Physics review: Vector quantities

OK... I decided to write up something for any potential GAMSAT sitters reading this site.

Quite commonly when someone begins learning physics, and doing problems, they sub numbers into formulas to get the answer. Yes, that's what you're supposed to do for numerical answers to get the solution, although sometimes there may be a few steps involved rather than just one step.

However, a common elementary mistake is to forget about directions. When we are dealing with vector quantities, we must keep in mind not only the magnitude, but also the direction. Otherwise, your answers will come out weird. Just as an illustration, if I'm facing south and initially travelling south at 3 m/s then turn around and travelling north at 6 m/s, it's different from if I was initially travelling north at 3 m/s and just sped up to 6 m/s. In the first instance, my change in velocity was 9 m/s north (i.e. 6-(-3), or 6+3), but in the second instance it was only 3 m/s north (i.e. 6-3).

In two or three dimensions, there are a few ways to approach vector problems. You can draw vectors and add them "head to tail", etc, and/or separate into orthogonal components and deal with each component separately. For one dimension, the main thing required is just to assign one direction as positive and label the signs of all variables accordingly (ie, opposite direction means that the variable's value is negative).

I'll just finish with an example now. Perhaps I'll just use the same example as I started off with.

"A person was travelling south at 3 m/s initially. He slows down, then turns around and then runs north, reaching 6 m/s. He does all this in 3 seconds. What is his average acceleration?"

Well, in this case, I'll assign north to be positive.

His initial velocity is south 3 m/s, and since north is positive, south is the opposite direction and is negative. So u=-3 m/s. (u=initial velocity)

He is finally travelling north, which was assigned as positive. So v=+6 m/s.

"Time" is 3 seconds; t=3 s (time is actually a scalar quantity, so usually it should be positive)

Anyway, so a=(v-u)/t=(+6-(-3))/3
=9/3
=3 m/s²

Since this is positive, and we assigned north as positive, this means that the acceleration's direction is north. Technically the acceleration needs both a magnitude and a direction; so just saying it's 3 m/s² isn't enough. Anyway, so from that, acceleration is 3 m/s² north.


Now, actually it still works if we assigned south as positive; you don't have to worry about which direction to assign as positive in two directions too much, although it's good to choose one where you don't have too many negatives, since it makes things easier.

Anyway, if I assigned south as positive instead (just to be different),

initial velocity=u=3 m/s south=+3 m/s (since south is positive)
final velocity=v=6 m/s north=-6 m/s (since north is opposite to south, where south is positive)

time=3 s

a=(v-u)/t
=(-6-3)/3
=-3 m/s²

However, since south is positive, and the answer is negative, that means acceleration is 3 m/s² north (ie, since south is positive, north is negative in this instance; and we have a negative answer). Same as before, 3 m/s² north, as long as you interpret the sign correctly by the constraints you gave initially.

A common mistake might be to just use magnitudes; forgetting that directions are important. Doing this:
u=3 m/s
v=6 m/s
t=3 s

a=(v-u)/t=(6-3)/3=1 m/s²; different from the correct answer of 3 m/s² north!

Plugging in magnitudes without thinking about directions only gives the correct answer usually when the direction is unchanged; ie if the person was initially going 3 m/s north and accelerated to 6 m/s north, or initially 3 m/s south and accelerated to 6 m/s south in 3 seconds, then the acceleration's magnitude would be 1 m/s². Not for the question given above though!

So... when doing physics and plugging numbers into formulas, remember to take into account directions! In 1D, this means assigning a positive direction, and figuring out whether a value takes a positive or negative sign, before solving the algebra for the answer. Otherwise, if you've got a few quantities in different directions, your answer is probably going to turn out wrong.

UniMelb MD 2012 handbook entry released

I read on a forum that the 2012 handbook for UniMelb has been released. This includes how they'll assess various subjects in MD next year, and more info about the second year of MD (as it was a new course, and it wasn't run this year for second year yet).
https://handbook.unimelb.edu.au/view/2012/MC-DMED

I notice they changed around the first year main subject slightly. Noticeably, now the one essay assignment is only worth 5% and not 10% anymore.
https://handbook.unimelb.edu.au/view/2012/MEDS90001

Yay! That makes me more happy and want to go to Melbourne more now. I hope I can get in.

[update] Last day of undergraduate classes for biomed!

Today was the last day for undergraduate classes at biomed for me, if all things go to plan. It has been a great experience at the University of Melbourne studying biomed here and meeting new friends.

After today, there is a "cram period" to follow; ie swotvac. For me, my exams are quite late, so I have more than two weeks before my first exam.

Although I would like to be able to finish my degree off for biomed, I want to continue on studying, and continue at Melbourne if possible. I hope I can get a medicine place here for next year. So since the release of offers should be 9-16 November, I should know whether or not I have a place in 12-19 days.

But until then, it's time to study for exams. I'll know whether or not I have an offer (major round) before my first exam. I hope I can get an offer from Melbourne in the middle of my exam study period!

[update] About 3 weeks until GEMSAS offers, 50% waiting over!

It is now about three weeks left until GEMSAS offers; a bit each way depending on whether it's closer to 9th or 12th of November. This means if you interviewed at Melbourne, about 50% of our waiting time after interviews is over!

During the past week, I handed in (online) what will hopefully be my last assignment for my undergrad course, and I completed an online test today. Now after there are no more assessments left until the exams. Just one more week of classes left this semester.

