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SF Physics 06 – Gravity waves & terminal velocity

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Reed Collge, Portland OR

The past two weeks have been ridiciously busy.  My eldest son and I worked the Livermore Airport Open House all day one Saturday – telling kids about rocks, fossils and lapidary while watching pilots do acrobatics with their airplanes.  The next morning we got up EARLY to catch a flight to Portland and spent all of Monday at Discover Reed College, flying home late Monday night.  I had never been to Oregon before and it was beautiful.  We were both struck by how green Portland was.  There are only two seasons where we live, brown (hot, dry summers) and green (if we’re lucky and actually get our rainy season).   Unfortunately we also happened to see the wildfires in California as we flew directly over them on our way home.

 

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Reed College

This past weekend I taught a 4-H Archery Leader course, teaching other adults and teens how to teach archery.  This is always a lot of fun because the people are great and we spend most of the weekend outside at the archery range.   In the picture below the teen in green is teaching the other, my younger son (who is pretending he doesn’t already know archery) proper shooting form with a piece of elastic.  Note the brown hills as mentioned earlier – I’m ready for rain!IMG_2668

Since I’ve been so busy I didn’t have a lot of time to prepare for class today and when I woke up to the news from LIGO I decided we would do some current events at the beginning of class.  I showed the following two videos and had a lot of discussion with the students on what it meant and why its so cool.

Next week the class is actually going on a fieldtrip to iFly so we spent the rest of class calculating our own terminal velocities (this was an activity that iFly provided us).  When you jump from a plane you do NOT accelerate forever, at some point the force of air resistance becomes large enough to cancel out the force of gravity (your weight) so that the total force on you is zero.  If the total force is zero, then your acceleration is zero and you fall at a constant velocity.  We talked about different factors that contribute to the force of air resistance – the density of air, your speed (think about walking versus running in deep water) and your area – if you spread out your body as you fall the air will resist your movement with a great force than if you move your body into a streamlined position.  There’s also a drag coefficient which depends on your shape.  Students estimated their surface area by assuming their body was made of a series of rectangles and ellipses, measured the length and width of each rectangle, calculated the area and added them up.   Students also had to find their mass in kg.  I have a scale with a switch on the bottom to change the units from pounds to kg so students were able to measure their mass in kg. Most found they had a terminal velocity of around 40 m/s or roughly 90 mph.  Next week when they are at iFly they will find out if their estimates were correct.

Here’s a website that steps you through some  calculations for terminal velocity,

http://www.softschools.com/formulas/physics/air_resistance_formula/85/

 

 

 

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SF Physics 05 – Pressure & Buoyancy

I started class with the following videos on fluids, density, pressure and buoyancy.

Before showing the last video, I crushed a few soda cans using air pressure, then showed the Mythbusters using air pressure to crush a steel tanker car!  For the demo, I put just a little bit of water in the soda can and then place it on a hot plate.

When I see steam coming out of the top of the can, I grab it with tongs and flip it upside into a bowl of ice water.  The ice water cools the can so that the water vaper (steam) inside the can condenses into a liquid leaving the air pressure inside the can very low compared to the air pressure outside the can which is what crushes the can.  The Mythbusters do the same but used a vacuum pump to lower the air pressure inside the tanker car.

The first lab the students did was “Pressure Differences” from Unit 1, lesson 5 in Science Fusion Module I.  Students had two straws, one placed in a beaker with water (and a few drops of food coloring) and the other is held horizontal so that they can blow air across the top of the first straw.  As they blow across the straw their lab partner watches the colored water move in the clear straw.IMG_2332

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When the student blows across the top of the straw, the pressure inside the clear straw decreases and the higher air pressure in the beaker pushes water up the straw.  This is how a straw works when you use it to drink as well, you suck out the air in the straw which causes low pressure and the high outside pressure pushes down on the liquid in the cup forcing it up the straw.

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The second lab was “Finding the Buoyant Force” , also from Unit 1, lesson 5.  For this lab students took a bit of clay and squished onto a string and found its weight in Newtons by hanging it from a spring scale.  They also filled a graduated cylinder about half way with water and recorded the volume of water.   Then they lowered the clay into the water, still hanging from the spring scale and observed that the force measured by the spring scale decreased as the clay was submerged in water.  The force decreased because there is now a buoyant force acting on the clay, pushing up on it.  When the clay is completely submerged, students measured the force on the spring scale and the water level in the graduated cylinder (volume of original water + clay).  Students then calculated the buoyant force by finding the difference in the spring scale readings (F above water – F in water) and they calculated the volume of the clay.

Since the density of water is 1 g/ml we can actually calculate the buoyant force since its equal to the weight of the water displaced by the clay.  Say the volume of the clay was 10 ml, that means 10 ml of water was displaced and that much water has a mass of 10 grams (density  = mass/volume = 1.0 g/ml for water so  every ml of water has a mass of a 1 g).  The weight of that displaced water is 9.8 m/s2 times its mass (10g = 0.010 kg) which equals 0.098 N. This matches the buoyant force they  found by using the spring scales.

We ended class by crushing the rest of the soda cans with air pressure…. because physics is fun.  Here’s a video of people crushing a 55 gallon steel drum with the same method I used for the can.

SF Physics 04 – Gravity

We started class today with the following videos:

And we can’t talk about the vomit comet and feeling weightless without watching OK Go.

