Them Physicists
I remember last year, Mr. Phoenix told us a story about a conversation he had with Ouzanova. They were discussing something about physics. So he wrote the equation on a chalkboard. Ouzanova then says, “That’s wrong,” and goes up to the chalkboard and adds the vector arrows above the variables. “There. Now it’s right.” The moral of the story is that physicists are a silly people, concerned with the most minute and insignificant details.
Yesterday, I had wrote a remake of a physics test. It was quite straightforward and I completed it without a struggle. Today, I get it back and find that I have quite a good mark and proceeded to find out what kind of stupid things I did to lose marks.
The first thing was when we had to find the height of the cliff in a projectile motion problem. Now, I’m sure we’re all familiar with this lovely equation:
Basically, you’re solving for the change in distance in the y direction. Since, in this particular problem, the object in question lands below where it started, the answer is going to be a negative number, even though in real life, heights of physical objects cannot be negative. So I solved for the number, got a negative result as expected, and included a note mentioning that the height of the cliff was indeed the opposite, that is, not a negative number.
And my esteemed teacher marks it wrong because it was a negative number.
Another good one is the interpretation of the acceleration-time graph. Now, when you deal with velocity, acceleration, and displacement, you deal with vectors. Vectors have both magnitude and direction, whereas scalars, like speed and distance, have only magnitude. So in interpreting the graph, there is one direction which is positive. In this case, that is north. Now, logically, when your direction is south, it is correct to say that the direction is a negative value, because you have already established that north is positive. If you so desired, you could have set south to be positive and north to be negative, but that’s up to you to decide.
So, in this lovely problem, it asks for the period of time in which acceleration is greatest. There are two sections in which the magnitudes are both 30 m/s2 but the directions are different. One is heading north and the other is heading south. By the convention that has been set, the one that is heading north is accelerating at 30 m/s2 [N] and the one that is heading south is accelerating at 30 m/s2[S] and therefore is accelerating (or decelerating, rather) at -30 m/s2 [N]. I put my answer down that the period with the greatest acceleration is the one that is heading north.
Of course, he marks it wrong and says that there are two periods in which acceleration is greatest. And of course, when I try and argue, he says something about how those are directions and not positive or negative. Of course, that runs contrary to everything we’ve learned in physics and geometry. If you have two vectors going in opposite directions, they cannot be the same. One must be greater than the other, because their directions are opposite to each other.
So there goes three percent on my test, which is one percent on my physics mark, which is 0.17 percent on my admissions average, which I need to be as close to or over 90 as possible, and I happen to be at 89.78788897889798798 or something, so 0.17 would help very much thank you.
Damn, it’s still not June yet.
darn — i hate when that happens.
you probably know this trick, and it might not be that beneficial, but what i’ve always done is
at the very top of the page always put
Let — be South/Down/Right and + be North/Up/Left — and in your answers have the +/- answer, and then translate it into relative position e.g. d = -45 m vs d = 45 m [south of pt a]
You probably know that trick — but i dunno, the first might not be that argueable, but it atleast gives the teacher the impression you understand that — the relatiev position thing ensures you get the marks…
ha — but you probably know all this stuff already — so i’m just being wordy and extraneous :p
the second question was asking you for magnitudes, which is why both 30 m/s^2 [N] and 30 m/s^2 [S] are correct. deceleration is a misnomer. as you noted, it is simply negative acceleration when speaking in terms of going forwards or backwards. you mixed velocities into your thinking, which muddled the understanding of the question. “greater than” or “less than” only pertains to magnitudes, as positives and negatives ONLY mean direction in physics.