Ladder problem

Below are snapshots of what we did during the discussion session on the ladder problem:

Clarifications for lab report 6

Worksheet pages to be handed in:

These are pages 73-75 (the template may be saying something different, from last semester).

Clarifications for lab report 5

Mistake in the template:

What was originally in the template should be changed to what is highlighted below:

GRAPH 1: Centripetal force vs radius

You must have three different curves on this graph, for the different values of the radius indicated p.65.

Next to each curve, indicate the corresponding value of the radius angular velocity.

GRAPH 2: Centripetal force vs angular velocity

You must have three different curves on this graph, for the different values of the angular velocity indicated p.65.

Next to each curve, indicate the corresponding value of the angular velocity radius.

Clarifications for lab report 4

Clarifications for lab report 4

1) Histograms

DO NOT USE EXCEL TO GRAPH THE HISTOGRAMS

Excel can do bar graphs, but they are not histograms. Draw the histograms by hand.
First choose a scale for your x axis (like you would for any other x axis)
Then choose a bin size; all bins must be the same width.

2) Averages

The averages you need to take are those of the "deltas", of the changes (in momentum or kinetic energy).

3) How to get the standard deviation algebraically is explained p.100 of your lab manual.

4) How much data must I use for each histogram?
At least the 15 lines on each page. Graph together the data corresponding to when the two carts had the same mass, and when the two carts had different masses.

5) What I explained in lab about the histograms and how to use them...
...in pictures! These are the snapshots of the board:

This is what each of your 4 histograms should look like, i.e. all that they should have on them (and it doesn't matter how high you draw the error bars, which are horizontal in this case):

Recall the definition of Delta:

This is what your error bars should look like or not, i.e. roughly how wide they should be compared to your histogram, so you can troubleshoot them. If yours look wrong, first check that you graphed them correctly, one standard deviation to the left of the average, and one standard deviation to the right; if that doesn't fix things, check your calculation of the standard deviation.

  

 And finally, once you've got the error bars, this is how to tell whether your data says that the momentum / kinetic energy was conserved in the collision at stake (the vertical line represents the y axis, i.e. at x = 0):

 

I hope that makes everything clear, but if you have further questions don't hesitate to email me ;-)

Clarifications for lab report 3

Clarifications for lab report 3

How to find the acceleration that you graph need to graph for graph 2 and 3. 

 

  VERY IMPORTANT.

 

Use the times and distances that you measured in the equation: d = 1/2 a t², solved for a.

Do NOT use Newton's second law to find this acceleration: if you did, your graphs would not tell you anything about whether your experiment confirms that law or not, so your graphs 2 and 3 would be meaningless. They are meant to allow you to check that the relationship between the actual accelerations, that is the accelerations measured experimentally, and the actual masses or applied forces, is indeed Newton's second law. So it is CRUCIAL that what you graph in graphs 2 and 3 be these actual accelerations; and those you find by using the actual times and distances that you measured, and the equation d = 1/2 a t².

Lab manual pages

No, you do not need to hand in the pages from the lab manual.

 

When finding the acceleration from graph 1, how can I make sure that I'm doing it right, ?

You can't be 100% sure... but 99% you can. Do the practice examples at the bottom of:

http://physicslabssdeclark.blogspot.com/2012/10/how-to-find-experimentally-value-of.html 

If you're getting them right, it seems you know what you're doing ;-) If not, go over the material on that webpage.

 

 

THE MAIL ROOM

THE MAIL ROOM

If you really must hand in a lab report late:

- don't email me to ask me what to do. Read this instead:

- You will lose points (-10% up to 4 hours late, -25% up to 24 hours, -50% up to 48 hours, too late after that).
- email me a scan of your report as soon as possible; any legible pictures are fine, or you can convert your doc file to pdf online, for instance here: link. Do NOT send me documents in a recent word format (I'm still running XP! What can I say I love XP.)
- Your report will be considered as handed as soon as: I receive by email a copy I can open AND your doc file is submitted into the D2L dropbox, whichever of those two events occurs LAST.
- hand in the paper copy in my mail box, which is in the mail room, PAS 254. It is open during office hours and looks like this:

Labs-General Advice

Taking data during the lab

- make sure that you vary the independent variable a lot, as much as you can given the set-up that you have. Its "range" - i.e. the difference between its lowest and highest value - must be as large as you can get it.

