Day 100: Radial Acceleration/Force Problems and Some Tension in the room

Advanced Physics:

We briefly discussed the answers to the Ranking Task I handed out yesterday.  Here it is:Unit 6 Ranking Task-The Merry Go Round –All Questions

Most of the students did very well on it.  In regards to the last question, I asked a series of questions aimed at gauging if the students understood a key concept about uniform circular motion and radial force.  Here it is:


Me: What is acting as the radial force in this example.

Ss:  The static frictional force.

Me: Good, which rider, D or F has a greater static fictional force?

Ss: They have the same static frictional force because they have the same mass and the we an assume he same coefficient of friction.

Me: Good. I see you have Rider D with a greater radial force than Rider C. Why is that?

SS: Ummm…. well….. let’s see…..

OK, it was not really like that for the last response, but it did take a bit of time for the classes recall that there is a difference between a required radial force and whether or not the force supplying it is larger enough.   I used an analogy. Let’s say I go to Starbucks and I want a coffee that costs $3.  I have some dollars in my pocket, but just because I have some dollars that does not necessarily mean that I have enough dollars.  I have to actually check to see if I have enough.  If I have $4, I’m good to go, but if I have only $2, I don’t have enough and I get no coffee.

They students worked on 7 problems in class in small groups.  No group finished all 7, so (by design) they will have some to work independently on.  I suggested which ones were more challenging that they should work with their partners on.  This type of approach (in class time to work) is a bit new to me, I usually do not allow extended periods of time for this.  I don’t think it is a bad thing, I just want my students to realize they will have to do some independent work too.

To see which problems they want to see presented tomorrow, I set up a Schooloogy poll.  The one(s) with the most votes are the ones we’ll go through.  Here is a screen shot of it.Screen Shot 2015-02-05 at 7.49.42 PM


General Physics:

Today was about throwing numbers into the net fore equations we have been writing as we draw force diagrams.  I gave them a series of situations, they solved for the tension force, then we checked it:

Sample Tension Force Problems #2

Even though it is hard to read, the last problem set the students up to do a static equilibrium challenge tomorrow.  Here is a better picture of the last problem:


They were given one of the tension forces (measured by the WDSS) and an angle.  Tomorrows problem is very similar, but with both strings angled.