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Homework Help: Need help getting screwed over.

  1. Oct 8, 2006 #1
    Simple introductory physics problems.. help please.

    Hi, I've just started taking high school physics in online school.
    I knew it would be a challenge without face-to-face interaction with a teacher where I can ask questions and such, but I didn't think it would be this lame. There's 2 components to each unit, an 'investigation' section where you're supposed to learn how to do the problems and an 'assignment' section where the actual coursework lies.

    Problem is... The investigation section is utterly useless, so I guess I'm going to be teaching myself a lot of physics... And I'm terrible at math. Nonetheless I'm willing to try so I hunted down this board and hopefully someone here can help me get through the first unit.

    It seems easy enough, 4 basic questions.

    Here they are:

    Display the answer to the correct number of significant digits.

    1. 142.47 + 30.2

    2. 342 x 12

    Isolate for the variable given. Show all of your work.

    3. A = (B + C) / D isolate for C

    4. 1/do + 1/di = 1/f isolate for f

    Now these look really simple, even to me, and I feel like a moron for not just being able to to do this in 5 seconds like I probably should. The investigation section doesn't tell you HOW to do these though. It just gives an example. For 'isolating variables' the entire 'lesson' is this, in entirety:

    344 = 0.6T + 332

    344 - 332 = 0.6T + 332 - 344 (subtract 332 from both sides)

    12 = 0.6T

    12/0.6 = 0.6T/0.6 (divide both sides by 0.6)

    20 = T

    So.. okay. That's great. Except it doesn't tell me anything, since each problem will be solved a little different. So now that you know where I'm coming from, hopefully you can help me actually learn how to do this.

    Starting with problem 1, I think this is the only one I actually got right and understood.

    1. 142.47 + 30.2

    It's my understanding that my answer must have the same number of significant digits as the number with the LEAST AMOUNT of significant digits in the problem.

    So, 30.2 is 3 significant digits, so my answer will be:

    142.47 + 30.3 = 172.77
    =173 (rounded up to get 3 digits.)

    This answer of 173 seems correct to me. If not, could someone please explain why not?

    Moving on to question 2. This one is giving me some trouble.

    2. 342 x 12

    This seems pretty straightforward, but from the investigation section, the teacher seems to want me to arbitrarily use scientific notation when multiplication is involved. Observe the example she gave me:

    "Find the area of a square which has a length of 10.1 cm and a width of 5.0 cm.

    10.1 x 5.0 = 50.4

    Express the final answer to 2 significant digits. Therefore the area of the square is 5.1 x 10^1 cm^2."

    What I don't get is this random moving of the .1 from the 10 to the 5.

    If I were to do this problem with my own logic, 10.1 x 5.0 = 50.4, to bring it to 2 significant digits we round the answer to 50. Ta da! But apparently thats wrong. Instead we randomly trade the .1 from 10 to 5 and make 10.. a power of 1? Isn't 10^1 still 10? Then what the heck is the point?

    Before I move on to the next 2 questions I want to get an understanding of these 2 under my belt. If anybody can spare some time to share a little enlightenment with me, it would be much appreciated.

    The main problem with this second question, for me, is that it is not explained under what circumstances you make 10 go to 10^1 and when you just leave it at 10. Its not explained either the rules of shifting the .1 from 10 to 5. I see that in this case it just makes the equation more simple, but with a more complex equation, I would have no idea how to implement it, because it seems more or less arbitary.

    Thank you so much anyone who can guide me.
    Last edited: Oct 8, 2006
  2. jcsd
  3. Oct 8, 2006 #2
    By the way, here's my actual attempt at the second problem:

    342 x 12

    342 x 12 = 4104

  4. Oct 8, 2006 #3
    Sig figs
    1)172.7 (Use the decimal rule)(when adding or subtracting numbers, the number with the least # of decimals shows the number of decimals in the answer)(in this problem 30.2 has one decimal so the final answer must have one decimal)

    2)4100 (Use the digit rule)(when multiplying or dividing, the number with the least # of sig figs shows how many sig figs are in the answer)(for this problem 12 has 2 sig figs so the final answer has 2 sig figs.)(4100 has 2 sig figs only the 4 and 1 are significant
  5. Oct 8, 2006 #4
    Regarding your scientific notation question, you've caught a red herring. the answer is 5.1 X 10^1 because they took the 50.4 and rounded it up (arbitrarily) to 51. 51 in scientific notation is 5.1 X 10^1. 5.0 X 10^1 should also be correct. 5.0 X 10 is equivalent, but probably not preferred, depending on the instructor. Probably they rounded up to spare confusion regarding a 0 following the decimal.

    It seems absurd to write it that way, but the text is trying to get you used to the notation with small numbers before trying to challenge you with larger ones.

    Since the text wants everything expressed as scientific notation, your (correct) answer to problem 1, 173 should be rewritten 1.73 X 10^2.

    Your answer for problem two is almost correct. As td04 told you, 4100 is correct. The proper scientific notation will be 4.1 X 10^3. Be careful: 410^10 is much larger than that: 1.3 X 10^26.

    Isolating for the given variable is merely algebraic manipulation. Find a way to put that single variable on one side of the =, with everything else on the other side. The point of these exercises is to force you to brush up on the algebra skills you will need through the rest of the course.
  6. Oct 9, 2006 #5
    Yo, Dazed. Does this online course tell you to buy a book?
    Unless their "lessons" suddenly turn into real lessons, I'd suggest investing in a good text.
  7. Oct 9, 2006 #6
    There are a lot of resources online that are excellent in lieu of purchasing a text.
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