# Frictional force on a block on a ramp that has a wall holding it up

Today in class, students were asked to draw a force diagram for the drawing shown on the right. The drawing shows a block on a ramp with a “wall” holding it back. The question that we couldn’t answer easily is,

‘Does the friction between the ramp’s surface and the block need to be accounted for in the force diagram.’

Then, we can take this a step further, and ask

‘Does the frictional force between the wall and the block decrease the normal force provided by the ramp?’

We couldn’t decide, so this afternoon I created a quick test to see if I could answer it. Here is my experimental design:

I started with just the book on the metal ramp. I placed the book against the force sensor, then tapped a bunch on the ramp above the book. I then collected data for ten seconds and recorded the mean of the all the data collected. I repeated this three times. Here are the results:

• The ramp is 73º.
• Mass of book: 1.85 kg

#### Book on metal track

Trial 1 Trial 2 Trial 3 Average
Mean 4.370 5.001 5.275 4.882
St Dev 0.0773 0.0695 0.0410

#### Book on sandpaper on track

Trial 1 Trial 2 Trial 3 Average
Mean 2.541 2.821 2.796 2.719
St Dev 0.0306 0.0129 0.0073

#### Other results

I tried measuring the force of static friction between the book and the ramp when the ramp was horizontal. My results weren’t very consistent, but I got 3.06, 3.96, and 3.47.

### Conclusion

The difference between the force exerted by the force sensor (acting as the “wall” in the problem) were significantly different when comparing the metal surface to sandpaper, and both are less than the “x component” of the force of gravity (“x” being parallel to the ramp), which should be 5.3 N:

$1.85&space;kg&space;\ast&space;9.8&space;\tfrac{N}{kg}&space;\ast&space;\cos&space;(20^{\circ})=&space;5.3&space;N$So, while the problem may be assuming you can ignore the friction force between the ramp and the block, I conclude that this is not wise unless the problem specifically states to ignore friction.

### Other thoughts

I noticed that on every trial, the value of the force decreased during the ten seconds I collected data (the LabQuest screen above shows negative numbers, so the force is getting less negative).

# Order of operations in math equations; a new take on explaining PEMDAS

A recent discussion on the NSTA physics teacher list, the question of “order of operations” came up.  In math, the order of operations defines which operations (addition, subtraction, multiplication, etc.) one performs first. A writer asked for solutions to the following expressions:

1. 2 + (3 x 2) + 3
2. (2 + 3) x (2 + 3)
3. 2 + 3 x 2 + 3
4. (2 + 3) x 2 + 3

Most responders use the correct order of operations (PEMDAS: Parentheses, Exponents, Multiplication/Division, Addition/Subtraction) and got the right solutions…but a few did not. Finish Reading: Order of operations in math equations; a new take on explaining PEMDAS

# Correlations and Scatter Plots

After a couple of hand-graphs, my students use their TI 83/84 calculator for graphing. We set the calculators so they give an “R squared” correlation, which we often use to help determine which regression type (linear, quadratic, power) fits the data the best. Many of my students have a hard time understanding the nature of the correlation, and I have developed a presentation that I hope will help them.

The slideshow above advances every ten seconds, but feel free to click the link above to give yourself control over the timing. Finish Reading: Correlations and Scatter Plots

# If a nucleus is full of positively charged protons, why doesn’t it fly apart?

In my physical science class this week, I realized the textbook was introducing the structure of the atom and positive and negative charges, but not answering addressing the clear question that follows the presented information is “what the heck keeps all those protons together in the nucleus?” The book does get around to the nuclear forces, but not until many chapters later when it presents forces.

This gap in reasoning seems consistent in many introductory chemistry/physical science books. In an attempt to bridge this gap, I have written a brief introduction to the four fundamental forces. My target audience is our 9th grade introductory physical science class, but it might also be useful in higher or lower grades.

Since my school has moved to 1:1 iPads, I formatted this document to fit well on a landscape mode (“hamburger”) iPad: 8.5″ wide by 5.5″ tall. If you want to print it, you can use a PDF reader to print two on a page, or take the Word document and edit it to your heart’s content.

# A radical idea: Treat teachers like plumbers…

This weekend, while attending a Model United Nations convention with my MUN students, I got to talking with a few colleagues and discussing how people become teachers.

