Help and hint . Q&A for HW
Topic: HW 1a #1
Student A: For the first question,A human hair has a thickness of about ** µm.. the
answer is wanted in Meters and Kilometers. My answer is incorrect, which is:
***10^-* meters, is it the format of the correct answer? How should i fix it?
Dr. Man: See intro to webassign question 6 and 7
Scientific Notation
Very large and very small numbers are easier to write in scientific notation. For example,
the number, 2360000 would be written 2.36 multiply 10^6 in scientific notation.
How would you enter this number in WebAssign using scientific notation to 3 significant
figures? (WebAssign uses e-notation for entering scientific numbers, for example, 1.23e-4.)
2.36e6Your answer is correct.
2.36 multiply 10^(-6) would be 2.36e-6
Topic: HW# 1a & HW# 1b
Student A: I feel that I have been answering the questions correctly but webassign marks
them wrong because of the orders of magnitude. I am stil not understanding what that
means. Can someone explain?
Student B: Well, the orders of magnitude is like what Dr. Man said in class
(E^2, E^12, E^-15, ect.) if your answers are off because of orders of magnitude, maybe
you're multiplying the values wrong? For hw1A #1 there are E^-6 micrometers in a meter
so you divide X micrometers by E^6. You can write it out with all the zeros or you can
use scientific notation, it'll take both.
Dr. Man: One thing is that webassig don't accept 10^5 or 10^-5, you should write E5 or
E-5. The other thing is that you need to multiply or divide the correct value. 1 km
is 1000m etc. Practice more unit converting questions.
Topic: HW 1a #2
Student A: I don't understand what a megacalculation is. I haven't bought the book yet,
but I've tried searching online and have come up with nothing.
Student B: Mega means 10^6
Dr. Man: These words can be found on a table in Chapter one. To read the book is
important. You need to know What Mega, Kilo, mili... stand for.
Topic: HW 1b #3
Student A: I answered question 3 incorrectly. What does "x" refer to in the first option?
Dr.Man: Read Chapter 2. x refers to distance or position, which has the dimension of
length. A refers to acceleration which is the velocity change per unit of time.
Topic: HW 1b # 5
Dr. Man: Actually, The second part is asking you what is the factor (ratio) of the old area
over the new area. Find (Area_old/Area_new)) If the new area is 0.5 times of the old area,
the answer is Area_old/Area_new=2. The new area is less than the old area by a factor of 2
Also pay attention to area= the SQUARE Of the length! You do not need to convert units or
anything in the second part. What matters is the RATIO.
Student A: My answer is within 10% of the correct value, and it says “this may be due to
roundoff error, or you could have a mistake in your calculation. Carry out all intermediate
results to at least four-digit accuracy to minimize roundoff error, but I still don't know what
my error is.
Student B: What the program is telling you is that while you are converting from one
unit to another, keep a few numbers behind the decimal point and then round off when
you have the final number. This will make sure that the answer you get is more accurate.
Dr. Man: Actually, is the error is between 1% to 10%, it can be round-off error, caused by
the fact that you didn't keep enough digits of values. For example, if the correct answer is
1.04 but you round it to 1.0, The percentage different between your answer and the correct
answer is (1.04-1.0)/1.04, which is about 4%. You cause 4% of error throughout the 3rd
siginificant figure, that is called round off error. So, as emphasized in lecture note of chapter
1, it is good to keep 3 to 4 digits of siginificant figures for your answer. 1.04 has 3 digits
of significant figures and that's enough. 1.0 only has 2 SF. However, if your answer is 1.7
and off by 10% at most. the correct answer could be within the range of 1.7-0.17 to 1.7+0.17
which is 1.53 to 1.87. Whether your problem is due to round off error, or the actual result
is different from your calculation for example: 1.55 or 1.81, you don't know. So double
check your calculation and keep 3 digits of siginificant figures. You will know whether
it's due to round-off or other mistakes.
Topic: HW 1b #6
Student A: I need help on part 3 of question 6, i don't get how to convert into km/h^2 b/c
we need to convert both the length and the time.
Student B: From meters to kilometers is easy because there are 1000 meters in a kilometer,
so you divide the number of meters by 1000m to cancel out the units. For the hr^2 just think
of it this way...an hour= 60 min, so an hour^2= 3600 min^2. Then one min= 60 sec, so a
min^2= 3600 seconds^2. This will help you covert from seconds^2 to hours^2.
Dr. Man: There are sample questions on the lecture notes about this kind of problem.
When you convert unit you can always safely multiply 1 to your initial value.
1km/1000m =1
3600s/1h =1
right? If you have *****m/s^2, and you want to convert to km/s^2, you multiply
1km/1000m, and the unit "m" cancels, you end up with ****/1000 km/s^2
When you need to convert m/s^2 to km/h^2, you need to multiply not only 1km/1000m,
but also 3600s/1h for twice...Why? see lecture notes and the book for more details.
If you still don't get it, check out the HW solutions, which will be posted on course
website, after it is due. You can also ask your webassign to release key to HWs when
they are due.
Topic: HW 1b Q8
Student A: The golf problem is simple: get the distance by adding d1 and d2 and find
the displacement by subtracting d2 from d1. The computer is telling me I''m off by
10-100%. I also used a possitive sign for displacement.
Student B: I had the same issue too. The total distance traveled by the ball is actually
**+** +**, since the ball also travels a d2 distance [in the negative x direction, back
into the hole.
Dr. Man: “get the distance by adding d1 and d2. Find the displacement by subtracting
d2 from d1.” Why do you think so? Both the two statements above were wrong. As you
said it is a simple question. Be PATIENT and use your finger or pencil to follow the trace
the golf ball. Then carefully find out the distance and displacement. Be careful to check out:
for the entire motion, wher is postion-initial and position-final? Pay attention to the figure.
Which part is d1 and which part is d2.