Electric Vehicle C

[rman]

Note: It is standard practice to record values to a precision with 1 uncertain digit. In this case, you may consider the "10ths" digit is the first uncertain digit , but the rule writers obviously felt it is the 100ths digit.

Having supervised this sort of event for many years, I can attest to the fact that timers are almost always within a few 100ths of each other. 10-20% of the time they are identical. Timers are usually aware when they have significant error and those times should not be used.

By eliminating the slowest and fastest results from 3 timers, many errors are eliminated. Using just the center time produces much more accurate results than combining them.

Without a doubt, there are errors in the system: however, stopwatches are the only viable method in the Science Olympiad environment.

I am not suggesting that there is a realistic way of timing with anything other then humans with stopwatches but I do wonder why they decided to go to 0.01 second as a significant number. I have done that sort of timing hundreds of times and I have almost never seen three timers come within a few hundreths of a second. I did a bit of research on this and the only thing I could find that seemed to quantify this in any way was the following. "One problem with all "human" timings is a small delay, the "personal equation" (PE), which is the human reaction time between the eye first noticing the event and the hand or voice responding appropriately. The "personal equation" (PE) for most people is around 0.3 sec, and often a random error or uncertainty of approximately ± 0.1 or ± 0.2 sec may need to be added". This was from an article about timing astronomical event with a stopwatch. The thing I believe that is significant here is the "random error" of 0.1 to 0.2 seconds. This is almost precisely what I have seen while timing swimming events. I would imagine that the random error would become even greater if the stop time could not be anticipated. In swimming the start time is done with a countdown, horn and strobe light, the stop time is highly predicatable because you see the swimmer approaching the wall. In electric vehicle the start time should be pretty predicatable because of the countdown, but the stop time cannot really be anticipated at all since the timers have no real idea about when the vehicle will stop. In an article I found on swimming timing they gave example numbers that typically showed from a few hundreths to a few tenths of seconds of difference between the timers. Interestingly the articles I happened across on Olympic swimming timing discussed the fact that they needed to use cameras that take several thousand pictures per second to get the accuracy needed to be assured that the true winner was given the gold.

[techy101]

hey guys, im new to science olympiad and am doing this event, i was wondering if the we can build it out of legos or is that illeagal? if it is what material do you suggest using? Thanks so much

[jazzy009]

it is not illegal to use legos.

[binary010101]

Legos are rather fragile, however. Usually, one made of wood would be sturdier.

[Untitled]

How often do you guys switch your batteries? We are currently using 2 d cells to run our ev. But we are having trouble in time consistency.

[jazzy009]

we have that problem as well. so we bought 50 batteries, used them all once, then separated times based on what a voltmeter read. its really annoying because you have to repeatedly switch batteries but it does save a few batteries as well as make your times more accurate

[fleet130]

We are currently using 2 d cells to run our ev. But we are having trouble in time consistency.

Since you are running on only 2 cells (approximately 3 volts) , you may not be getting enough current to produce the torque required for consistent results.

When the torque is low, friction in the vehicle and dust, cracks, bumps etc. on the track have a much larger effect on the time. You might try increasing the battery to 3 or 4 cells and see if the time consistency improves.

[andrewwski]

Current is drawn. You can draw just as much current from 2 cells as 4, provided the batteries have enough charge.

Adding more batteries only increases voltage. Which increases speed. Unless you wire them in parallel, in which case you have the same voltage but more charge (current per unit time). You can still draw the same amount of current from less charge, it just won't last as long.

[fleet130]

You can draw just as much current from 2 cells as 4, provided the batteries have enough charge.

It ain't necessarily so!

You're forgetting to take into account the resistance of the motor and the batteries themselves (to keep it simple, ignore inductance/impedance and consider the motor to be a single resistive load).

For motors, torque (force) is related to current (amps) and speed is related to EMF (volts). Since Acceleration=Force/Mass and the mass of the vehicle is constant for any given configuration, acceleration depends on current.

If the motor has a resistance of 3 ohms and the cells each have an internal resistance of one ohm, the total circuit resistance would be 5 ohms. Two 1.5 volt cells connected in series (3.0 volts total applied) would produce a current of .600 amp ( current=volts/resistance).

Four cells would make the total circuit resistance 7 ohms and an applied voltage of 6.0 volts. This would produce .857 amp of current.

If the vehicle requires .500 amps to start in motion, 2 cells will provide only .100 amps to accelerate the vehicle and 4 cells will provide .357 amp of current for acceleration.

Changes in the force necessary to move the vehicle will have a much large effect on the acceleration of vehicles powered by 2 cells than on vehicles powered by 4 cells. The effect of the lower acceleration at the start is compounded as the vehicle travels down the track.

[Untitled]

We are using only 3 volts to power the ev because it helps with the straightness and braking accuracy. Is there a magic number of how many runs till you switch your batteries? Btw we are using a mechanical ev.