I posted your lab report grade and your midterm exam grade on BlackBoard

Just a note. I will probably have the tests and reports graded by late Monday or Tuesday. As soon as I finish, I will post the grades on blackboard.

Note that I am switching around the order of the upcoming labs. The next one will definitely be friction (after the midterm). After that, I haven't decided.

Just to be clear. This week we are doing the second part of projectile motion. The week after that is the midterm (October 16). This should be fine since the last day to withdraw (if that is what you want to do) is October 23. The formal report is also due on October 16. This will give you some grades before the last day to withdraw.

I will admit it. I am scared. I am afraid of the midterm coming up, because I have this strange feeling that many of you are not prepared.

My fear probably comes from the lack of questions. No one asks for help. Maybe this is because everyone already has a good grasp of the material. However, my experiences from preivious classes suggests that this is because you are not doing your work. I could be wrong.

Let's take a little poll (anonymous).

Just a reminder. If you don't read the lab info before lab, it it just going to be that more confusing in class. This next lab uses a spreadsheet to solve a basic kinematics problem. Actually, you could probably do everything you need to do BEFORE even coming to lab if you wanted.

Since we are going use a spreadsheet, if you have a computer you would like to bring, that would be good. I will have plenty of computers to use, so you don't need one. If you do bring a computer and you don't have a spreadsheet installed, I can recommend Open Office - it's free.

For those of you that like to see stuff, here is a video I made of creating a calculation in a spreadsheet.



If you want to watch this, do it before class. First, the computers I have don't play this too well (they are old). Second, I hate hearing myself in the video.

Logger Pro is a software package that you will use in class. It can control a motion detector that will allow you to get position-time data for a moving cart. Here is a video tutorial on how to do this.

Acceleration in Logger Pro from Rhett Allain on Vimeo.

You might want to make the video full screen so you can see the details better.


Speaking of graphs, here are two extra links

- Graphing with kinematics I
- Graphing with kinematics II

In Newton's second law lab, there are two cases.

In the first case, the track is level. The second case has the track inclined. I am going to draw a free-body diagram (two actually) for the inclined case. I think this will help you out some with the lab and with your lecture course perhaps. Here is a diagram of the situation.



Here is the free body diagram with the forces on both objects.



A couple of important things to note:
- The magnitude of the tension on the cart is the same as the magnitude of the tension on the hanging mass
- The normal force on the cart is normal to the track. Since the track is not horizontal, the normal force is not vertical (I see this mistake all the time)
- I have chosen to have the x-axis parallel to the track. This is a good thing because the acceleration of the cart is in the x-direction AND the acceleration of the cart in the y-direction is zero.
- This x- and y-axis does not apply to mass 2, which can have a normal axis.
- The tension in the string is NOT the weight of mass 2 (unless it is balanced, but that is a special case).

Ok, now for the hint. Let me look at Newton's second law for the red cart. I can write Newton's second law as two equations:



What forces are acting in the x-direction? The answer is: the tension and part of the weight of mass-1. I can write the x-forces equation as:



Note that θ is the angle the track is inclined. If you can't see where that componet comes from, draw a picture to help with your geometry. The y-forces equation will be:



Now for the hanging mass. The y-force equation for it is:



There is one more trick. Since the two objects are connected by a string, they must have the same magnitude of acceleration. So, the a's would be the same. Now you have a case where you have two equations two unknowns and you should be able to finish from there.

Hope that hint helps.

If you are wondering how to put error bars in your graph in Excel, check this out:

http://blog.dotphys.net/2009/01/tools-error-bars-on-graphs/

This shows how to add error bars in Logger Pro (a program that you will be using starting next week) and Open Office. Open Office is free and has a spread sheet program that is essentially equivalent to Excel (and free, did I say free?)

If you make a graph (and you will in lab 3) and you want to find the slope, how do you find the uncertainty in the slope? Here are two methods that you could use.