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January 2018

R2 Physics 19 – Waves

Students read 9.2 Musical Instruments in How Things Work the Physics of Everyday Life and watched the following videos before class.

For the lab I found this handout for Lab 8: Waves & Sound on the University of Oregon Physics 101.  We  got through the first couple of labs in the handout.  Students took a large slinky, or coil spring and played around… I mean practiced making wave pulses and standing waves.

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Then they stretched out the slinky to a certain distance (2 meters in this case) and timed how long it took a wave pulse to travel down the slinky and back (4 meters total).  Knowing the distance traveled and the time it took, they could calculate the wave speed, v = distance/time.    The second experiment had them set up a standing wave like in the photo below and calculate the wavespeed by using the wavelength and the frequency of the wave (number of oscillations per second).  In the photo below the wavelength is 2 meters, the same as the distance between the end points.  To find the frequency they timed how long it took for 10 complete oscillations of the standing wave (moving from the position as shown in the photo, to the opposite side and back to the same position).  From that time they could find the time for one oscillation, T,  and the frequency is just one over the period, f = 1/T.  The wave speed is equal to the wavelength times the frequency.  They found the wave speed was the same for the wave pulse and the standing wave.

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They then stretched the slinky or coil spring to increase its length and the tension in the spring and repeated the standing wave measurement and found the wave velocity increased with increased tension.   The experiment was repeated once more, using half the slinky and the original distance so the tension would be roughly the same as when stretching the slinky twice as far and they got the same higher wave speed as expected.

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R2 Physics 18 – Pendulums

The high school class did exactly the same lab that I had the middle school class do yesterday – determine what attributes (length, mass, amplitude) of a pendulum determine its period, the time it takes for one complete oscillation.

Students were asked to read Chapter 9.1 Clocks from How Things Work, the Physics of Everyday Life and watch the following videos:

Students measure the time for 10 complete oscillations and divided by 10 to get the period of the pendulum for different amplitudes, different masses and different lengths.  A couple of groups had trouble because they changed more than one thing at a time so they had to go back and redo the measurements.  If you’re trying to determine if mass affects the period then you only want to change the mass of the pendulum bob and have the other variables (length and amplitude) stay the same.

I did have the high school students make a graph of the period as a function of length. Most of them had data over a large enough range that they could tell the graph was not quite linear.   I showed them that now that they have this graph if I asked them to make a pendulum with a period of 1.5 seconds, they could look on their graph and determine the length of the pendulum they would need.

 

 

SF Physics 15 – Oscillations

Unit 1, Lesson 2 of the Science Fusion Module J is about the properties of waves and one of the quick labs that you can download from the online resources is a pendulum lab.  In this lab, Investigate Frequency, students make a simple pendulum and measure the period as a function of the length of the pendulum.  Since its difficult to stop and start a stopwatch for just one complete swing of a pendulum, students measure the time for 10 complete oscillations and then divide by 10 to get the period for one oscillation.  I actually made the lab a little more free form and asked the students to make a pendulum that had a period of 1 second.  We talked about how grandfather clocks worked and what would happen if the pendulum was swinging really fast, or too slow and how that would affect the clock.  I asked the students what might affect the period of the pendulum.  They came up with three things to try, the mass of the bob, the amplitude of the swing,  and the length of the string.  IMG_4255

I had them use ring stands (with clamps and rods) to support their pendulums but the Science Fusion lab suggested taping a ruler to a table so that the end of it was sticking out past the edge of the table, then hanging a thread from the ruler with washers tied to the other end of the string, serving as the pendulum bob.  I have two nice sets of hanging weights so the students used those, which made it easy to change the mass of the bob.

I also printed out big protractors on cardstock so they could measure the initial angle when they start the pendulum.  Students found that changing this initial angle did not change the period of the pendulum, nor did changing the mass.  But when they changed the length of the pendulum, the period changed noticeably.  The longer the pendulum, the longer the period.   Students shortened or lengthened their pendulums until they got a period of 1 second.

