Gravity Vehicle C

[mrsteven]

I think I'm going to make sure the event supervisors can measure the height of my vehicle accurately. It would be unfortunate if they measured it incorrectly because they can't access it.

I was about to recommend retired1's idea, make a slot in the track next to where the car drops that a meter stick or tape measure can run through

[_HenryHscioly_]

If I were to let my vehicle just roll, roll, until it stops. Is the deceleration from start to end decreasing, increasing, or constant?

I was told that it is constant, and once the vehicle reaches 0m/s, drag/friction/deceleration drops to 0.
I know that air resistance is proportional to the square of velocity, so I thought that the deceleration would decrease as velocity decreased.
IF air resistance was not taken into account, and then if it were, would decel be decreasing, increasing, or constant?
I am only taking my first physics class this year.., so If you could not use BUNCH of equations to explain, that would be nice!
Thankss

[iwonder]

This was brought up last year, but in my mind or resistance has alway been negligible, so deceleration is constant(constant friction in axle/bearing/etc).

[mrsteven]

This was brought up last year, but in my mind or resistance has alway been negligible, so deceleration is constant(constant friction in axle/bearing/etc).

agreed unless you make a sail or something insane on your car

[chalker]

If I were to let my vehicle just roll, roll, until it stops. Is the deceleration from start to end decreasing, increasing, or constant?

I was told that it is constant, and once the vehicle reaches 0m/s, drag/friction/deceleration drops to 0.
I know that air resistance is proportional to the square of velocity, so I thought that the deceleration would decrease as velocity decreased.
IF air resistance was not taken into account, and then if it were, would decel be decreasing, increasing, or constant?
I am only taking my first physics class this year.., so If you could not use BUNCH of equations to explain, that would be nice!
Thankss

It's all of the above as follows:

Your vehicle starts at 0 velocity, 0 acceleration. When you release it, it accelerates down the ramp to some maximum velocity. In order for the velocity to change, there must be some sort of acceleration, thus the acceleration for some small fraction of time is increasing from 0 to some max value. Without getting into the gory details, we can safely assume the acceleration 'maxes out' well before the velocity. Which means for some small amount of time then the acceleration will be constant and at a positive value (since velocity is still increasing). Once the vehicle leaves the ramp, it will begin to lose speed due to friction and air resistance. Since the velocity will be decreasing then, the acceleration needs to be negative, thus the acceleration then quickly decreases from a positive value to a negative value. Again, we can easily assume this happens much quicker than the velocity change, thus the acceleration will become essentially constant for a while (at a negative value). Once the vehicle comes to a stop, the acceleration must also come to zero, thus for a portion of the run at the end the acceleration will actually INCREASE slightly (in order to go from a small negative value to zero)

[JimY]

Chalker,

I recall in this thread last season that some posts were discussing optimum shaped ramps that allowed the maximum linear velocity to be reached in a minimum amount of time. I translated this to mean that these ramps were shorter in total length than, say, one that is circular in shape with a near vertical section at the top, as this shape would have a longer total length than one that is, say, more like a quadratic or cubic function. At nationals last season, did you happen to notice if the top teams had ramps that were relatively similar in shape? If so, can you share what they had in common? I downplayed the optimum ramp idea last season, mainly because once someone posted the actual shape, the change in direction at the bottom from more vertical to horizontal just seemed too extreme and might cause repeatability issues with steering. Then once I saw the results, it seemed like accuracy was far more important than speed, which is what I anticipated all along. Since I wasn't there, I'm just asking what you are willing to share on ramp shape.

Thanks.

[chalker]

Chalker,

I recall in this thread last season that some posts were discussing optimum shaped ramps that allowed the maximum linear velocity to be reached in a minimum amount of time. I translated this to mean that these ramps were shorter in total length than, say, one that is circular in shape with a near vertical section at the top, as this shape would have a longer total length than one that is, say, more like a quadratic or cubic function. At nationals last season, did you happen to notice if the top teams had ramps that were relatively similar in shape? If so, can you share what they had in common? I downplayed the optimum ramp idea last season, mainly because once someone posted the actual shape, the change in direction at the bottom from more vertical to horizontal just seemed too extreme and might cause repeatability issues with steering. Then once I saw the results, it seemed like accuracy was far more important than speed, which is what I anticipated all along. Since I wasn't there, I'm just asking what you are willing to share on ramp shape.

Thanks.

I didn't run the event at Nationals, and only saw a handful of ramps when I was walking around (which all seemed to be a rather 'normal' slightly curved version). However I know the people that DID run the event lurk on scioly, so I'll defer to them if they want to chime in...

[retired1]

From scioly.org/wiki/gravity_Vehicle pg 2 says that it is a Brachistochrone curve. I checked this on a couple of math sites and they agree.
The picture from the scrambler site is not quite correct. The math to make one is tough, but one reference says where to place a circle of a given size to get nearly the identical curve. Sorry, I do not remember where. Our wild guess last year came close and it was strictly eyeball.

[iwonder]

The velocity at the bottom will be the same no matter what type of ramp is used, so I fail to see how it's worth the trouble to build a specially shaped ramp when one can be approximated by something similar to an L. Am I missing something simple here?

[mrsteven]

The velocity at the bottom will be the same no matter what type of ramp is used, so I fail to see how it's worth the trouble to build a specially shaped ramp when one can be approximated by something similar to an L. Am I missing something simple here?

If I recall this conversation last year, it resulted in saying that the curve did not matter as long as the end of the ramp was as close to horizontal as possible to turn all of the possible gravitational energy from the ramp into forward velocity.
So fancy curves or not, just make it horizontal at the end