Again, expecting something to be really hard and wasting the whole period on it, and burning away time that was supposed to be spent on the practice finals, is a terrible choice. And it is a choice. It's a choice that was made when the student decided that NOT ALL definitions don't matter. And that happened over a matter of more than a week. Knowing definitions are what make this a 5-minute quiz solution. If you disagree, it means you didn't learn all definitions. I put then in writing as a checklist.

 

It was a three force problem with a two-torque balance. So that's as simple as these problems can be. But people who refuse to change their prioritization of definitions will never see that. (In this case, it's definition of where the force is drawn, definition of origin, and definition of leverage as it relates to lever-arm. And definition of which torques are out of the page and which are in. And torque equilibrium. This is all old news by now, which is why I'm saying 5-minute solution.)

 

A perfectly done FBD would make all of the following simple and quick to see. But not everyone does that; not everyone has trained themselves to know where forces go, and not everyone has trained themselves to identify leverages by definition. So all the people who don't do those things, they will take infinite time solving this quiz and still have nonsense, because they live in a world of making simple things impossible. Those who don't live in the world of impossibility, because they prioritize the definitions I tell them to, will do the following in just a couple of minutes, because they will have made a simple FBD in about 1 minute.

 

I gave a known mallet force vector located a given number of centimeters from the pivot axis. The values of these quantities were (0.94 kg)g and 6 cm, (0.88 kg)g and 8 cm, (1.69 kg)g and 5 cm, or (1.175 kg)g and 4 cm.

 

I told people to solve for an unknown ruler mass, and I told people that the ruler was 100 cm long.

 

So a person who has followed directions over the last 2 weeks has placed the unknown mg vector at the 50 cm mark on the ruler. Then the two-torque balance expression (using the givens (0.94 kg)g and 6 cm as an example) became:

 

(Torque-CCW) - (Torque-CW) = 0

 

Torque-CCW = Torque-CW

 

"[(0.94 kg)g](6 cm) = (mg)(50 cm)"    to cancel the g and solve for m would be a pretty good solution, but it is wrong.

 

A quality F.B. diagram shows that the leverage that goes with mg is not 50 cm. It is 50 cm times the cosine of 20 degrees. Working out that the leverage would be 50 cm times the cosine of 20 degrees is something that would be done on scratch paper, and it's a component job that I've shown countless times in class by now. (It uses SOHCAHTOA.) If you don't do the manipulation on scratch paper to get components like 50 cm times the cosine of 20 degrees, but I've done it in front of you countless times by now, well, when are you going to start doing it? (And those who don't know that such a component is important, well then you don't know the definition of leverage, and that's what I'm talking about. I've explained it many times. When are you going to change this if you haven't yet?

 

Correct last step:   [(0.94 kg)g](6 cm) = (mg)[(50 cm)cos(55)]        Cancel g, solve for m.

 

Answer: m = 120 g

 

This was a 5 minute quiz.

 

1. D

2. A

4. B

5. B

6. C

7. C

8. C

9. A

10. D

12. C

13. C

16. A

17. A

18. B

19. A

21. C

25. B

26. B

27. D

31. B

32. C

33. D

34. D

35. B

36. B

39. A

40. A

41. C

42. D

46. B, D

47. B, C

48. C, D

 

Know this site:

 

lots of Free Response Practice material there. And rubrics.

 

On Tuesday, I'm handing out a mock AP Physics 1 practice multiple choice test that I'm not allowed to post.

 

Attached here are a couple of old AP multiple choice tests and answers. Questions not relevant to the course have been cut out of the original test documents. If you have 1993 or 1998 test documents that don't say the word "Abridged" in their file names, discard what you have and open up these instead. I've reduced these to 4 pages each for you to eliminate busywork for you.

 

 

 

22. T = 392 N, Pole force horizontal = 339.5 N, Pole force vertical = 0

 

24. a) f = 267.5 N, n = 1300 N, b) 0.324

 

20. b) T = 342.6 N, upward friction = 683.3 N, rightward normal = 171.3 N, c) 5.14 m

 

28. 2.8 m

 

In the meantime, here's a checklist to see if your study has included all relevant things. One page and thorough. It mentions a lot more problems than I mentioned before. You can do as much practice as you like or have time for to build strength.

 

As I said in class, your job is to challenge yourself with the 4 or 5 Chapter 8 problems that I highlighted. Where those problems involve components of vectors, you're doing strong work. Should be a fun topic to end the semester on.

 

Conservation of Energy and Momentum Test - Original Questions - I said I'd post these. If you screwed up this very simple test, it's imperative that you look at this now to get ready for the final.

 

 

And remember the ruler problem that I did as a demo? The one where the two little masses sitting on the ruler were 3.7 grams and 7.4 grams, and the whole thing balanced on 48.2 cm, and the little masses sat on 0.5 cm and 99.5 cm? Remember that? You were supposed to calculate the mass of the ruler from all of that. That answer was 90.65 grams. I made one mistake. I should have had one of you walk the ruler to the scale and weigh it so that we could yell PHYSICSSS!!! Anyway, if you didn't do that problem in class as I instructed, you have a chance to catch up, because this paragraph is very descriptive. Go through the FBD equilibrium method, with torques, and practice by proving this answer.

 

 

 

These sample MC attachments: this is to get you used to CollegeBoardish exam questions in multiple choice. (There will be some multiple choice questions on the final taken from old AP exams.) Take it like a test, doing the questions that have been covered in class and skipping topics we haven't gotten to yet. See how it goes. I posted exams from 1993 and 1998. Do this early. People who only use these multiple choice exams when the final is days away are people who get ZERO benefit from them.

 

 

This is the site that has year after year of Sample Free Response Exams. Go to the site, scroll down to the test from year 2006, select "Scoring Guidelines" for that year, and there you'll see how they did the rubric for the block-slab-friction mock test that I gave you on Wed. 12/6.

 

Note: for people in AP Physics 1, the site will have useful problems for practice even though it is from the AP Physics C exam.

 

There is also Help on #50 there. It's in the document labeled as #58.

 

#56 is my favorite.