For the lab I used Activity #4, Listen to gravity, like Galileo did, from “Gravity” a physics lab from Ellen McHenry’s Basement Workshop.  Students tied washers to a piece of string, either equally spaced (50 cm) or with the distance between washers increasing (10 cm, 30 cm, 50 cm, 70 cm, 90 cm) as they tied them to the string.  If you hold one end of the string up high (standing on a chair/stepstool) and let it hang to the floor, what do you think you will hear when you drop the string with equally spaced washers? Will the noises be equally spaced in time or when they the ‘dings’ get closer together as the string/washers accelerate?  What about the second string with the washers spread out at increasing distances?  Will the dings be further apart as the string falls, or when they will be equally spaced?  After the students think about it a bit… and untangle their string of washers (this took almost as much time as tying them all on to the string) they actually drop them and listen to the pattern of sound the washers make.  We dropped the string of washers on to a cookie sheet to make a nice loud ding.

The string with the equally spaced washers hit the ground at shorter and shorter time intervals since the washers that fell longer are accelerating for a longer period of time and going faster when they travel the last 50 cm before hitting the cookie sheet.  The washers that were tied at greater and greater distances hit the ground at roughly equal time intervals. Here’s a slow motion video of the string with equally spaced washers.

Slow motion video of the second string, with washers at increasing distances but hitting the pan at almost equal time intervals.

Its kind of hard to hear when you’re doing the actual experiment so having someone take a movie so you can watch/listen in slow motion is useful.

SF Physics 03 – Forces

Lesson 3 in Science Fusion Module I is about forces and Newton’s Laws of Motion so I started class by talking about forces and showing a few videos.  The first two lego stop motion movies were made by students (not mine).

I also showed this video of Felix Baumgartner and pointed out his velocity to the students and we watched as it quickly increased to over 800 mph.  But after a few minutes he started to slow down until he reached about 130 mph and he stayed roughly at that speed until he opened his parachute.

I asked the students why did his speed slow down as he was falling?  We talked about air resistance and how the thickness of the air and therefore the force of air resistance changes with altitude.  The force of air resistance also depends on your speed – think about trying to run in waist high water versus walking slowly, its much more difficult to run through the water than walk – same thing with air resistance.  At some point as you fall the force of gravity and the force of air resistance balance out so that the net force on you is zero, therefore you’re acceleration is zero (F=ma)  and you fall at constant velocity.

To demonstrate Newton’s 1st law I built a lego car with a smooth top so the minifigure sitting on top could move easily.  I gave the car a push and when it hits a rock the passenger continues traveling until she hits the rock herself.  I took the video below with the video physics app and if you play it at a slower speed (hit settings and put play speed at 0.5) you can see the motion a little easier.  The red dots show you the position of the minifigure’s head in each frame.

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We did Unit 1, Lesson 3 S.T.E.M. lab, Newton’s Laws of Motion, which means we built straw rockets.   Students wrapped paper around a stiff plastic straw and then taped the paper to make the body of the rocket.  They then twisted the end to make a  cone on one end.  Some added fins and other stuff to their rockets.  They also tried using more or less paper, thicker paper, etc.  To launch the rockets we had stiff straws in plastic bottles with clay holding them in place and blocking air flow, so that when you squeeze the bottle air shoots out of the straw.  We had a variety of plastic bottles to try.  The goal was to get their rocket to travel the furthest.  I believe the longest flight was 140 some inches.

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The student above has put fins on his rocket and is about to launch it by squeezing the air out of the bottle.  The air exits the bottle and pushes on the front end of the straw rocket accelerating it.

SF Physics 02 – Acceleration

Today’s lab was Acceleration and Slope from Unit 1, Lesson 2 in Science Fusion Module I. You need online access to retrieve the labs for Science Fusion. If you buy the homeschool package through the Homeschool Buyers Co-op then you will have access to the labs.

The lab suggests making a ramp out of metersticks (place two beside each other with a small gap between them) but I happen to have a huge marble run set so I had the students build ramps from the Chaos Tower set.

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Two ramps built from Chaos Tower set.

Half the track is flat against the table and the other half is elevated at one end to make a ramp.  Students release the marble from the top of the ramp so that its initial speed (velocity) is zero.  I asked the students what they thought the marble would do and they all realized it would speed up or accelerate when going down the ramp.  Some said it would slow down when it reached the flat track, which is technically true since there is friction, but with this set up we can ignore the friction and the marble should continue at nearly constant speed once its on the flat part of the track.

Once the tracks were built, students released the marble from the top of the ramp and timed how long it took to get to the bottom of the ramp.  They did this 5 times and found the average time for going down the ramp.  Then they repeated the experiment but measured the time the ball was rolling on the flat part of the track.  So the marble is rolling down the whole track 10 times but the students measure the time spent on the ramp the first 5 times and then measure the time spent on the second half of the track the last 5 times down.  Students also had to measure the length of the flat track.

The goal of this experiment is to find the acceleration of the marble on the ramp.  Acceleration is the change in velocity over the change in time.  We know the initial velocity is zero because the marble starts from rest.  The final velocity of the marble can be calculated from the data on the flat track.  Velocity equals distance covered (length of flat track) divided by the time it took to travel that distance (flat track time), v = x/t.  Students found the marble was traveling at roughly 100 cm/s on the flat track.

Now we can calculate the acceleration on the ramp because the change in velocity is final velocity minus the initial velocity (which was  zero), divided by the time it was accelerating (the time it took to go down the ramp), a = v/t.  Accelerations vary depending on the steepness (slope) of the ramp, so students got values between 50 and 100 cm/s2 .  They then repeated the whole experiment with a steeper ramp and found that the acceleration increased.

This is a pretty straight forward lab and helps the students learn to take averages, keep track of their units and use their calculators for some simple calculations.  It also easy enough to do at home with any kind of marble track, or even hotwheels ramps and cars since all you need is a stopwatch (or clock app on your phone) and a ruler.

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