Tuesday, September 2, 2003

Sci Tech observation

Teacher: Tony Bertucci

Time: 8:15 Ð 10:01

The teacher checked attendance. 30 out of 31 students were in the class (17 females).

He talked about changing the student passwords the next day. He also talked about backing up the work students do on 2 disks and about taking care of it (he suggested making a patch for the disks inside the log book or getting a hard case)

The teacher reminded the students of the list on the board of things in the computers that are not working (so it can be fixed).

Then the teacher said that today they are doing prep for the lab tomorrow. "If you don't understand something, raise your hand and ask" he emphasized couple of times. 

(The lab tomorrow is an efficiency lab where students calculate the % efficiency of a system of a ball rolling on a ramp, by calculating the kinetic and potential energy of the ball. The teacher explained about the lab and wrote down on the board the main points).

First, Mr. Bertucci asked, "What is efficiency?" (a student: "speed and quality of work"). Then he asked what is mechanical efficiency (students were puzzled). He reminded the students that a few days ago they talked about the efficiency of a fireplace (if we have as much as trees that we can put in the fireplace and no hit goes up the chimney, then we have 100% efficiency). To get the efficiency you compare how much you get out to how much you put in, he said. Then, he gave another 2 examples; he talked about the potential energy in the car's gas tank, and bout the energy of a light bulb that turns into hit.

Then, he told the class the purpose of this lab: "to determine the % efficiency of a simple system"

He asked: "what is a system?" (Things that work together), " what is simple means?" (Not too complex), "what is simpler, a mouse trap or a nuclear bomb? A kite or an airplane?"

Then he went over the materials that the students are going to need:

• A stopwatch
• Ramp
• 3 balls of different masses (he asked "what is different masses means?") (a steel ball, a wiffle golf ball,  and a regular golf ball)

Then he explained about setting up the ramp (he also drew a ramp picture on the board)

Setting up the ramp:

Groups can have different heights of ramps. Determine the height so the ball rolls down and hits the floor without bouncing. You want a smooth transition of the ball from the ramp to the floor. To get the height, use a chair, a backpack, or booksÉ you want something stable.

Mark a starting points (where you going to put the ball every time). Measure the height. If you change the height, youÕll change the energy input.

Then the teacher explained about the data that the students needs to collect:

The distance (D) in cm

The height (H) in cm

The Masses of the golf, wiffle, and steel balls (M_gb, M_wb, M_sb) in grams

The radiuses of the balls (r_gb, r_wb, r_sb) in cm

The teacher explained that they are going to put the balls on the ramp, holding it with a pencil or a pen (not fingers) so they could get the same type of release (as close as possible) each time. Each group is going to time the ball.

Mr. Bertucci wrote on the board the concept of "parallax distortion" and explained it (you get a distortion of measurement if you measure something at an angle). He gave an example of parallax distortion (when you ride in a car as a passenger and look at the speedometer, the speed you observe is different from the speed that the driver sees). Therefore, when timing the ball, the students need to move with the ball.

The teacher explained that the class is going to group up to 3 students in a team. One will be the timer, one will be the ball person, and one will be the recorder.

There are going to be 10 trials for each ball and they should gather the information in a table:

The teacher asked: "why are we taking the average? Why not measuring the time just once? Are we going to get the same time every time?"

Then, the teacher started talking about kinetic and potential energy. "What is the different between kinetic and potential?" he asked (the students learned about it last week). "We got a ball rolling down the rampÉso we have a circular motion and a motion in a straight line".

He wrote the following formula on the board:            KE_rot =  ½ I ω²

KE_rot  (Kinetic energy rotation)

I = the moment of inertia

Question that he asked:

"How many times do we have to use the formula?" (3 times, just for the average measurements)

"What is "the moment of inertia" means?" (He gave an example  - rolling a car that wonÕt goÉ does it take more energy to get rolling the car or to keep it rolling? The instant that the car starts to roll is the moment of inertia)

The teacher explained that "You have to overcome friction and overcome gravity" in the moment of inertia (repeating what students said). Also he explained that calculation of I is different for different shapes. In our case we have a sphere.

I _sphere = 2/5 Mr²

(M is the mass of the sphere, and r is the radius of the sphere)

ω =       the distance in radians

                        t_avg

(t_avg is time average)

He asked: "what is a radian?" (A unit of measurement), "how many degrees there is in a circle?", "what is the relation between circumference and radius?" (Circum = 2Πr)

He explained that

 Distance in radians = number of turns X 2Π

(Number of turns means number of turns for the sphere to cover the distance)       

The teacher explained that in order to find the number of turns, the students need to take the circumference (2Πr) and divide it in the distance (1 meter = 100 cm).

A student asked the teacher to explain the formula of KE_rot again

Mr. Bertucci then explained that they also have to calculate KE_{transitional.}

 KE_trans = ½ MV²

M is mass

V is velocity (V = distance/t_avg)

The teacher explained that the students also need to find the potential energy.

PE = Mgh

M is mass, g is gravity, and h is height (at starting point).

Gravitational constant = 980 cm/sec²

Mr. Bertucci then asked " what are we trying to accomplish on the first place?" (The % efficiency)

% efficiency = output/input x 100 = KE total/PE x 100 = (KEtrans + KErot)/PE x 100

Mr. Bertucci then asked the class "do you now feel like you can take a test about it?" (In his class, students are supposed to be ready for a test anytime)

He asked "What is a conclusion?" (It is what you decided all your data and calculations means). "In this case the conclusion would be: the % efficiency is ___."

"Do you measure how efficient the ramp is or the efficiency of the ramp, floor, ballÉ(the system)?"

All this took place in the first hour. In the second hour, the teacher asked the class to continue working on there mechanical drawings (using the DeltaCad program). About 10-15 minutes into the second hour the class was interrupted (there were some problems with the server last week and programs had to be downloaded). The students had to stop working on the computers (a computer for each student). The teacher decided to show then a video of projects done in a class similar to theirs (a video he planned to show the class latter this week). Even though the teacher was not in the room most of the time of the video, most of the students were attentive to the video (some were talking periodically in the back of the class). All students were fully attentive at the first part of the class (the lab prep).