Here are the basic steps to becoming certified as a teacher in California:

1. Pay lots of money to get a BA/BS at your favorite university. If you want to be an elementary teacher, you may be able to take some education courses, but if you want to be a secondary teacher, you probably won’t have any room for them since being a “Highly Qualified” teacher means you need a degree in the subject you want to teach. Finish Reading: A radical idea: Treat teachers like plumbers…

# Next Generation Science Standards, second draft published

The second draft of the Next Generation Science Standards were released this week. You can find them all here:

www.nextgenscience.org/next-generation-science-standards

You can search them here:

www.nextgenscience.org/search-standards-dci

I have exported pages containing all DCI Arranged Standards – Public Release High School only.

# Results of online grading survey

Last week I posted a survey of online grading practices. I announced the survey through several email lists I participate in [1], as well as on my Facebook “wall”. The survey was open for five days, and resulted in 164 responses. I make no attempt to pass this survey off as statistically accurate representation of all teachers/schools–the respondents come from a class of teachers that is actively pursuing professional development and fairly tech savvy–but the data do provide in insight into the practices and views of a relatively widespread group of teachers/schools (see Question 9 below).

The entire data is available to see on a Google spreadsheet here.

[this page last updated 4 Dec 2012 at 4:45 PM Pacific Time]

## Summary of responses

Question 1: Of the respondents, 96% post assignment scores in some way.

Top four systems:

1. PowerSchool (37)
2. Edline (15)
3. Skyward (12)
4. Infinite Campus (10)

Complete list here.

Question 2:What is your school’s/district’s policy on posting assignment scores? View comments here.

Question 3:What portion of your school’s teachers do you believe post assignment scores/grades online?

Question 4:How would you rate the satisfaction of the teachers at your school with the value of your system? A majority of teachers at my school find…

Question 5:How would you rate the amount of time you put into communicating with individual parents? [N/A responses–those who do not have a system or have only taught with online grading–are not included in the pie.]

Question 6: How satisfied are YOU with the general impact of posting assignment grades online? (Please reflect on how the system has impacted your teaching. If your system has some annoyances–mine sure does–please try to overlook these when answering this question). View additional submitted comments here.

Question 8: Is your predominantly school (check all that apply)?

Question 8: Locations Approximate location of responders based on ZIP Codes provided (not all submissions had ZIP codes, and some are just the central ZIP code for the city, not for the specific school). The markers in the Atlantic Ocean are from ZIP codes that geocoded to Europe.

View Locations of responders in a full screen map

### Footnotes

1. Email lists include:

• National Science Teachers Association (NSTA) Physics list
• NSTA Pedagogy list
• Modeling Instruction list.

# Survey of teacher use of online grading systems and their implementation

If you teach at a middle school or high school, please take a couple minutes to answer this survey. I will post results here in mid-December 2012. (Apologies for the multiple scroll bars.) I’ve reached the capacity of the free Survey Monkey site (100 responses!), so I’m making a copy of it. If the survey don’t appear below, it means I’m tweaking the code–try back in a few minutes and the survey should be back live.

Create your free online surveys with SurveyMonkey, the world’s leading questionnaire tool.

Grading tends to be the biggest time consuming task for teachers. For those teachers who want students to complete lab write-ups that truly reflect discover and learning, providing feedback on labs can be an even more daunting task. This year I started using a system that has significantly sped up grading the mechanics of the lab, allowing me to spend more time on their analysis. I use address labels with seven check boxes to allow me to quickly look over a lab to see what parts are missing. Then I’m able to go in and make more detailed comments about the sections I want to focus on.

I use Modeling Instruction (this is an older site, AMTA will be launching an updated site in the next week or so) in my class, which focuses on analyzing data to develop models and reach conclusions. In most labs students plot data to look for trends, then develop the equation of the line/curve. As you can see above, my check list focuses on their data organization and plotting skills. Again, this frees me up to spend my time on their analysis and conclusions.

As I grade, I check off each item that I see completed, and can circle items the student hasn’t included (e.g. those dreaded missing units). Then I peel off the label and stick in on their report. The label is only an abbreviation, and needs to be backed up with more detailed expectations for the check list items. I provide students with a longer “Lab grading guidelines” page that they can refer to. My latest version is shown below:

Copies of Word documents for both of these can be found here (both are .docx format):

# Newton’s Laws and shopping carts falling out of truck

Watch the following video, the try to answer the questions below it:

1. What is happening to the truck and the carts when the carts start following off the truck?
2. What causes the carts to fall off the truck?
3. While the truck is moving, the left row of carts doesn’t fall off the truck. Can you explain why they don’t?
4. At about 7:57:52, the truck stops moving and yet the carts keep falling out. Explain what is causing this.
5. When the truck is stopped, the left row of carts starts falling off the truck. Why do you think this happens?