Hacker News Reader

Comment by nonrandomstring | Hacker News Reader

Brings back memories. We had a particularly strict physics teacher who only awarded full marks if our answers were numerically correct, with correct units, and we proved (with a little marginal aside simplifying all mass, length and time) that the dimensions were also correct.

View on HN

Hmm, that's very standard where I'm from. You're never supposed to operate on bare numbers, but carry the units along in your equations, which automatically proves that the answer has the correct units if you didn't make mistakes. To get full marks you're definitely not allowed to just assume the units in your answer.

Agreed. A plain number as an answer could give you zero points. It is always UNITS UNITS UNITS. 10 what? Kg? Apples? kW?

I remember actually a chemistry test where I did not use units in an important calculation but only in the answer. If I had used units I would have realized that I totally fucked up. I went belly up. :-(

Dimensional analysis was part of my degree but honestly, I remember little.

I had a high school physics teacher who would have the instructions "Answer in SI units unless specified" at the top of his exams, but there was always a last bonus problem that referenced the previous answer and promised extra credit equal to a full problem for the answer in "most creative or interesting units (with work shown)". It was a competition in the class for the extra credit and he'd typically award 1-3 of the answers full extra credit.

I agree, this was a basic sophmore year concept at my mechanical engineering department.

I used the HP48 family of calculators for most high school and college science classes, so it was easy to keep track of units throughout the entire chain of calculations with automatic conversions and a degree of type checking (not quite fully automatic dimensional analysis). I developed a very strong habit of always being explicit about units for any quantity, and was appalled at how lenient many students and instructors were. Worst were the textbooks that defined obscure domain-specific constants without explicitly stating the units, often forcing the reader to reference several pages to be sure of the units in use for that context.

Similarly, I would have never survived college physics without a cheapo CAS calculator that could tag and symbolically manipulate units. I simply made far too many algebra errors without it.

My chemistry teacher was good like that.

Sadly my HS physics teacher often gave us unreasonable values on homeworks and exams so you couldn’t sanity check easily. Eg - a kangaroo jumps 245 meters high, or the barrel of a gun is 1.5 kilometres long, etc.

Why do people do such silly things? Part of working assignments is to grow an intuition, and no kangaroo jumps hundreds of meters. Such crap just reinforces the notion that what you do in school is completely meaningless and divorced from reality.

Reality includes jet packs and moon walks where such numbers can be relevant. It also includes virtual realities like Fortnite in which such jumps are mundane. And the oddity can add interest to the problem while helping to train the intuition with examples outside of the normal box.

then make them relevant. ... don't say kangaroo jumps 500 metres, say an alien from mars jumps x martian units and ask the student to derive a conversion factor between the martian and earthling units.

or some problems with time dilation.

BUT don't call it "kangaroo jumps 50000 km and we need it cause reality says so"

It can also help you understand what physics you should be using / taking account of.

A metal ball travelling 1m at 3 m/s and I’m pretty confident I can get a good approximation using the basic Newtonian equations.

An irregular shaped object like a Kangaroo travelling 50.000 km at presumably many hundred of meters of per second and I need to think about a lot more! Not least that this kangaroo os travelling about one and a quarter times around the world so the spherical nature of the world needs to be taken into account!

Same here, our high school physics teacher drilled that into our heads and 35 years later I still carry through units or do a separate calculation every time.

Reply on news.ycombinator.com