I will now describe how I think you should It's About Time this year. I did this event during my senior year, and we medaled regularly, so I think I have some amount of authority on this topic.
Written Test
- Compile a formula sheet. Practice and get good at doing the math and physics calculations quickly and accurately. You might want to have someone who is good at these things in this event.
- Look at lots of practice tests. It helps if your school's team somehow has sets of tests from many invitationals, but you can still find some on scioly.org, I hope.
- Making the binder
- Find a lot of information. If you have time, make the information easy to read and quick to access before you print them out and stick them in the binder. If not... well, I guess a lot of printed web pages is better than nothing.
- Approaches on how to make information easy to read and quick to access
- Sometimes you just have large reference tables full of numbers. Um, yeah.
- Take notes on a computer - to be printed and put in the binder. When you update the notes, don't just write it in pencil/pen on the printed notes. If you have time, also update the computer version and print out new versions.
- Put tabs in the binder. I ended up cutting Post-It notes into thin slices, because I had so many tabs.
- Which topics should I study?
- Read the rules and past practice tests.
- Um, check scioly.org.
- Make sure your partner is also on board. The test has to be completed in a short amount of time, so you will probably have to split up the test, so both of you have to be good.
Clock
I think you should use a pendulum or spring
I think you should use a pendulum or spring.
We got two objectives: 1) optimize how well you do on the timing part of the test, and 2) minimize the amount of effort/time/money you spend on making the clock, as those resources could often be devoted to other events.
Okay, there's also 3), which encompasses a bunch of miscellaneous personal reasons - the desire to challenge oneself, to build something cool, to satisfy one's curiosity, to have fun, etc. This may push you to build something more complicated then necessary. Could be a good or bad thing! You figure out how to balance personal reasons with the team competition.
- Why water/sand clocks are bad
- Flow rate/viscosity may vary depending on temperature and humidity, and how in the world are you going to compensate for those variables during an competition? (Granted, the variation may be small, but still.)
- Risk: if you spill, you might get penalized.
- In the competition I helped out in, a team got penalized for refilling the upper reservoir of their water clock in the middle of a time interval. D:
- (Disclaimer: Never actually tried making one of those; this might not be accurate.)
- Advantages of pendulum or spring clocks
- Can be built to be quite robust
- Small variation in period
- Can be done pretty well with relatively little effort, though it is possible to make more complicated ones.
Basic Design
Okay, from now on, "we" means me and/or the two other guys that worked on the event with me.
- Have a sturdy base and frame to hang the pendulum/spring from. Make sure it doesn't shake/wobble.
- Base
- We just used a flat slab of wood for the base. But it wobbled. So we also taped a washer to the bottom of the slab at a strategic location. But it still wobbled sometimes. Blargh. In the end it didn't kill us though.
- In some competitions, you might actually have to put the clock on a desk that is slightly tilted.
- Maybe make something like a tripod? I dunno.
- Frame
- Um, make sure the frame is heavy and sturdy enough that the motion of the pendulum/spring doesn't affect it too much. Although maybe it's okay for the frame to shake a little. Not sure.
- Pendulum/spring
- Controlling the period
- Let T denote period. Then...
- Pendulum: T = 2 pi sqrt(L/g) for small amplitudes, but is greater for larger amplitudes.
- Vary L
- Mass on a spring: T = 2 pi sqrt(m/k). As far as I know, this does not vary depending on amplitude, as long as Hooke's law holds.
- Vary m, the mass. Or vary k by using different springs.
- What's a good period to use?
- Naively, one might want to make the period 0.1 s, since that's the precision that the competition specifies. But that could be hard to build and hard to count.
- I think anywhere between 0.5 and 2.0 s is doable. We ended up consistently getting within 0.2 s of the target time by estimating fractions of a pendulum swing. Remind me to talk about estimating fractions of a period later.
- I think we did something like 1.5 s with a pendulum, so ~0.75 s for one swing. But then we had another pendulum with a period of 0.8 s.
- Setup
- Pendulum
- Mass hanging from a string
- String is somehow clamped/fastened/secured from the top so the whole thing, y'know, swings like a pendulum.
- Have a horizontal bar or a hook; tie the string to that with an appropriate knot.
- Clamp the string with something
- Mass at the bottom of a rigid bar
- We didn't do this because I tried and there was too much friction at the axle and strings were so easy...
- (I know some teams have pendulums that extend below their base. Therefore, you would have to set the clock on the edge of a desk or countertop. Um... I guess this approach works, but I suppose there's the slight risk where you are unable or not allowed to put your clock at the edge during a competition. It's also just a bit more inconvenient.)
- Spring options
- I saw Harriton's clock. They hung a mass on a spring and hung the spring on the frame. In their clock, the mass bobbed up and down.
- Make sure the mass is massive enough (or that the spring has little enough damping, or that there is little enough friction) so that even after 300 s, the thing is still oscillating.
Testing the Mass-on-the-Spring
I have no idea. Never did it. I would assume that it'd be like pendulum testing, except the period is more constant, so your life is easier?
Testing the Pendulum
Preface/disclaimer: I don't actually know if this is the right way to do things. But it's the thing we did and it kinda worked for us.
