Simple Science Physics Question that pissed me off

[Vicboy]

Okay, imagine you have a big test, and you feel you might get a high grade.

then you come across Part V - #1.

The question is

Sally drove with an average speed of 45 kph over 40 minutes. How far has Sally went?

Over it all until the hundreths digit.


This isn't homework help, but merely to see if you guys would have gotten the wrong answer or not.

And believe me, if you were me, you'd answer it in a way that my teacher wouldn't accept.

Go ahead.

 

[Nbah]

? 40 minutes are 40/60 -> 2/3 hours.
=> from the formula, 45x2/3=30 km. So Sally drove 30 km for the 40 minutes.
Maybe I'm wrong, dunno, but we get these kinds of questions in the fifth grade =/. Unless I overlooked something, it should be simple, any kid should be able to do it ><

 

[Vicboy]

Anyone else wanna answer this?

 

[TheDivineBoss]

Her speed is: 45 km/h.

She drove for 40 minutes, so 2/3 of an hour.

You have to do:

45|??
60|40

45 x 40 / 60 = 30 km

The answer is 30 km according to me.

And it's often like that you gain points on test by not having the correct answer, but how you got it.
If you only put on your paper ''The answer is 4'', then you get rarely the full points. But if you say ''2x2 = 4, so the answer is 4'', you'll get the full points.

 

[Vicboy]

If you only put on your paper ''The answer is 4'', then you get rarely the full points. But if you say ''2x2 = 4, so the answer is 4'', you'll get the full points.

I'd understand that, but mine is least expected...

 

[Nbah]

And it's often like that you gain points on test by not having the correct answer, but how you got it.
If you only put on your paper ''The answer is 4'', then you get rarely the full points. But if you say ''2x2 = 4, so the answer is 4'', you'll get the full points.

True, true. I've done that sometimes and occasionally I don't even get ANY points, because the teacher says I just copied it from someone else, without the explanation.

 

[NoobinJass]

40 mins = 2/3 hour so
2/3 * 45 = 30 km.... am i right??

 

[Vicboy]

You know what guys, that's the same answer I gave, 30.

But, it seems that since I stated "Over it all until the hundreths digit."

I was suppose (I know this is bs) to convert 2/3 into a decimal.

Thus it becomes 0.666666 then over it all to 0.67.

So the answer ends up being 30.15

Now, tell me, that's not an accurate answer...

 

[Razorbrain]

okay, ill tell you, that is not the correct answer

 

[Vicboy]

okay, ill tell you, that is not the correct answer

actually, rounding off 0.666666 to 0.67 is acceptable.

 

[Mechanical Man]

Don't calculate the 40/60 first because you will cause rounding errors. Use division only in the end. And I see people here confusing miles with kilometres.

 

[TheDivineBoss]

Don't calculate the 40/60 first because you will cause rounding errors. Use division only in the end. And I see people here confusing miles with kilometres.

Tell me about it. I thought kph meant something else.

Googled kph => 1 kph = 0,277777778 m / s

Vicboy, please mention some background information when asking something.


If I'm right and didn't confuse the /3,6 with the x3,6 (those are the methods to calculate from m/s to km or the other way around)
0,277777778 x 3,6 = 1 km/h

So basicly she goes 1 km/h, and did that for 40 minutes, so let's do that table again:

01|??
60|40

1 x 40 / 60 = 0,6666666667 km or 666,67 meters in 40 minutes.

Amiritenoaw?

Edit: And shut up about the commas, some of us are Europeans.

 

[Masiah]

Considering that a simple physics question like this discriminates the friction of the contact between the tyres and the ground, the friction between the car and air, the fact that keeping a constant speed of 45km/h (or any other) is nearly impossible and many other factors in order to simplify it, I think that considering that 2/3 = 6,666666666... = 6,67 is apropiate. Ergo... tell your teacher to give you full marks and then to f*** off.

 

[Vicboy]

tell your teacher to give you full marks and then to f*** off.

Hey, he's a nice teacher and he's considering it...

BTW, for TheDevineBoss, I was doing the same thing that I encountered in the test.

 

[PurplePoot]

Average speed of 45, not constant.

Also, teach your teacher something about rounding - that is, you round the answer, not during the calculation - that way, you get a rounded number but don't lose precision.

 

[~Void~]

Velocity = 12.5 m/s
Time = 2400 seconds

Distance = VT = 30000m = 30km

 

[Teh_Ephy]

You know what guys, that's the same answer I gave, 30.

But, it seems that since I stated "Over it all until the hundreths digit."

I was suppose (I know this is bs) to convert 2/3 into a decimal.

Thus it becomes 0.666666 then over it all to 0.67.

So the answer ends up being 30.15

Now, tell me, that's not an accurate answer...

That's the kind of significant figures calculation that is used in chemistry, which is a load of fucking shit. Straight from the mouth of my Physics lab teacher last semester, calculating sig-figs at every stage can result in upwards of 30% difference from not doing sig-fig calculation until afterwards. Your teacher just blew up the space ship.

 

[PurplePoot]

You never calculate significant figures at ever step. Not even in Chemistry, at least from every experience I've had, as well as people referring to higher level (University) chemistry.

It's just stupid loss of precision. Significant figures are supposed to represent uncertainty, not create it...

 

[Japut3h]

It's simple as hell, took practically no time to figure, and I'm not going to post the answer because most likely a ton of people posted it already.

 

[Eleandor]

What was the interrogation about?

Pure mathematically seen, 30 is the correct answer.

But the title of this topic is physics, and no matter what the question is, according to the physics rounding rules 30,15 would be the correct answer. As she literally stated you had to round down to the 2nd digit, you should have paid attention to it especially considering the ridiculous difficulty of the question. In fact, seeing how easy the question is I'm pretty sure it was a test about rounding according to physics rules, and frankly, at such a test it's pretty obvious you have to answer the physically correct answer...

The reason why it's not mathematically calculated is related to the information given by the precision of the speed measuring.
The speed is measured in km/h. The given information is in minutes.
The precision of a measurement in hours is different from the precision of time in minutes, hence why you have to calculate 2/3 before you can calculate the total distance.

 

[PurplePoot]

But Eleandor, you don't round during calculations - only after them.

 

[Teh_Ephy]

You never calculate significant figures at ever step. Not even in Chemistry, at least from every experience I've had, as well as people referring to higher level (University) chemistry.

It's just stupid loss of precision. Significant figures are supposed to represent uncertainty, not create it...

My physics lab teacher mentioned not doing sig-figs until after calculations because, literally, he had Chemistry students that would do that and get the wrong answers.

Eleandor: 45km/h and 40min are both given without zeroes to the asked level of precision, implying that the numbers are exact, and that any calculations done with them have infinitely many significant figures; 2/3 can be inferred to be acceptable as a ratio and not a rounded decimal, aside from the fact that in physics, significant figures (as PurplePoot mentioned) are to be calculated on the final result, not the intermediate steps.

 

[Vicboy]

I guess you guys know how I'm feeling about it.

 

[PurplePoot]

Just explain this to the teacher?

 

[Vicboy]

Just explain this to the teacher?

Yeah, he's considering it, so explaining it will just get him irritated..

Just wanted to see how you guys would answer it.