Gravity Vehicle C

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Re: Gravity Vehicle C

Post by retired1 » October 10th, 2012, 3:03 pm

For us old folks, would you expound on "starting as high as possible" and on "sliding mass" and how your achieved that.

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Re: Gravity Vehicle C

Post by iwonder » October 10th, 2012, 4:25 pm

He wrote a very detailed post in last year's thread, around May I believe, it's basically a mass that starts on a set of rails, falls from as high as possible, and lands in a 'pocket' on the bottom of the vehicle, right?

And no, I would never try and machine titanium ;)
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Re: Gravity Vehicle C

Post by Balsa Man » October 10th, 2012, 4:42 pm

retired1 wrote:For us old folks, would you expound on "starting as high as possible" and on "sliding mass" and how your achieved that.
With all due respect, I'm first going to say is what I get to say many times each year when a new kid pops up 15 pages of discussion into an event, and says, "so folk, how do I make a good one of __ fill in the blank___?"

Go back and read- that's one of the great values of this forum- the accumulated discussion and knowledge.
Being an old person myself, I'm not going to re-type my discussion of how we did this - it is there from last year, and there's a link to a video.
Quality is not very good, but.....its there. Not going to go into anymore detail on how, because the guys asked me not to. They're ok with coach sharing .....cool things we discover/develop up to a point, and we're at that point on the 'gravity supercharger.' -because it does work.

There's also a lot of discussion on the value of starting as much mass as high as possible, and getting as much mass as possible down as low as possible - the basic physics of where the energy comes from that translates into horizontal velocity. Understand the context- last year, there was a 1meter ceiling limit- nothing could stick up above a meter, prior to start. This year, starting ceiling is a new varible. At whatever starting height/ceiling height, what determines the velocity you come off the ramp at is how far the center of mass falls- as the vehicle rolls down the ramp, and transitions to the floor. That's why you want as much mass as possible as high as possible - i.e., as close to your ceiling as possible when you start, and as much mass as possible down close to the floor when you roll off onto the floor. The first part of the equation is easy- build light, hang a big hunk of mass on the back end of the vehicle- that gets center of mass as high as you can, and if it's at or below axle level, it's down close to the ground when you hit the floor. But with mass concentrated behind the rear axle, load on the front wheels will light- and the puppy will want to wander. We started with those basic physics. We found the problem of wandering with a heavy rear weight bias. We asked the question, so how can you have most of the mass at the back when you start, and low and in the middle when you finish? The answer was obviously, "hmm... it has to move." We figured out howw that could actually be done. There are other ways. Bottom line, with the 2.5 kg weight limit last year, we picked up a goodly portion of the kinetic energy of over 1.5kg falling 8+cm (compared to just starting w/ the moveable mass already in the center of the chassis. Pretty good energy shot - enough for ....a few tenths of a second faster to 7.5m
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Re: Gravity Vehicle C

Post by illusionist » October 10th, 2012, 5:23 pm

Balsa Man wrote: With all due respect, I'm first going to say is what I get to say many times each year when a new kid pops up 15 pages of discussion into an event, and says, "so folk, how do I make a good one of __ fill in the blank___?"

Go back and read- that's one of the great values of this forum- the accumulated discussion and knowledge.
Being an old person myself, I'm not going to re-type my discussion of how we did this - it is there from last year, and there's a link to a video.
Quality is not very good, but.....its there. Not going to go into anymore detail on how, because the guys asked me not to. They're ok with coach sharing .....cool things we discover/develop up to a point, and we're at that point on the 'gravity supercharger.' -because it does work.
Yup, Balsa Man's right. If you look somewhere around page 49-51, there should be a pretty good summary of the problem and solution of the "supercharger". Balsa Man really has shared a ton of useful information and insights that his team has learned. I wouldn't ask for more details about the specific mechanisms other than maybe the physics (I haven't taken physics yet, and so I'm just learning from what I can find online).

About the physics behind it all, there is A LOT of discussion about falling masses and transitioning between vertical and horizontal velocity during the first 25% or so of the 2011-2012 Gravity Vehicle forum. Definitely worth reading (or re-reading).

