2/2/10 Class

Today, we wrapped up kinetic and potential energy and began our exploration into electrostatic force.

We finished off Chapter 5 with one last problem involving a bungee jumper jumping off a cliff. Using the given information, we had to calculate various things such as the max velocity, the point of max velocity, and the spring constant of the bungee cord. The big insight used to solve this problem was realizing that . Using this, you can set to find the spring constant of the bungee cord and through that the rest of the answers.

After a short break, we began our journey into the wonders of electrostatic force with a hands-on experiment involving "magic" tape. We put layers of tape on the desks and quickly ripped the top layer off and observed the way this top layer of tape interacted with its surroundings (i.e. other pieces of tape, paper, etc.). When two top tapes came in proximity with each other, they tended to repel each other, demonstrating a new type of force: electrostatic force. Other properties of the tape we observed was that it attracted pieces of tape from the bottom, rather than repelling them. Mr. Burk also showed us similar interactions between a plastic rod and paper as well. This energy comes from the displacement of electrons in the objects being rubbed together; when electrons (which are negative) are lost or gained, the charge of that corresponding object becomes more positive or more negative. When two objects have opposite charges, they will attract each other.

That's what we did in class today. Next scribe is Margaret.

Wedensday November 4, 2009

First Mr. Burk told us that we needed to be doing our self assessment and our reassessment that he gave us last week. Then he gave us our tests back with our corrections graded. Then we continued working on the lab where we analyzed the Mythbusters' efforts to determine whether a dropped bullet and a fired bullet would hit the ground simultaneously. They did, which makes sense, because vertical force, velocity or acceleration is independent from any horizontal force, velocity or acceleration. I couldn't find a video of this on the internet. We calculated the velocity of the bullets and the time it would take for them to fall to the ground, using velocity versus time graphs and the equations , and . Our homework is to do 4G and make sure that it is right, because Mr. Burk will grade for correctness. The next scribe is Paxton

10/28/09 Class

We spent the majority of class time today going over HW 4F and the concepts covered by said homework assignment. Problem 1a involves drawing a free body diagram for a 40 kg chest sitting on a 20° ramp with a man pulling on a rope attached to the chest at a 30° angle. The question asks you to solve for the minimum tension force the man must exert on the chest to keep the chest from moving (this problem disregards friction) using a graphical method. First you should solve for the only force you know at first: the gravitational force. Using the equation

, you can calculate the downward gravitational force to be about 400 N by calculating . Once you draw this into your FBD, you should be able to see

another fact; both the normal force and the tension force have to act in set directions. The normal force acts perpendicular to the surface while the tension force acts at 30° to the surface. Because the chest is not accelerating (or moving for that matter), the net force must be zero and when the vectors are drawn tip-to-tail when you add them up, they should form a loop. Therefore, you can draw lines along the points where the vectors should pass through (for example, the tension force vector must pass through the tail of the gravitational force vector). Once you have done this, you can see the length's of the normal and tension force vectors. Now you simply need to measure the length of the tension force vector and use your scale to find the approximate magnitude of the force (it should turn out to be about 150 N). The following shows what your finished vector drawing should look like:

For problem 1b, you are supposed to solve the problem using the component method. The three sets of components are , , and (these could be derived by using sin, cos, tan trigonometry). Knowing that the net force is zero, you can add up all the x-components and all the y-components and both sums will be zero. Knowing this and after solving the equations for the tension force, the tension force turns out to be about 158 N (very close to the graphically found answer from 1a).

This is what we did in class today. Next scribe is Jason.

10/16/09

We started off class today by reviewing the concept of the two rooms. There are two sets of rooms, the first of which has one room in the middle of deep space and the other falling with the acceleration of g, or approximately 9.8 m/s^2. From the inside of these two rooms, there isn't an experiment that can be performed to distinguish the two. For example, if you release a ball in the air in either elevator, it will appear to stay in one place. In the deep space room, this is because it is truly weightless. For the falling room, this occurs because the ball accelerates downward at the same rate as the room and so appears to not move in relation to the room and everything in the room. The other set of rooms has one sitting on the surface of the Earth and the other in a deep-space rocket with an acceleration of 9.8 m/s^2. Einstein believed that these two rooms should be indistinguishable from the inside as well (for example, a ball dropped in the room on the Earth will accelerate downward at 9.8 m/s^2. The ball in the rocket room would attempt to keep a constant velocity due to its inertia but would appear to accelerate downward because the room is accelerating constantly so the floor is actually striking the ball. The ball drops on the surface of the Earth due to its gravitational mass while the ball in the rocket appears to fall due to its inertial mass). However, if he was to go by this theory, then light would need to behave the same in both rooms as well. A laser shined in the rocket room would attempt to move in a straight line, but since the room is accelerating constantly, it actually strikes a point on the wall slightly below where it would strike if the room had a constant velocity. Therefore, the laser would need to behave the same on the surface of the Earth, bending below a completly straight path. Because of this, Einstein believed that gravity must have some affect on light, a new idea for his time.