- Time the pendulum
- This is the part which was very time consuming for us compared to other people.
- Easy solution would be: find the pendulum's average period by timing the thing with a stopwatch, counting swings over five minutes and then dividing the time by the number of swings. At competitions, simply count the number of swings and multiply by the period.
- The issue with this is that the pendulum increases its period as the amplitude of the swing gets smaller. For the pendulum that we built, if we followed this approach, we'd get errors on the order of 0.5 s.
- What I did
- Found a lap timer - a timer that can record lots of time points. So I could start it, and then press "lap" again and again.
- At first I used an online stopwatch, but it was annoying to parse the times. I later just wrote a lap timer in Java with a more convenient output format.
- Sat down next to the lap timer in a comfortable position.
- Started the pendulum and the lap timer at the same time.
- Every single time the pendulum swung to its leftmost or rightmost point, I pressed lap.
- I stopped after five minutes.
- This is super boring but I thought the sacrifice was worth it.
- You might be able to do this in an automated way by recording data with a sensor - perhaps a smartphone sensor. A teammate found that when the metal mass of our pendulum swung back and forth, it caused light levels and magnetic fields detected by our strategically placed Android devices to vary periodically. And if you have time series data from that, you might be able to analyze it using WWZ (weighted wavelet Z-transform), which can be done in VStar... But this is all speculation and sounds unnecessarily fancy.
- Do multiple trials.
- Analyze the timing data
- I put the data from multiple trials in Excel.
- I subtracted each row from the row above to find time between swings.
- I made graphs to see how the period changed over time. Unsurprisingly, it decreased.
- Table: Eventually, I produced a table that went up to five minutes. The vertical and horizontal axes were number of swings. The numbers in the table were the times that those number of swings correspond to.
- The times were not just multiples of the period, but were averaged from the lap time data, so they accounted for period change over five minutes.
- This also meant we had less arithmetic to do - didn't have to multiply period by number of swings.
- Caveat: If your pendulum mass is hanging from a string, then the period will probably change over the course of days or weeks due to the string stretching and contracting. So you might want to retest a few days before a competition. This is kinda annoying.
- Fractions of a period - You need to account for those, or you will not be as precise as you can be.
- Find the times corresponding to fractions of a period. When you're measuring a time interval with your pendulum, don't just round the number of swings - instead, maybe be as precise as "35 and a quarter swings."
- Keep another table of how long 1/8, 1/4, 3/8, etc. of a swing is, so you can add those in instead of doing more arithmetic during a competition.
- Handle reaction time in a consistent way by always starting and stopping in some kind of consistent way.
- Practicing
- Play random time intervals and try measuring them with your clock. See what the errors are. Try to find ways to improve.
Data Table
As mentioned before, if you have a pendulum, you should probably print out two tables: one that says how much time each number of swings corresponds to; one that tells you how long fractions of a swing is. Here is an example:
Check with the event supervisor at impound. The supervisor would either want you to impound your tables along with your clock, or he would allow you to keep the tables in your binder but then use them during the clock part.
Even if you don't want to time your pendulum's every swing for five minutes and just found the average period, I still recommend making and printing out a multiplication table of your average period. That way, you don't have to waste time on or screw up the arithmetic during a competition.
Fancier Designs
As mentioned before, you might not want to do fancier things unless you really want to and have the time and already have a backup clock in case the fancier one fails. Some possibilities:
Pendulum with an Escapement
If you are doing It's About Time, you better know what an escapement is! Anyways, the energy from the gradually lowered mass means that your pendulum has approximately constant amplitude and period throughout the entire five minutes. Yay! One of us did make an escapement pendulum; the gears and escapement were 3D printed. It generally worked pretty well. Cons: Sometimes the escapement would just stop inexplicably. Also, we had to wind it back up at the end of each run.
Other
- Gravity escapement - I saw a team with one.
- Deadbeat escapement - Supposedly better than any run-of-the-mill anchor escapement. But harder to build. Maybe 3D print one? I designed one and 3D printed it, but it did not work. It just got jammed. :(
- Complicated K'Nex clocks - I've seen a couple of those. They might work well, but they look quite time-consuming. Hmm. Maybe a viable option.
- Their periodic noises may distract and disrupt the counting of other teams, but they're not loud enough to disqualify you. Yay?
Redundancy
At states, our device had not one but two pendulum. One of them was a regular pendulum - a mass hanging by a spring from a horizontal bar. The other one was the 3D printed escapement pendulum. So each one of us operated/measured one of the pendulums, and we obtained two independent measurements on each interval, so hopefully that meant we were more accurate? On one of the trials, the escapement pendulum ran out of string and stopped prematurely, but fortunately the other pendulum still yielded a time measurement. Hooray for redundancy?
Conclusion
Good luck on this event. In closing, I have a few random bits of advice for you. Don't...- butcher a Newton's cradle and use the balls as pendulums. Unless you're very very desperate.
- worry about what other people think about you. If your heart tells you to move you body side to side or nod your head up and down to match the motion of your pendulum/mass-on-a-spring, go ahead and do it!
- forget to have fun. Except when you're behind on time and/or thrown into the event and/or completely screwed. Then, just try your hardest to mitigate your impending failure. :P (lol jk but really tho)
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