For Physics 121, on labs and on randomized problems I critique your work and expect you to make the appropriate corrections and, if asked, to resubmit your work work otherwise respond to my suggestions. If your first attempt was reasonable, then once you have done this to my satisfaction you have an A. If your goal in the course is less than an A, let me know and I'll let you know when your work meets the appropriate criteria. After a few homework assignments and labs you will understand what I expect.

The homepage lists the CD lectures by their numbers for each week.

When you save in your word processor (e.g., Microsoft Word--not one of the stripped-down versions like Wordpad (which on some computers does give you the option) or Notepad (which never does)), one of the boxes will have a place where you choose File Type. Go into that box and choose Rich Text. Then save.

You are welcome to use Excel for graphs, and should brush up so you can do so. Use scatter graphs--they are the only type that we use for most scientific work, because they are the only type that respect the scale of the x axis. However, when it is more convenient you can also hand-draw the graphs and simply describe them in your reports to me.

Question:

I am finished with problem 1 which was assigned with week one. All I need to know is how you would like it done. I.E. in a visual form of how I solved the problem (like the introductory problems) or in a mathematical form. I can do it either way, so just let me know. More work to come soon.

For the first assignment, both ways would be a good idea, so I can get a good picture of how you are thinking. On later assignments a briefer synopsis would usually be sufficient, though I'll accept any level of detail you wish to communicate.

Student wrote:

I really did not understand the formula fro working most of these problems and could not find anything on disk for some of them. Never the less i attempted them. I would fimd it helpful if you told us when and where to use our disks and what to use on the disks that coorasponds to what we are doing.

My response:

You made a very good attempt on this problem, and after reading the notes and correcting a couple of steps you will understand a lot more than if I had given you a formula to plug numbers into.

Formulas have two functions, one of which I consider very bad.

• The bad thing is that they allow people to get the right answers without understanding anything about the physics, in the process allowing physics teachers the illusion that since their students are getting the right answers they know some physics.
• The good thing we can do with formulas is use them as symbols for things that we do understand, which reduces the misapplication of formulas to situations were they don't apply, and is based on understanding of the science and not just the mathematics.

As a result I generally ask you questions that will stretch you a bit.

• This is different than what you experience in high school, were the only questions we ask are ones we can reasonably expect a good student such as yourself to be able to answer correctly at least 90% of the time (this is in my opinion necessary and appropriate in high school courses but not in college work).
• Here you will typically be asked questions that I expect a good attempt on, something like 70% success from the typical student. You learn more from 70% success on a challenging problem than from 100% success on an trivial one.

On the present quiz (see student's responses below) I expect a Physics 121 student to usually get the first question, based on the very first problem set where we deal with rates.

• I expect the student to then be able to calculate the average velocity, based again on the first problem set.
• I expect that the typical student will then usually mess up the calculation of the rate at which velocity changes, because this step requires that the student put together the knowledge that

• we must here divide the change in velocity by the change in clock time,
• the velocity changes from 0 to the final velocity (which we just calculated by doubling the average velocity in this special case),
• and that the units will be (cm / sec) / sec, which can be expressed as cm/sec/sec or cm/sec^2.

• I fully expect that at least some detail in this process will get left out, but that with a little correction the student will then put everything together and achieve at least the basis for a full understanding of the situation.

• I expect that student will be much less successful on the last problem, which involves the entire reasoning process of the previous series, and that most students will end up just calculating the average velocity, at which point I will tell them that this problem is just same as the previous without the step-by-step breakdown in questions.

• This alerts the student to the fact that I use this strategy a lot, giving a broken-down problem before a problem which just asks the question.

You did somewhat better than expected on the problem, in fact very nearly getting the last problem. Your work to this point looks very good. I'm confident that if you follow up on my notes you will understand this situation very well, which is very good for a week's work.

See my notes below.

Dave Smith

**

If the velocity of the object changes from 3 cm / sec to 13 cm / sec in 7 seconds, then at what average rate is the velocity changing?

13cm/sec-3cm/sec=10cm/sec, then (10cm/sec)/(7sec)=1.43cm/sec/sec

A ball rolling from rest down a constant incline requires 4.6 seconds to roll the 54 centimeter length of the incline. What is its average velocity?

54cm/4.6sec=11.74cm/sec

An object which accelerates uniformly from rest will attain a final velocity which is double its average velocity. What therefore is the final velocity of this ball?

2av=fv, 1*2=2 ** if average velocity is 11.74 cm/sec, then final velocity is 23.48 cm/s. **

What average rate is the velocity of the ball therefore changing?

2-1=1 ** in the first problem you subtracted the velocities to get the change in velocity, then divided by the time interval to get the rate of change. You should do so here. Velocity starts at 0 and ends at 23.48 cm/s. You can easily finish from here. **

An automobile accelerates uniformly down a constant incline, starting from rest. It requires 14 seconds to cover a distance of 106 meters. At what average rate is the velocity of the automobile therefore changing?

106m/14sec=(7.57m/sec)/14sec =.54m/sec/sec

**

106m/14sec=(7.57m/sec), not (7.57m/sec)/14sec. Don’t use the = sign for anything but equality.

Then (7.57m/sec)/14sec =.54m/sec/sec, which is a good attempt at the rate of velocity change.

However, since we have uniform acceleration from rest the final velocity is double the 7.57 m/s, and so therefore is the change in velocity. So the correct answer would be double the answer you got.