On a similar note, we also discussed the basics of Einstein's theory of general relativity. Before Einstein's time, Newton's theories regarding gravity were widely accepted; Newton stated that the force that makes things fall and the force that keeps things in orbit were one and the same, the force we know as gravity. However, not understanding how gravity worked, he more or less saw gravity as a mysterious force that large objects used to grab smaller objects (i.e. the Sun grabs the Earth and keeps it in orbit). However, Einstein proposed a new theory in the early 1900s and found a way to understand gravity. He thought of the first four dimensions as a single plane of "space-time fabric." Large, heavy objects, such as planets or stars would create dips in this space-time fabric which would alter the paths of things moving through the dip. Therefore, the force associated with gravity is really caused by objects being influenced by dips in the space-fabric. The following picture shows the Sun bending the space-time fabric and how this affects the surrounding planets:

As you can see in this picture, dips in the space-time fabric can not only change the paths of planets, but it can also influence the path of light. Today, this phenomenom is seen when we are able to observe stars that should be physically blocked by a large object, say the Sun. The following picture shows the light from a star bending around the Sun:

Point A in this picture is the actual position of the star. But because light bends around the Sun, we perceive the position of the star to be Point B, which would be the actual position of the star if the light was traveling in a straight line.

Finally, we also talked about frictional forces. Unlike normal forces which act perpendicular to a surface, frictional forces act parallel. However, these two kinds of forces are strongly related; the size of the frictional force depends on the size of the normal force (i.e. the harder you press two surfaces together, the greater the amount of frictional force). The general term "friction" can be divided into two different types of said force: static friction and kinetic friction. Static friction occurs when two surfaces have no relative motion, for example if a block is on a slanted surface but doesn't move. When surfaces have relative motion, kinetic friction occurs. You can see this when you slide a block along a tabletop. Friction is often quantified by using something called the "coefficient of friction." This is defined as the frictional force between two surfaces at any given moment divided by the normal force at that moment. The higher this number, the more frictional force for a smaller normal force (i.e. a big frictional force). After the friction talk, we split into groups and performed experiments to find the factors that affect or don't affect frictional force (we tested factors like mass, normal force, surface area, velocity, etc.).

These are the things we did in class today. Next scribe is Eliza.

Tuesday October 6th Physics Class

In class today we started off by getting a few handouts. We got the chapter 4 reading, HW 4a, HW 4b, and a packet for the lab.

Next we expanded our knowledge of vectors. To subtract a vector A from a vector B, add the opposite of vector B. The opposite of a vector is just the same vector going in the opposite direction. You can also subtract vectors by putting them tail to tail.

http://mathforum.org/~klotz/Vectors/subtraction.html

We also found that to find -(vector A + vector B) you add the two vectors and take the opposite of the resultant.

After our discussion of vectors we started a new lab. Our first task was to find out how many Newtons are in one Spring Unit. After we did that we stretched an uncalibrated spring different amounts, measured the force, and graphed force vs. stretch of the spring (∆x). This lab is to be continued.

At the end of class we turned in the vector lab. Homework is 4a.

10/2/09 Class

The first thing we did in class on Friday, was to get back our 3E and 3F homework. We then talked about how the earth has a huge mass, which means it has a huge inertia. The moon and sun can change the velocity of the earth, it's always changing because we're orbiting. We exert force on the earth, because the earth exerts a gravitational force on us, but the force we exert is not enough to change the velocity of the earth. Footprints are proof that we put force on the ground. THe normal force of us on the earth equals the gravitational force of us on the earth, but if we are in quicksand there would be less normal force.
We went over many of the homework problems. The solutions are on the blog. Just dropped, a heavy dart and a light dart have the same acceleration. a=fnet/m, and if the darts were shot by a gun, the mass would be larger for the heavy dart. So it would have a larger acceleration. This happens because by the time the dart has left the gun, it already has a larger velocity.
When two vectors are added, the sum can be anything from zero to the sum of the lengths of the vectors. If you're confused about how to arranget the vectors, ask yourself "Is this something I could walk?"
The vector activity is due Tuesday. The vector activity sheet shows us how to do vectors on the calcuatlor, but be comfortable doing this with a ruler and protactor before you use the calculator. Our test is tomorrow. Study. Don't forget about the Unit 3 Review Sheet for practice.
The next scribe is Margaret

