There always seems to be common mistakes that if one does a calculation without thinking a few steps ahead you might get the incorrect answer even if you know the answer is correct. This page shows common mistakes that have happened in recent semesters. If by chance you think you have done the calculations correctly then take a look below because you may have run into either a significant figure or a rounding error problem. At the bottom of this page is a listing of common question that have been asked and the answers you might get if you send in the same problem report. Try this page first and then use the trouble report if you really are in need of it.
Watch your significant figures when you submit your answer. Here is an example:
Find the vertical height of a falling object with the given information:
Use the equation: y – yo = vot – (1/2)gt2 . Solving for y, you get: y = (vot - (1/2)gt2 ) + yo .
Plugging in the given numbers you get:
y = [(0.0000 m/s)(0.27 s) - (1/2)(9.80146 m/s2 )(0.27 s) 2 ] + 1.41 m
y = 1.05273678 m
When entering the correct answer into CHIP152L, remember to use the same number of significant figures as the least precise of the quantities being multiplied or divided. Remember that this only holds for multiplication and division. The addition and subtraction rule uses the same number of decimal places in your answer as the least amount of decimal places in the given information. How do you record this answer in CHIP152L?
CHIP152L will accept y = 1.1 m. The quantity 1.1 has two significant figures, which matches that of 0.27, the least precise of the qantities being multiplied.
CHIP152L will not accept any of the following:
y = 1.05 m (too many significant figures)
y = 1 m (not enough significant figures)
y = 1.052737 m (too many signifianct figures)
If you are asked to calculate a number, like the value for y above, and then you need it to be used in another problem, do not use the value you entered into CHIP152L. Use more digits than you might think necessary because there may be a rounding error in your answer. Here is an example:
Using the values given above to calculate U, potential energy, of the object at the given height.
Here is a calculation using the answer typed into CHIP152L with the correct significant figures, but it will give you the incorrect answer for U.
U = mgy
U = (0.0494kg)(9.80146 m/s2 )(1.1 m)
U = 0.53 J
Here is a correct calculation.
U = mgy
U = (0.0494kg)(9.80146 m/s2 )(1.05273678 m)
U = 0.51 J
As you can see, using the more accurate value for y that is not typed into CHIP152L gives you a different answer by about 4%. The second calculation does not round in an intermediate step and is what is needed to be done in order to get the correct answer. You should carry more digits throughout your calculations than is necessary. Decide on what the significant figures should be after you have the final answer. Watch for rounding errors when doing calcultions that involve a number already calculated. Refer to the lab manual, page 14, for more information about rounding errors.
If you have similar problems with either significant figures or rounding errors, the following may be an answer you will get. These questions and answers have been taken from actual CHIP152L trouble reports that have been sent in because of problems.
Problem: In the prelab I was unable to get values for the kinetic energy by using the exact formula in the book. I asked others on their methods and they used the same method.
Answer: It appears to me that your problems are caused by (a) rounding in an intermediate step and (b) not having the correct number of significant figures in some of your submitted answers.
Problem: In the prelab the answer that CHIP152L is looking for is not accurate. I have every number but that one right, and by working backwards you should be able to get the correct answer even if the calculations don't work out. I did this also and CHIP152L still will not accept my answer.
Answer: Your answer to the pre-lab question is close but not correct to three significant figures. My guess is that you have rounded in an intermediate step and this throws your answer off.
Problem: I did my prelab but it does not show up in the grade book. I apparently did not receive any points for it.
Answer: The reason that CHIP152L gave you no points on your prelab is because you did the assignment after the due date which is as soon as you begin the experiment. Even if you are ill, in whcih case you should speak to your laboratory GTA about it, the due date in CHIP152L is set for that same time.
Problem: I used the equation in the lab manual but did not get the correct answer. I checked my significant figures and I have no rounding errors. What am I doing wrong?
Answer: You are correct that you are using the correct significant figures and there seems to be no rounding errors, however, look closely at the units for the given data and what units your answer should be in.
Problem: I was unable to get values for the kinetic energy by using the exact formula in the book. I asked others on their methods and they used the same method. To get the velocity, I used v=at, and solved from there. I converted my masses to kilograms and plugged everything into the equation, but the answer always came up wrong. After driving myself crazy for hours, I decided to write this report.
Answer: I just tried the prelab question with your unique set of input numbers and CHIP152L accepts all of my answers as correct. It appears to me that your problems are caused by (a) rounding in an intermediate step and (b) not having the correct number of significant figures in some of your submitted answers.
In particular, your value for acceleration is off by about 1%, although it is within the window accepted by CHIP152L. However, the answer to B, for example, needs to be within 0.4% to be correct to three significant figures and the propagation of an acceleration off by 1% throws your answer off by more than 0.4%.
Problem: Problem 1 part 2 is wrong i believe. I don't see any other way to do it than to use v2=v2 + 2ax. It works for the same problem with an initial velocity, but not starting from rest. I believe that it is wrong.
Answer:The reason why your answer was not accepted by CHIP152L was because it does not have the correct number of significant figures. How many significant figures does a have? What about x? The result should have the same number of significant figures as the term in your calculation that has the least number of significant figures.
