Gravity Vehicle C

[Balsa Man]

Okay, so I get that the distance that the center of mass falls is what influences the velocity, but I still can't seem to grasp that a ramp shaped like this (turned 90 degrees so that the the longest side will be vertical) will result in same speed as a ramp shaped like this.

Since the first ramp is steeper and allows the mass to fall most vertically down without being deflected horizontally, won't it result in greater speeds? I've been re-reading your posts, but I still dont get it... >.< Surely the difference in ramp shape between the first and second images has an impact on velocity right?
Edit: You've said that the "almost vetical" thing doesnt affect horizontal v, but rather how high and low the center of mass is. I understand the yes-or-no part, but I dont understand the why.

This isn't related so much to my planning for the event right now, but just personal learning.

Okay, another question, relating to the rules. It says that the release mechanism must be a pencil (line 3.h), but it does not state unlike last year's mousetrap rules that the pencil must be used in a vertical manner. So based on that line of the rules, I can use the pencil to pull out a release mechanism right? I just want to confirm it. And I know this isn't the place for official clarifications, these are solely opinions, etc.

I certainly understand why you're asking (ramp shape, V, t)
Did you read the paper I suggested looking at back on Sept 21? -
"What seems to be at question here, and it's the hard part to get one's head around, is whether that means an object coming off the brachistochrone curve ramp will have a higher horizontal velocity than the object off the flat ramp. Psychology/human perception bumping into physics. It would seem so, but, as I now understand it, that is not how it works

Here's an interesting paper- see page 2 onto 3 particularly.
http://uweb.cas.usf.edu/~drohrer/pdfs/Rohrer2003M&C.pdf"

When we first started thinking about the physics of this event in September, I had the same.....impression/perception. Studied b-curves, etc. Then I checked with my sons- one working on his PhD in math (w/ a strong physics background), one doing undergrad astrophysics, and my stepson doing undergrad physics, and I found this article. Also, as noted did small ramp test. My conclusion at this point is a) within limits that matter (small fraction of a second for ramp travel time), shape does not matter - will not get you more than that small fraction of a second, and it not get you different/greater horizontal V.
Sorry, but I don't understand what you're referring to "the yes or no part."

[chalker]

Can someone go more in depth about the possibility of using elastic devices? I am a little confused as to what would be allowed and the extent they would help the vehicle.

Are you referring to rule 3.c.? Just some general guidance, there is no such thing as perfect energy conversion. You always lose a little bit of energy whenever you switch from one form (e.g. gravitational potential) to another (e.g. stretched elastic). Thus if you want the maximum energy to get to propelling the vehicle forward, you should minimize any intermediate steps.

[illusionist]

Okay, so I get that the distance that the center of mass falls is what influences the velocity, but I still can't seem to grasp that a ramp shaped like this (turned 90 degrees so that the the longest side will be vertical) will result in same speed as a ramp shaped like this.

Since the first ramp is steeper and allows the mass to fall most vertically down without being deflected horizontally, won't it result in greater speeds? I've been re-reading your posts, but I still dont get it... >.< Surely the difference in ramp shape between the first and second images has an impact on velocity right?
Edit: You've said that the "almost vetical" thing doesnt affect horizontal v, but rather how high and low the center of mass is. I understand the yes-or-no part, but I dont understand the why.

This isn't related so much to my planning for the event right now, but just personal learning.

Okay, another question, relating to the rules. It says that the release mechanism must be a pencil (line 3.h), but it does not state unlike last year's mousetrap rules that the pencil must be used in a vertical manner. So based on that line of the rules, I can use the pencil to pull out a release mechanism right? I just want to confirm it. And I know this isn't the place for official clarifications, these are solely opinions, etc.

I certainly understand why you're asking (ramp shape, V, t)
Did you read the paper I suggested looking at back on Sept 21? -
"What seems to be at question here, and it's the hard part to get one's head around, is whether that means an object coming off the brachistochrone curve ramp will have a higher horizontal velocity than the object off the flat ramp. Psychology/human perception bumping into physics. It would seem so, but, as I now understand it, that is not how it works

Here's an interesting paper- see page 2 onto 3 particularly.
http://uweb.cas.usf.edu/~drohrer/pdfs/Rohrer2003M&C.pdf"

When we first started thinking about the physics of this event in September, I had the same.....impression/perception. Studied b-curves, etc. Then I checked with my sons- one working on his PhD in math (w/ a strong physics background), one doing undergrad astrophysics, and my stepson doing undergrad physics, and I found this article. Also, as noted did small ramp test. My conclusion at this point is a) within limits that matter (small fraction of a second for ramp travel time), shape does not matter - will not get you more than that small fraction of a second, and it not get you different/greater horizontal V.
Sorry, but I don't understand what you're referring to "the yes or no part."

Okay, I'm beginning to understand this now. I talked to a physics teacher and she was able to explain it pretty well (not that you didn't). By the "yes-or-no" I meant that I got that shape doesn't matter. I just didn't understand why until now. Thanks for spending time to explain this.

[Balsa Man]

Glad its been helpful - a good example of what this board is good for.

It is all somewhat non-intuitive; easy to get confused.

Good luck on your GV!

[questionguy]

I was going over last year's powerpoint of Mousetrap Vehicle and something interesting that I saw was that the thicker the axles the faster your vehicle will go. When considering this, is it more effective to build your vehicle with say, 1/2 inch axles with no bearings, than 1/4 inch axles with bearings? The wheels on both options would be the same.