Monday September 28

We started out today in class going over vector addition. We discussed how vectors represent two quantities, force (or velocity, acceleration, or displacement) and direction. Then we continued working on our vector packet. We sketched and then added the vectors of two spring units pulling a block as illustrated below.

This is the arrangement of the block and the springs. The springs are the gray lines.

This is an illustration of using the tail to tip method of adding vectors. The two gray lines represent the springs and the blue line represents the actual force exerted on the object.

Then, we took a washer and three spring scales and pulled on the washer with all three spring scales so that it wasn't moving. Then, using a giant circle with degree measurements on it and the reading on the spring scales, we found the force and direction that each of the spring scales applied to the washer. We graphed and then added these as vectors, which should have looked something like this.

Our Homework was worksheet 3F, which had 2 pages. The next scribe is Rahul.

Class on Wednesday

In class Wednesday we went over a lot of new material with forces. One of the main concepts was that in the absence of forces an object moves with a constant velocity, that was one of the things we were all supposed to understand after that class period. We started off going over that the general wasy to describe forces is that forces require something to exert the force on and something that exerts the force. From now on, we will be describing a force as, The force of something on something. We also learned how to describe such a generic term such as gravity, the non-contact gravitational force of the earth on the block. Then we went over a system schema, how to draw them and what forces and objects they should include. The free body diagram was another diagram that we learned how to incorporate with forces, and the slope of the rays from the dot if the surface is sloped. Also instead of writing out the non-contact gravitational force of the earth on the block every single time we learned that you can do this, Fg, E-->B and that is telling the exact same thing. The class on wednesday covered a lot of material, and all of it was very interesting. The next scribe is... Joe for the class on Friday which we already had.

September 10, 2009

The first thing we did in class today was take a reassessment over graphs of motion. Then Mr. Burk showed us a picture of a green blob in the air, which we determined was a tennis ball falling. We determined that the tennis ball was moving because it was blurred, and because one side of the blob was more translucent than the other, we determined that the tennis ball was accelerating. We used the shutter time, the diameter of tennis balls and the constant acceleration caused by earth's gravitational pull to calculate the average velocity of the tennis ball. For this, we used the equation , finding the change of x by measuring the start and end of the green blob, finding the difference and then subtracting the diameter of a tennis ball from that. Then we made a velocity vs. time graph and found the area under the graph to discover the height from which the tennis ball was dropped. After that we were done analyzing the picture of the tennis ball, so we started on a lab. The goal of the lab was to find the relationship between force and acceleration. We did this by pulling a cart along a table and measuring the force acting on it and the carts acceleration. We measured the force acting on the cart by pulling the cart with a set number of springs, because each spring, stretched to a constant distance, exerts a constant force on an object. We measured the acceleration of the object using the motion sensors. Finally, we graphed the force on the cart verse the acceleration that the cart was experiencing and found that they were directly proportional to each other, when friction was taken into account. Our homework was worksheet 3B. The next scribe is Matthew, unless he has already gone. I'm not sure.

September 2, 2009

These are the things we did and talked about in class today:

We talked about yesterday's double class period and how successful it was. We decided that it worked well; not only was it an enjoyable (more or less) way to spend two hours, everyone learned a lot and gained a better understanding of the material that we will be assessed on this coming Friday.

Mr. Burk talked about why assessments in this class should not be stressful in the least. First of all, it is very, very difficult to score an 100 on the first try (if you show mastery of a concept, you will receive a 3 out of 4 and only receive the 4 out of 4 after showing mastery a second time). Also, if you were having a bad day and somehow just couldn't seem to show understanding of a concept on the day of the assessment, there will be numerous opportunities to improve your grade. This method of testing is effective in that you won't go far in this class or in life by just memorizing equations and doing textbook problems over and over again. You must truly understand a concept to succeed, which this method of testing attempts to measure.