So... last week of learning biomed next week, if all goes to plan. I hope I can get this degree completed well. But I don't feel like leaving university just yet. Hopefully I get to stay studying at uni for the next four years after this year in medicine, and I hope to do that at Melbourne. It has been nice studying here for the past three years. Three weeks left to find out.

[update] About 4 weeks until GEMSAS offers...

About four weeks to go now. So about 1/3 of the wait for GEMSAS offers for unimelb interview people is over. It is a bit frustrating knowing that last year by this time people would already have received offers from some of the consortium unis with interviews including UniMelb. However, that said, the GEMSAS is a fairer system than last year. I hope those next year don't have to wait so long, due to them not having to put too much new coding for their software etc. Leaving offers this late gives applicants little time to arrange things like honors or applying into other courses next year if unsuccessful.

Anyway. My last assignment for this year is due on the coming Friday. Then after that there is no significant assessment until the exams. I don't particularly like assignments, and it is quite relieving to see that the only assignment in the first year MD course in UniMelb, if they don't change it next year, from the main subject is worth only 10% and no more.

[update] About 5 weeks to GEMSAS place offers

There's about five more weeks until GEMSAS med school offers now. It's getting closer and closer, but if it was last year before the GEMSAS system then it would only need one or two more weeks of waiting. I suspect they really want to do a lot of double checking with the new GEMSAS system being in place just to make sure everything is right. Hopefully those applying next year don't have to wait so long.

Anyway, I also got my exam timetable. This time, all my exams are in the second half of the exam period for my university; late second week and third week.

[update] UQ officially releases cutoffs for 2012 entry.

Within the past few days (or today?) UQ has released their cutoffs for 2012 entry. http://www2.som.uq.edu.au/som/FutureStudents/MBBS/Admissions/DomesticStudents/Pages/DomesticStudents.aspx

It seems like the unbonded cutoff is a GAMSAT of 73; which I've alluded to before in comments. Before I predicted it to be 74-75 on the assumption that there were going to be 25 unbonded places, but it seems like there were in fact 40 (this number is derived from the "GEMSAS letter of death", by subtracting the number of total UQ offers by the number of MRBS and BMP places). So that explains why my prediction was slightly inaccurate.

Interestingly though, there was no cutoff given for unbonded rural applicants. It seems too that the cutoff for rural and non-rural BMPs were both the same (66*, meaning everyone with 67 GAMSAT got a BMP if they put UQ first, but if they got 66 they were ranked by GPA and those with lower GPAs missed out). Maybe this means that even with the 25% or more rural quota in place, there were enough high quality rural applicants so that at least 25% of them were already above the non-rural cutoff, so there was no need for a lower rural cutoff. I wonder if this will be repeated next year.

So that's that. I thought it was interesting information, so I blogged about it. I think my Melbourne interview went reasonably, although I can't be 100% sure (and other people probably did well because of the repeat qs too), but if it didn't then I hope there is still a UQ unbonded place left for me from someone withdrawing. Ah the UQ (lower preference) issue...

[update] back to uni tomorrow, final 4 weeks (sem 2, 2011).

Tomorrow, I'm back at uni for what will be the final four weeks of undergrad coursework if all goes to plan. I just finished one of my assignments worth 5% too, and made an online submission. Now there's only one assignment left, which is worth 15%.

Meanwhile, I'll have to do quite a bit of catching up with course content before the end of semester exams. Thankfully, I only have three subjects.

BMP: Bad Movie Physics

Tonight during dinner, some random TV program was playing. Through watching it, I noticed quite a few physically impossible events. It reminded me of some advice that my year 12 physics teacher told our class during orientation, before the end of year break of year 11. I don't remember the exact words, so I'll paraphrase:

"When you watch movies [during the break], watch out for bad movie physics."

It's probably a good exercise to do. My year 11 physics teacher and year 12 specialist math teacher also mentioned some movies having instances of bad physics as well, although they didn't tell my classes to look out for them as an exercise. There's quite a few instances of them, and just now I searched Google and came across a site called Intuitor Insultingly Stupid Movie Physics, which has a list. Sometimes it's deliberate like Harry Potter, although some other times it seems like they are oversights. To list a few:

Outerspace Explosions: Since there's no atmosphere in outer space, unless there is any conducting piece of solid or liquid connecting the exploding object to your spacecraft, there's no way you'll hear the explosion.

Superman: Even if he could fly, by catching the people being saved in the air, there is hardly any time for the speed of the person to slow down, so there will be a very big force. Funny he hasn't injured anyone.

Shuttle in space slowing down without brakes (and not against gravity either): There's no air resistance if there's no atmosphere, so the shuttle should just keep going.


Anyway, have fun spotting them out yourselves in whatever movies you watch.


(Note: In the medicine admissions context, BMP means Bonded Medical Place)

[update] About six weeks to GEMSAS offers...

The offers from the GEMSAS consortium are expected to be released 9-16 November. In six weeks, it will be 12 November, so there will be about six weeks until the offers are released.

This is a long wait. Last year, applicants only needed to wait about two or three weeks for a Melbourne offer after their interview. It seems like they delayed the Melbourne DDS offers too for some reason, although I didn't apply. I wonder why they predict that GEMSAS will take that long to process interview scores though.

Anyway. It seems like I've fallen behind in terms of studying, due to practicing for interviews instead. After the assignment is done, it's going to be catch up time. Uni starts again in two days, and it seems like today is the last day of "standard time" too... daylight saving starts tomorrow in Victoria and New South Wales.