[fishman100]

I was going over last year's powerpoint of Mousetrap Vehicle and something interesting that I saw was that the thicker the axles the faster your vehicle will go. When considering this, is it more effective to build your vehicle with say, 1/2 inch axles with no bearings, than 1/4 inch axles with bearings? The wheels on both options would be the same.

Well you would naturally want to stick with the bigger axle (watch your weight limit though), but wheels without bearings would cause more friction. If you can find a 1/2" bearing then I would use that.

[bwy]

I was going over last year's powerpoint of Mousetrap Vehicle and something interesting that I saw was that the thicker the axles the faster your vehicle will go. When considering this, is it more effective to build your vehicle with say, 1/2 inch axles with no bearings, than 1/4 inch axles with bearings? The wheels on both options would be the same.

As an extra note, the vehicle only went much faster with thicker axles in the mousetrap car because of the winding of the string around the axle. In this case, it wouldn't matter quite so much, because it's not propelled by a wound string.

[Balsa Man]

I was going over last year's powerpoint of Mousetrap Vehicle and something interesting that I saw was that the thicker the axles the faster your vehicle will go. When considering this, is it more effective to build your vehicle with say, 1/2 inch axles with no bearings, than 1/4 inch axles with bearings? The wheels on both options would be the same.

As an extra note, the vehicle only went much faster with thicker axles in the mousetrap car because of the winding of the string around the axle. In this case, it wouldn't matter quite so much, because it's not propelled by a wound string.

bwy is absolutely correct- the effect of bigger axle on a MTV was ONLY in power/speed from the trap pullong a string wrapped around it. This has nothing to do with, no relartionship or meaning to the question of speed of a gravity-propedlled vehicle- apples and oranges.

You want axles big enough to be stiff enough to carry the load.
Bearings will work better (i.e. will suck energy out at a lower rate than no bearing)
Going back to earlier discussions on "what matters" - within a reasonable range, bearing/axle size will not meaningfully change the rolling friction

[questionguy]

I was going over last year's powerpoint of Mousetrap Vehicle and something interesting that I saw was that the thicker the axles the faster your vehicle will go. When considering this, is it more effective to build your vehicle with say, 1/2 inch axles with no bearings, than 1/4 inch axles with bearings? The wheels on both options would be the same.

As an extra note, the vehicle only went much faster with thicker axles in the mousetrap car because of the winding of the string around the axle. In this case, it wouldn't matter quite so much, because it's not propelled by a wound string.

bwy is absolutely correct- the effect of bigger axle on a MTV was ONLY in power/speed from the trap pullong a string wrapped around it. This has nothing to do with, no relartionship or meaning to the question of speed of a gravity-propedlled vehicle- apples and oranges.

You want axles big enough to be stiff enough to carry the load.
Bearings will work better (i.e. will suck energy out at a lower rate than no bearing)
Going back to earlier discussions on "what matters" - within a reasonable range, bearing/axle size will not meaningfully change the rolling friction

Thank you all for the advice, I think i'll stick with the bearings. As for vehicle design, does it make a significant difference if the car is in the shape of say, a dragster, versus a rectangular block of wood?

[Balsa Man]

Yes, depending on what you mean by shape.

If you’re thinking about aerodynamic drag, a streamlined, dragster-like shape would have less than a .....chunky block of wood. Shape, and cross-section (frontal area) would determine that. Would that be a “significant” difference – more than a fraction of a second? Possibly, depending on how streamlined dragster-like, or how chunky block of wood-like. The effect; the difference in run time, would be significantly less than the difference in bearings/no bearings.

If you’re thinking about wheelbase length and where the center of mass is, yes, it could make a significant difference. There has already been pretty extensive discussion of how and why in this thread- worth re-reading. In this context, dragster implies, to me, long wheelbase, weight low (as in close to the axle line), block of wood implies shorter wheelbase, weight higher (part of the “block” well above the axle line).

As has been said many times, the energy going into the vehicle, which primarily determines its horizontal velocity, depends on how far the center of mass falls – “h”; height over which gravity acts. You want to maximize h – center of mass as high up when you start, and low down to the ground as you come off the ramp as you can..

If the weight is evenly distributed throughout the chassis, a long wheelbase is going to have less h than a short one- its going to take h off at the top. If its, say 50 cm long, then the center of mass is going to start 25cm down the ramp (the midpoint of the wheelbase). If its 25cm long, the center of mass is going to start 12.5 cm down the ramp; you loose 12.5cm of h. However, if you take the wheelbase too short, you are going to loose linear stability- it will tend to randomly kick off the intended ‘run line.”

If the weight is not evenly distributed, then it depends on how you’ve done that. Again, its center of mass that drives things.

If you have a long, or short wheelbase, and you concentrate the weight toward the back (as in build light, and put a chunk of mass back toward the back- a piece of metal, a rock, whatever), you gain h. How much gain depends on how far back, how much mass, and on how much the parts of the chassis in front weigh, compared to the chunk of mass. The tradeoff here is, if you concentrate the mass too far back – make the center of mass really close to the rear axle, you again are inviting linear instability. Maximizing h at the bottom says you want the center of mass as close to the ground (once the vehicle is on the floor) as you can; at, preferably below the axle line.