Mr. Burk gave us a cartoon printout which at the end had some physics problems involving velociraptors and velociraptor food (i.e. people). We spent about 20-30 minutes working the first of the problems out on the white board. It was a fairly difficult problem that involved calculating the time it would take a velociraptor to catch a person and we needed to use two graphs to solve it, a position vs. time graph and a velocity vs. time graph. The objective was to find the point in time where the person's position was equal to the velociraptors position. This was accomplished by finding the position on the position vs. time graph where the lines representing the person and the raptor intercepted. This could also be accomplished by using the velocity vs. time graph and taking the areas under the two lines (which is equal to displacement) and finding the time where the raptor had traveled 40 meters more than the person (the person had a 40 meter head start).
*By the way, I tried to upload pictures of the work Mr. Burk showed on the white board but I couldn't transfer them into the text box.

We then went over the first page of Homework 2D. The first half wasn't very difficult; it gave you a x vs. t graph and you had to translate it onto v vs. t and a vs. t graphs. The 2nd half was more confusing, showing information on the v vs. t graph. While translating that into terms of a vs. t wasn't very hard, moving that onto the x vs. t graph was much more difficult and took very careful thinking (one of the difficulties lay in realizing that a negative slope on a v vs. t graph can still show forward motion as long as the line still lies above the origin line).

That's all we did in class today. Tomorrow's scribe is Dylan (for real this time).

Class notes for Friday, August 28

When class started on Friday, we reviewed homework 2b. Mr. Burk explained to us that the area in a velocity vs. time graph represents the displacement. THe slope in a velocity vs. time graph is the change in velocity over the change in time. THe change in velocity is found by subtracting the initial velocity from the final velocity.
We then talked briefly about the corrections policy. Mr. Burk asks that we generally turn in our corrections within a week. It is perfectly fine if you choose to not make corrections. Talk to Mr. Burk if you have any questions.
Next Mr. Burk talked about some of the big ideas from Wednesday's class. The position vs. time graph of the falling basketball made a parabola, while the grpah of position vs. time squared made a straight line. A square root of the position vs. time graph would have also made a straight line.
Mr. Burk spoke with us about the double interval method. The double interval method is used for estimating instanteneous velocity. You make a position vs. time data table and then find the change between those points. Average V=change in x/change in t. You want to make this as small as possible. The double interval method gets you a line closer than average velocity because you use two intervals instead of one.
After this we broke into groups and did the Understand the Motion Sensor lab. We will finish this lab during the next class. Our homework was to finish the last 2 pages of homeowork 2B, complete the weekly evaluation on webassign, reread 2.7, and do the basketball lab. Don't spend more than 10 minutes making the graph on excel. Email Mr. Burk if it takes you any longer. Monday's scribe will be.... Eliza.

mary elizabeth

8/26/09 Physics Class

In class today, after we got back various old homeworks, we looked at a constant velocity model. The model showed two cars, A and B, both traveling at a constant velocity. Our task was to find the equation of each of the cars. The trick was remembering that the slope of a line is not simply y over x. Although this worked for car A because it went through the origin, it did not work for car B.
Next we discussed the graph of the falling basketball. We all knew that the graph was not a straigt line. Instead, the graph formed a parabola or a quadratic. We all knew that the equation of a parabola is y equals x squared. Mr. Burk asked us how we could prove that. This led to a discussion of the meaning of directly proportional. If a line is directly proportional it is straight and it passes through the origin. We realized that if the y-axis of a graph was position(x) and the x-axis was time squared and a line on the graph was directly proportional, it meant that position was a function of time squared. So to prove that position was a function of time squared in our lab, we squared all the original numbers and made a new graph to see if it formed a straight line. And..... it worked! This method is called linearization.

After that we took another look at our original graph. We had already agreed that the original graph was not a straight line. But when we zoomed in on certain time intervals that consisted of only 4 or 5 points, we noticed that they seemed to form a straight line. Mr. Burk even used a ruler to show us that the points really did make a straight line. We did this with points at the beginning of the graph and points at the end of the graph. The difference was, that the points at the end had a steeper slope than the points at the beginning. We concluded that over a small time interval, the position seems to follow the constant velocity model. At a later time the slope is steeper because the average velocity is greater. This is called instantaneous velocity. Our next discussion was, how do you find instantaneous velocity? You find the average velocity over a "small enough" time interval. How small is "small enough"? - As small as possible. We used the double interval method to do this. To find the instantaneous velocity of a point, simply find the average velocity of the point before it and the point after it. That concluded today's class.