We had some time at the end of class to watch the following videos.

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R2 Physics 17 – Phase Changes

Students were  asked to read 7.2 Water, Steam, Ice in How Things Work the Physics of Everyday Life and to watch the following videos before class:

I started class with a lecture on phase changes and how it actually takes a lot of heat to melt ice into water, and even more to change water from its liquid phase to its gas phase (boiling water).  We talked about how perspiring can cool you off, the sweat evaporating takes heat away from your skin, leaving you cooler.  And we discussed how when you are constantly adding heat to a pot of boiling water, the temperature of the water won’t change, all the heat is going into the changing the phase of the water.

IMG_4094I happened to have some sodium acetate trihydrate left over from last year’s chemistry

hot ice

class so I made a super saturated solution using the recipe from this website, 30 ml of water (I used distilled water but not sure that’s necessary) and 170 g of sodium acetate trihydrate that I bought from Steve Spangler’s website.  I mixed them together in a large beaker on a hot plate and stirred over medium heat until all the crystals dissolved.  I then covered the beaker loosely with foil (just to keep stuff from falling into and causing it to crystalize) and let it cool.   Playdough to Plato has a nice description of making hot ice and the physics behind it.   Its a supersaturated liquid and would prefer to be a solid
at room temperature, so when its poured on to a plate over a few solid crystals of sodium acetate trihydrate it immediately crystalizes into a solid (photo).  It also releases  heat energy when going through this phase change and becomes quite warm, hence the term hot ‘ice’.   Here’s a nice video showing different things you can do with hot ‘ice’.

After that demonstration I put a piece of dry ice on a paper plate in front of the students and asked them what was happening to the ‘ice’.  Dry ice does not melt like regular ice, it goes directly into the gas phase from the solid phase, which is called sublimation.  But if we put a piece of dry ice in a closed container the pressure will build up in the container and the carbon dioxide will no longer go directly into the gas phase and we can observe liquid carbon dioxide.

Here’s a video of dry ice in a centrifuge tube with the cap on tight.  You can see the dry ice begin to melt into a liquid and promptly begin to boil (at room temperature).  When the lid is opened at the end of the video, you can hear a hiss as the built of gas is released and you can see the the carbon dioxide turn instantly back into a solid.

We did this lab last year in Chemistry and took a slow motion video, the container was a plastic pipette and we let the pressure build until it popped.

In both of these videos you see the containers are placed in larger containers of waters, this is just to make it easier to see the melting dry ice because the pipetter/centrifuge tube get pretty cold and water from the air will condense on them making it hard to see inside them.  It also contains the ‘explosions’ if the pipette pops.

At the end of class we used the dry ice for various fun ‘experiments’, dropping bits of dry in beakers with soapy water to make lots of bubbles and using the cold vapor in an airzooka.

SF Physics 14 – Waves

IMG_4077We’re moving on to Science Fusion Module J, Sound and Light. The first lesson is on waves.  I started class by asking the students what they knew about waves and wrote everything they came up with on the board. We talked about different ways to describe or characterize waves – how big they are (amplitude), how many wiggles they have (frequency) and the type of wave, longitudinal (oscillating in the direction its traveling) versus transverse (oscillating perpendicular to the direction of travel).  I explained how earthquakes produce both types of waves (P waves are longitudinal and S waves are transverse) and scientists use data from those waves to figure out what the center of the earth is like.   For the most part the students just played with the long slinkys, producing wave pulses and standing waves.  They also measured the speed of a wave pulse and found that the wave traveled faster when there was more tension on the coil.  71OPk6GFY9L._SL1500_There’s a nice demonstration kit with a large slinky and long helical spring that I have and its definitely money well spent.

At the end of class we watched the following videos:

I was hoping to getting sand patterns on some metal plates with my son’s cello bow but we couldn’t get it to work.  I think our plates were too thick and the one thin piece we had was too big and drooped, letting the sand fall off.   See the video below for instructions.

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