote:Balsa Man, when you said the sliding mass produced faster times, you were comparing it with a fixed-in-the-center mass, right? It should not improve your time over a low fixed-in-the-rear mass, so the benefit you are getting over the latter is really in accuracy rather than speed, is it not?
Yup, that was what I was getting at with my question.
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### Re: Gravity Vehicle C

illusionist wrote:
Flavorflav wrote:Balsa Man, when you said the sliding mass produced faster times, you were comparing it with a fixed-in-the-center mass, right? It should not improve your time over a low fixed-in-the-rear mass, so the benefit you are getting over the latter is really in accuracy rather than speed, is it not?
Yup, that was what I was getting at with my question.
Yes (mainly) and no.... Faster when the mass starts at the ceiling/max height (and slides down into the "bay" in the middle of the chassis), than when you start with the mass already down in the bay (center of mass falls farther). In that the (moving) mass is positioned a few cm above the plane of the chassis, if you were to release the vehicle w/ the mass in that position (and not sliding down into the chassis), it would, of course, be above the chassis at the bottom, meaning you'd loose those few cm of drop height (hence lower V off the ramp). However, if it were positioned in the plane of the chassis (i.e., "low fixed in the rear mass"), at/off the back, you'd get the same V as you would with the moving mass setup. But with the mass concentrated in/off the back, straight-line capability is degraded, so, compared to a.... configuration that gives good straight-line performancee, it's faster. It's really a combination - optimization - of both V and accuracy.
Len Joeris
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illusionist
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### Re: Gravity Vehicle C

Balsa Man wrote:
illusionist wrote:
Flavorflav wrote:Balsa Man, when you said the sliding mass produced faster times, you were comparing it with a fixed-in-the-center mass, right? It should not improve your time over a low fixed-in-the-rear mass, so the benefit you are getting over the latter is really in accuracy rather than speed, is it not?
Yup, that was what I was getting at with my question.
Yes (mainly) and no.... Faster when the mass starts at the ceiling/max height (and slides down into the "bay" in the middle of the chassis), than when you start with the mass already down in the bay (center of mass falls farther). In that the (moving) mass is positioned a few cm above the plane of the chassis, if you were to release the vehicle w/ the mass in that position (and not sliding down into the chassis), it would, of course, be above the chassis at the bottom, meaning you'd loose those few cm of drop height (hence lower V off the ramp). However, if it were positioned in the plane of the chassis (i.e., "low fixed in the rear mass"), at/off the back, you'd get the same V as you would with the moving mass setup. But with the mass concentrated in/off the back, straight-line capability is degraded, so, compared to a.... configuration that gives good straight-line performancee, it's faster. It's really a combination - optimization - of both V and accuracy.
Okay. So it's wrong to think that the vehicle gets an extra boost or kick because of the mass slamming into the vehicle's bay, right? All it does is just give the vehicle a faster acceleration when the mass contacts the bay?
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iwonder
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### Re: Gravity Vehicle C

All the mass slamming into the vehicle does is effect a quick transfer of energy from the mass to the vehicle, this causes quick acceleration, but it's the same effect as the whole vehicle(the center of mass) starting from the height of the mass.

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### Re: Gravity Vehicle C

iwonder wrote:All the mass slamming into the vehicle does is effect a quick transfer of energy from the mass to the vehicle, this causes quick acceleration, but it's the same effect as the whole vehicle(the center of mass) starting from the height of the mass.
Yup. Part of the design is getting a good elastic collision (lexan plate on the striking edge of the weight, lexan plate on the struck edge of the bay, plates aligned for good contact)- transferring as much of the energy of the falling mass into the vehicle as possible. Like what happens in billiards when you shoot the cue ball into another ball; cue ball stops, other ball heads off... But in this configuration, at the time of contact, the weight becomes part of the vehicle. One of the fun engineering bits was timing the release of the vehicle so just before the mass hits, the vehicle is freed to roll.
Len Joeris
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### Re: Gravity Vehicle C

Assuming that you used some variation of balsa man's "accelerator" and you have a very light and strong front end, a long car would have about a third of a meter head start on a short car. This could amount to as much as 5% less time on a short track with all else equal, namely a true track and a good brake system. Our team ran a very very compact car and it ran true and fast. I suspect that we will try a long car to see if it will track true. If it fails, then back to a shorter car.

I like the wing nut brake for the simplicity and ease of use.To reduce the friction, I chase the threads on the all thread and could re-tap the wing nut if it was still a bit too much friction. Silicon and / or graphite should reduce to friction to very little.

I wonder if a third axle would be legal?

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### Re: Gravity Vehicle C

Not commenting on the legality of such a design, just out of curiosity, what would the utility of the third axle be?
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### Re: Gravity Vehicle C

chalker7 wrote:Not commenting on the legality of such a design, just out of curiosity, what would the utility of the third axle be?
As a guess, maybe he plans on using it for the paperclip.. Other than that i have no clue... Now, i'm really curious to why someone might have a third axle
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### Re: Gravity Vehicle C

How does a 3rd axle help with the paperclip?

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