Tomorrow's scribe: Mary Elizabeth

8-25-09 Physics Class

Today, with our 2 hours of class, we learned many things that are going to better help us understand what we will do later this semester. First, Mr. Burk handed us back our Bogus Wkshts with our out of 2 grade. Next we talked about position/time graphs that was on the 2A HW. The homework showed us how to record what distance someone is going in and how fast through work on the paper. Then we got into the heart of the class; first thing was that we split up into groups of 3 and drew on our whiteboard how to graph a position/time graph of a car going at a constant rate. Then we expressed the same thing with and equation and words. THEN, if you were really good at that, you made a velocity/time graph of the car and an equation: v(t)=vo. Easy!

As a class we had two discussion questions: what is the meaning of slope in a v/t graph? and what is the meaning of area? The slope is the acceleration of the object and the area is the displacement because v times t is m/s times s which cancels out the seconds leaving you with m - displacement Thanks Joe.

The last part of class was deticated to the basketball/tickertape lab. In groups of 3 we wanted to see the acceleration and velocity of a falling basketball starting from about 6 ft. in the air. We recorded the basketball's position in incremints of 1/60th of a second and put it on an Excel Spreadsheet. Then we took it to the lab to make some final touches before turning in work and getting the HW and printed it out. That's it -- tomorrow's scribe is gonna be Sana.

also here are some pics of the lab:

(actually it's up top)

8-24-09 Scribe Post.

Today in Physics, we turned in our FCI worksheet and Bogus worksheet. We also went over the Bogus worksheet just discussing in general why everything was impossible. Afterward we worked on getting data for our Position vs. Time Graphs with a Ticker Timer and a Buggy in groups of three or four. Our Buggy was named "Gangsta Mobile". We created two sets of data. For the first we placed a meter stick down and put the buggy in a parrallel line next to it. We then gave each person in our a group a job. One person was the timer and the other people would markwhere the buggy was at after 1, 2, 3, 4, and, 5 seconds Then we turned on the buggy at the same time we started. Then the timer would call out the seconds when they occured and the markers would line up where the buggy was when the time was called. That way we could use time as a stable, independent, x variable and distance as a not as exact, dependent, y variable. Then we plotted both axis on graph paper and wrote an equation for the graph in a f(x)= mx + b format using units for the x value as well. The second graph was about attaching a piece of ticker tape to the buggy and putting the ticker tape under a ticker timer and turning both on and getting a strand of ticker tape with holes made by the ticker timer and being pulled along by the buggy. The ticker timer was 1/60th of a second so every six dots was 1 tenth of a second therfore we can see how far the car moves in one tenth of a second by measuring the distance betweeen six dots. Because the buggy is moving at a constant rate, the distance between the six remains the same for all dots on the paper. Then we created a graph for this experiment as well using the Guidelines given out by Mr. Burk. Everything we didn't finish with the graphs was for homework. Then we went into lockdown because a fugitive was on the run from the police and had escaped into the forest near our school. We were locked in our Physics Room for the rest of the third and fourth periods and spent time chilling and socializing with the Physics students of the class next to us. Finally they gave the all clear and the homework was Homework 2-A. Read 2.4, And finish the graphs. Next scribe is ... Gaston Quantz

ScriWednesday, August 19 was the first day of physics class. The first thing we did was talk about how we know the world is round. Flat Earth Society members believe it to be flat, and we discussed how we know it is round. We wrote short essays on how we would convince a Flat Earth Society member that the world is round for homework.

We talked about how knowing something and understanding it are two different things. We filled out the front of a worksheet about something we understood. Everybody picked something that they understood well. Some of the things that people chose included sports and music. The worksheet asked questions such as what was the thing we understood, how did we come to understand it, and how did we know that we understood it. After we took a few minutes to do this, we divided into groups of three and shared with our partners the things that we understood. Each group wrote down their member's ideas on a whiteboard. After we finished, we put the whiteboards at the front of the room so that the class could take a look at what everybody had said.

Some important papers that we received on Wednesday included a syllabus, a sheet called Physics Concept Assessment Log, and one called General Instructions for Written Homework. We also received the first chapter of our reading. For homework, we wrote our flat earth essays, read chapter 1, and filled out the Course Policies Worksheet. We also became familiar with the course website, where there will be important messages and information for our class.