The stuff above was directed at any member who might be new to the event or this forum.
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Re: Gravity Vehicle C

Post by retired1 » October 10th, 2012, 6:20 pm

Just came from there. I did not know that this site existed last year, so when I found it this year, I looked at a couple of pages of last year and did not find much of value. I got back to pg 45 and there was a lot of useful information.

I watched a lot of the nationals as an interested visitor and did not see Bear's team run. Will have to haul out the pictures.

The winning team had a rathe poor first run but had a fantastic second run.

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Re: Gravity Vehicle C

Post by iwonder » October 10th, 2012, 6:32 pm

I'm fairly certain bear isn't on a team ;)

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Re: Gravity Vehicle C

Post by illusionist » October 10th, 2012, 7:01 pm

retired1 wrote:Just came from there. I did not know that this site existed last year, so when I found it this year, I looked at a couple of pages of last year and did not find much of value. I got back to pg 45 and there was a lot of useful information.

I watched a lot of the nationals as an interested visitor and did not see Bear's team run. Will have to haul out the pictures.

The winning team had a rathe poor first run but had a fantastic second run.
Just to clear up some confusion, Bear/bearasaurus was the event supervisor at nationals. He's a grad, and isn't part of any team. Balsa Man (above), is coaching a division C team.
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Re: Gravity Vehicle C

Post by fishman100 » October 10th, 2012, 8:08 pm

Currently I'm trying to compare last year's scores to this year's scoring formula to see what a "good score" from last year is this year (I'm assuming that the height of the ramp is 100 cm when I put scores from last year through this year's formula). I'm also trying to run some numbers through this year's formula with the current specs, however I'm having some trouble because I can't calculate the speed of the vehicle because of the change in mass. I searched through last year's thread and found some equations, but the numbers I came up with don't make any sense (I'm going to say right now that I haven't taken physics yet). Here's what I have for calculations so far:

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)).

Taken from: Link and Link

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

Taken from: Link and Link

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 I'm doing wrong (I also don't really know what I'm doing :P ) but since the max weight has been reduced this year I'd like to find the correct equations so I can see how that affects the speed of the vehicle.
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Re: Gravity Vehicle C

Post by iwonder » October 10th, 2012, 8:40 pm

Ahh, it seems like you're trying to use kinematics when you want to use energy equations... Remember that the ramp is curved, so more than 1 meter distance, and that the force that's acceleration the vehicle(parallel to ramp surface) changes in a curved ramp.

The easier way. We know tha at the top of the ramp(1m), the vehicle has a certain potential energy, calcuated by the equation u=mgh, so we know that u=(1.5)(9.8)(1)=14.7J. Since this is equal to the kinetic energy at the bottom(by the conservation of energy), and we know that Uk=.5mv^2, we can deduce that 14.7=.5(1.5)v^2, solving this, v=sqrt(4.9), that's your maximum velocity(excluding friction).

Now lets get rid of the numbers. Because the energies are equal, we can say that mgh=.5mv^2. The mass terms cancel out, then we can multiply both sides by 2, yielding 2gh=v^2, lastly, we take the square root of both terms, and we end up with sqrt(2gh)=v, which is the equation to calculate the max velocity of the vehicle falling from any height.

Notice how the mass no longer matters, since none of this includes friction, it isn't very obvious at first, but we want to push the mass of the vehicle to increase its momentum and make sure that friction doesn't slow it down as fast as it would for a lighter vehicle.

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Re: Gravity Vehicle C

Post by _HenryHscioly_ » October 10th, 2012, 11:11 pm

v=sqrt(14.7*2/1.5)=sqrt(19.6)

about the "gravity supercharger", in regard to rule 5.e
In your opinion, what is the first sentence trying to say?
my opinion. at first, I was thinking, the car can't split into two cars...? lol, that would be interesting
A car that changes shape..or something similar, I would possibly term it as, "moving as a whole" since its not separating or anything....
Like when we walk with a group of people..the group is moving as a whole..?but the people can move around and change positions within the cluster of people, but the group is still moving as a whole...?
but when I thought about having a mass moving before the vehicle starts moving, it could possibly be seen as, not moving together..not moving simultaneously?and therefore not moving as a whole?

Also I was wondering, does anyone know why the national rules clarification doesn't open right at the beginning of the season but in mid-October?

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