# Definition clarification of vector mag/direction/rant

1. Nov 25, 2009

### GreenLantern

Okay so I feel this is a bit of a weak first post in a forum but I haven't yet found a place for comment that wouldn't just be useless clutter in the forum.

So I am currently a second semester Mechanical Engineering student and have been getting A's on every test in every class this semester...until now. I took my third to last physics test on friday and received it back on monday with a 76%. As i started to look through the test I saw that I lost 12 points (total from two separate problems) because of one simple thing.

On two questions he asks for a vectors magnitude and direction. Now the way I've always considered this was it could be answered in two forms. A resultant vector magnitude and a direction given in terms of an angle or unit vector OR it could be given in terms of vector components. At least this has been gravy for my calc/engineering classes...

What do you all think?
is:
$$\vec{F}$$=[5$$\hat{j}$$ + 3$$\hat{k}$$]​

considered magnitude and direction of a vector?

heres one of the actual problems (at the beginning of the test he specifically mentioned vector as the answer for it)

sorry its so massive....but am I really wrong in this? I got the correct components but is this really considered as not an accurate answer for the magnitude and direction of the magnetic field warranting a loss of 7/20 points? And should I even care enough to try and get it changed because this is the third out of three tests with which the one with the lowest grade gets dropped (this test score wouldn't count no matter wether i received the 12 points or not)

thanks everyone (hope this is the right forum)

-GreenLantern

2. Nov 25, 2009

### arildno

No, you shouldn't care to try and get another grade, because you won't get it.

The magnitude of a vector is a scalar quantity, you have not identified it.

Whether you had given the "direction" in terms of a UNIT vector, or as an angle it makes with, say, the positive x-axis is of less relevance.

You have shown a proficiency at basic math calculations (hence more than 50% correct), but you betray a hazy understanding of standard definitions of terms.

I fully agree with giving you just 65% correct on that question.

3. Nov 25, 2009

### D H

Staff Emeritus
Absolutely not. The magnitude of the vector you wrote above $$\sqrt{5^2+3^2}\approx 5.83$$.

The question very specifically asked for the magnitude and direction of the total magnetic field at the origin. You didn't answer the question.

4. Nov 25, 2009

### cesiumfrog

I see why the OP feels harsh, since in some form the OP has indeed given the precise "magnitude and direction" (as opposed to just the strength, or just the general direction).

But if I were marking the exam question, I wouldn't give full marks unless it clearly terminated in a plain English sentence that not only answers but re-acknowledges all parts of the original question (and suggests a full conceptual understanding): "The total magnetic field at the origin has magnitude ... and direction ....", with the correct number of significant figures (if not a full uncertainty analysis) and correct units.

The OP had completely neglected units of measurement (a number is meaningless on its own), used an inappropriate number of significant figures (showing a lack of respect for uncertainty, one of the most important facets of measurement), and worst of all, expressed an answer using undefined symbols (who knows how you expect j,k to be interpreted, why not at least use the same symbols as provided?). In the real word, such a response would be completely useless, since a person who did not already know how to themselves answer the question they asked, would still not be able to properly interpret all of it from your response.

Last edited: Nov 25, 2009
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