Clarifications for lab report 2

If a graph does not correspond to a straight line, don't fit a straight line to it! 
Fit a curve by hand, after printing your report.

Units:

Convert all the distances to meters (since you are using Excel, this should be easy even though you recorded them in cm).
Generally speaking, distances must always be expressed in meters, not cm.
Note that if you were using the ln-ln graph merely to find the power in the power law, this would not affect your answer - which is why it didn't matter too much for lab report 1. But this time it does. The values I gave you for alpha assume that meters have been used.

How do I find the slope and the y-intercept?

Crucial question indeed! Since now you are graphing in Excel, you need to read them in the equation that excel gives you for the best fit line. Which means it is really really important you get excel to give you this equation!

BTW: for a graph done in Excel, the graph does not need to take the whole page; that's necessary for graphs done by hand because on those it reduces the error involved in reading the values of the y-intercept, etc...

Confused about how to graph the R vs H graph?

What you need is only the average, the upper bound point and the lower bound point for each value of H, which would correspond to the following in the example I used in the template:

for example:

first column represents H (first value, 20.5, second value, 32.5); second column represents lower bound, average and upper bound for each of these two values.

0.2        0.17 (average value of R for H = 20.5)

0.2        0.21 (upper bound value of R, i.e. for upper error bar, for H = 35.9)

0.2        0.15 (lower bound value of R, i.e. for lower error bar, for H = 35.9)

 and something similar for H = 0.3 (in this example), and for all the other values of H.

You do not need to graph 18 values of R for each value of H with 5 (or whatever you have) values of H. 

However the averages need to be calculated using the 18 values.

Should all the data appear in the table?

No, what you MUST have in your tables are the values of what you need in order to graph (H, averages of R, values used to draw the error bars for R, ln R and ln H...).

If you already have tables in an electronic format that contain more than that, that is if you had a laptop with you and typed the values directly into it, then don't waste time "trimming" the extra data out of these tables, include it since it's there. But if not, definitely don't waste your time adding all the 18 values for each R in a table/tables, only give what you must.

TO FIND THE Y-INTERCEPT AND SLOPE FROM THE GRAPH

Y-intercept:

The y-intercept must be read off the graph, on the appropriate axis, not figured out using calculations. 

If the y-intercept does not appear on the graph, change your scale and regraph until the problem is fixed. 

Be careful when you read it that you are looking at the y-axis, i.e. that is that you are looking at the line through x = 0, sometimes it lies in the middle of your piece of paper rather than on the left hand side.

Slope:

To find the slope, use "rise vs run", (y2-y1)/(x2-x1), with "point 1" and "point 2" being points on the best fit line – just make sure those don't happen to be data points, that is that they don't correspond to values that you measured in the lab, or put another way, values that you have on your table.

So the order in which you need to do things is:

- indicate the data points (functions of what you measured) on the graph paper. Then forget about your table, you should never look at it again!

- draw a best fit line, that is straight line that fits those points, NOT a "connect the dots" zigzag thing. And only use those points, don't take the origin into account.

- Once you have the best fit line, imagine that is all you've got, don't even look at the points you used to find it. Calculate the slope in the same way as you would if you were just given that one line and the axes, and told "There, here's a straight line. Find its slope".  That is, choose two points on that best fit line (again, just make sure those don't happen to be data points, point you put on yourself).  Find the coordinates of these two points by reading these coordinates on the axes, and then use these coordinates to find the slope by calculating "rise over run".