# Day 76: Checking the Speeder and State Trooper Challenge

We finished Wb’ing the statics problem started the day before and took the conceptual part of the summative test.

General Physics:

Today was spent setting up and checking the Speeder and the State Trooper Challenge Lab.  Here are two samples:

# Days 77 and 78: I’m guilty… squeezing in the tests

It’s the week of Christmas and we have a full day and a 1/2 day.  In Advanced Physics, the students completed the problem set part of the Unit 4 Summative test on the full day, then we discussed the conceptual part taken last Friday.  In General Physics, the students took the problem set part of the Unit 2 Summative Test and on the half day, took the conceptual part.  Each test had a seasonal question or two.  Advanced:

From General:

# Day 75: Working Backwards and The Speeder and the State Trooper

Today was a WB session on a set of 5 ‘static equilibrium’ problems.  The students had been working on them for nearly one week, a few each night.  I truly believe it is important for the students to present the solutions to problems they have solved, (key word here is solved) which is one of the reasons I still WB problems.  It allows me to listen to the explanations and do some on-the-fly-formative assessment on the level of understanding.

Theses are the longest and most involved problems I have given them, here are two examples:

Sometimes these problems can look intimidating, especially one like #3 above.  To help with this, I have told my students I want them to become like Pavlov’s Dogs, no, not to salivate when they hear a bell, but when they are presented with a problem/question that has the word force in it, the classically conditioned response should be to draw a force diagram WITH the net force equations.  From there I drive at solving the problem by ‘working’ backwards.

SIDEBAR 2: Our chemistry and physics teachers use the acronym PIPES for our problem solving approach.  P- State the problem, essentially this is what you are solving for, the unknown. I- Information, what are the givens. P/E: Plan and explain HOW you will solve the problem, what steps will be followed. S- Solve it showing all your work.  The work backwards approach comes in at our P/E step.

Here is an example P/E and working backwards from problem 4 above:

P/E: 5. Find Ft from the Fnet x equation Fnet x=-Ft + (-) Ff + Fg x=0

4. Find Fg x from Trig (sine of the angle) and Fg

3. Find Ff from Ff – COF*Fn

2. Find Fn from Fnet y equation: Fnet y = Fn +(-)Fg y=0

1. Find Fg y from trig (cosine of the angle) and Fg

So start with step 1 on the bottom, then go up to step 2 …

Some students have ‘bought into’ the approach, but others are resistant, I’m not sure why though.

General Physics:

Today was the first part of a challenge lab we call the Speeder and the State Trooper. Here is the basic scenario:

So today was about using the motion detector to determine the velocity of the dune buggy and the acceleration of the phan cart.  Once the data is gathered, the students can choose to solve it graphically ( with a position-time graph) or with the kinematic equations.

# Day 74: Frictional Force Post Lab and an AWESOME Free fall movie

We had the post lab discussion for the frictional force experiment. This is not a tough discussion for the students because of the way the experiment was initially set-up.  Essentially, we varied the normal force and measured the frictional force for a given set of surfaces.  The graph is linear and it is different for the two sets of surfaces we tested… conclusion= the types of surfaces do make a difference.  This is something most of the students fully realized, but now the  slope of the linear graph provides a way for us to quantify the ‘surface effect’.  That’s how we conceptually defined the slope, then gave it the proper name, the coefficient of kinetic friction.

Another important aspect of this experiment is figuring out why there are two types of friction, kinetic and static, (and why they are not the same value). We have been drawing force diagrams for these for close to two weeks, now we are in a position to explain it.  I simply complete one trial of the experiment the students had just completed (making sure there is a large mass on the block). The students watch the data gathered in real-time on the screen and I ask them to determine when the block finally starts in motion.  Here is what the resulting graph look like (with my added scribbles):

It is easy for the students to see that the block does not start in motion until just after the sharp peak. So this leads us to a second coefficient of friction for each set of surfaces, the static coefficient.   A number of student groups commented that they saw the peak also and wondered about it.

