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

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.

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.

# Day 71: WB the Fg experiment and pound some problems

I forgot to mention in the last post that I make it a point to write gravitational force on the object by the earth a fair number of times so that I ‘get sick of writing it’… as do many of the kids.  I explain that there is a common name for the gravitational force exerted on an object by the earth, we call it weight.

We WB’ed the gravitational force experiment results.  As part of the discussion, we realize the slope of the linear graph is not equal to 1.0.  Many of the students were probably thinking, big deal… but it is.  It shows that weight is not equal to mass; they are completely different quantities and can not be used as synonyms.  All of you know the slope of the linear graph turns out to be 9.8N/kg for all three substances.  I tie this back to what is really giving the substance its mass, protons, and neutrons (I’m waiting for a student to say Higgs Boson…). Like many modelers, I define this as the ‘gravitational field strength for the earth’.  This naturally leads to the gravitational field strength for other massive objects like the moon. If we did the experiment on the moon that value changes, but not the number of protons and neutrons in the sample… so mass is constant.

We also worked through a sample tension for problem that allowed them to see how we use Fg and the net force equations we have been writing as part of our force diagrams.  Here is the problem:

They were to determine the unknown mass and the tension force on the left.  They were given he tension force on the right and the angle it makes with the horizontal.  Once both problems were solved, we used a balance and the WDSS to check the answers.

General Physics:

We decided that we would allow the kids to work through a practice sheet on solving ca problems with the kinematic equations.  It was the same 5 problems they had solved graphically… again, they already know the correct answers and it allows them to see which method they prefer.  I usually dislike this type of lesson plan, but one student this is an awesome thing to do, just let the pound some problems and ask questions when they have them.