General Physics:

We started the period by watching this short movie clip, which is totally awesome:

I had attempted to demonstrate this with a piece of paper and a textbook, but this is much better.

Following this we, WB’ed a set of free fall problems.

# Day 73: Friction and Freefall

Today all that we accomplished was to gather data for the frictional force experiment.  Here is a picture of our basic setup:

This was one of the two different sets of surfaces… it was to simulate a car tire on concrete. I asked our autos teacher to attach some real tires pieces on a block, but the size was a bit prohibitive. The other was he classic wood on wood.  Tomorrow we whiteboard the results.

General Physics:

We shared our values for the acceleration of an object in free fall, then worked a sample problem.  I used the ‘ol catch a dollar bill and you keep it’ problem to introduce the problem.  Challenge a student to catch a dollar bill as it slips through his/her fingers.  The elbow and wrist stay on a table and Georges head gets lined up with the fingers.  The discussion naturally comes around to that the student can’t react fast enough…. so how fast would you need to react to catch it?  Do a free fall problem and find out…. about 122 ms (~7.4 cm from the edge of the dollar to George).  If there is enough time, the students can catch a falling ruler and calculate their own reaction time.

# Day 72: I’m sensing some friction and ‘Tom Petty’ visits

Well, a Monday morning… what better way to start than with a force diagram assessment.  Here is the one we did today:

The majority of the students did pretty well, but there are still a few that need to come in for help, but just will not… like the two that had a frictional force but NOT a normal force !

After the assessment, we had a pre-lab discussion related to the frictional force.  We decided that we could quantify the (kinetic) frictional force if we pulled a block with a string with a force sensor.  Two conditions had to be met.  First, the string had to be parallel to the surface so there were no components to the tension force. Second, it had to be pulled such that the block had a constant velocity.

We discussed the factors that might affect the value of the frictional force… the usual suspects showed up; mass, weight, surface area, velocity, types of surfaces.  We discussed that we could not change the mass without changing two other variables and that is a major no-no in a controlled experiment.  Changing the mass will also change the weight and the normal force exerted on the block by the surface.  After some guiding questions, we decided to look at the normal force exerted by the surface because we can change that without changing the weight and mass.  We just need to add an extra normal force that points down on top of the block… we could just push on it, but that is hard to keep constant and measure.  A student (thankfully) suggested putting a known mass on top of the block, but still continue to define the system as the block.

To save some time, I demonstrated how to gather the data and showed that for us, there is no effect with the velocity or the surface area.  Tomorrow the students will vary the normal force on a given set of surfaces.

General Physics:

I collected the one problem assessment I sent them home with last Friday– solve a constant acceleration problem with the kinematic equations. I messed up a bit because the one I made up (based on a goal-less problem) was too hard for most of them.  Now I will need to give them a another to show they really can do it.

We then looked at an example of constant acceleration, namely freefall. I bridge this from our ramp lab.  We sketch the stack-o-graphs for increasingly steeper ramps until we have a vertical ramp.  To gather the data, we use the picket fence and photogate. I do not worry about if they fully understand how the photogate gives us the v-t graph, just that it is the slope of it that we want.  As part of this, we add washers to the bottom of the picket fence to increase the mass to see if there is any effect.  Tomorrow we share results.

# 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.

# Day 69: Believe it or not… shortened classes again.

Today we had another shortened set of classes.  This time because we went through the registration process with the freshman, sophomores, and juniors.  We had 42 minutes, so there was that…

The students took a force diagram assessment.  Here is what I gave them:

It was very timely because it was shown on TV the night before.  We also had the pre-lab discussion for gravitational force lab.

General Physics:

This group also took an assessment, this one on solving constant acceleration problems using a velocity-time graph.  After it was completed the students worked on an activity to develop the three kinematic equations for constant acceleration.

# Day 70: Gravitational Force Lab and Kinematic Equations for ca

This is really part 2 to a  previous post… the day after Neil deGrasse Tyson!