## Demonstrating Rotational Inertia

Have you ever struggled to describe Rotational Inertia to your students? Even worse, have you ever struggled to understand Rotational Inertia yourself? Did you know Rotational Inertia is the same as Moment of Inertia? Yeah, I’m with you there. I did not know the name had been changed until recently. However, I do think Rotational Inertia is a more logical phrase than Moment of Inertia. Well, if you would like some help with the concept of Rotational Inertia, then I highly suggest the Rotational Inertia Demonstrator from Arbor Scientific because it is an easy way to demonstrate the concept of rotational inertia. The demonstrator is composed of three pulleys of different sizes all centered around the same axle. Attached to the pulleys are four spokes on which four masses can be placed. The distance from the axle, or axis of rotation, of the four masses on the spokes can be adjusted.
In order to understand rotational inertia, we should first review the equation for rotational inertia of a system of particles:
The rotational inertia of a system of particles equals the sum of the quantity of the mass of each particle times the square of the distance each particle is from the axis of rotation. While the Rotational Inertia Demonstrator does not appear to be a system of particles, the equation for the rotational inertia of a system of particles helps us to understand how the rotational inertia of the demonstrator changes when we adjust the locations of the four adjustable masses. The closer the four adjustable masses are to the axle, or axis of rotation, the smaller the “r” value in the rotational inertia equation and the smaller the rotational inertia of the demonstrator.
We also need to review the Rotational Form of Newton’s Second Law of Motion to better understand rotational inertia. The net torque acting on an object equals the rotational inertia of the object times the angular acceleration of the object. Please remember torque and angular acceleration are vectors.
Notice the similarities to the Translational Form of Newton’s Second Law of Motion. The net force acting on an object equals the inertial mass of the object times the linear acceleration of the object. Again, remember force and linear acceleration are vectors.
Force is the ability to cause a linear acceleration of an object.

Torque is the ability of a force to cause an angular acceleration of an object.

Torque is the rotational equivalent of force.

Rotational inertia is the rotational equivalent of inertial mass.

Angular acceleration is the rotational equivalent of linear acceleration.

But, what does it mean that rotational inertia is the rotational equivalent of inertial mass? Inertial mass is the measurement of the resistance of an object to linear acceleration. Therefore, rotational inertia is the measurement of the resistance of an object to angular acceleration. In other words, the greater the rotational inertia of an object, the more that object will resist an angular acceleration. Referring to the rotational inertia demonstrator, the farther the four adjustable masses are from the axis of rotation, the larger the “r” value in the equation for the rotational inertia of a system of particles, therefore the larger the rotational inertia of the demonstrator. The larger the rotational inertia of the demonstrator, the larger the resistance of the demonstrator to angular acceleration. In summary, the larger the distance the four adjustable masses are from the axle, the larger the rotational inertia, and therefore the larger the resistance of the demonstrator to angular acceleration.
This is demonstrated below by hanging a 100 g mass from the largest pulley in two simultaneous demonstrations. In the demonstration on the left, the four adjustable masses are close to the axis of rotation and therefore the rotational inertia of the system is smaller. In the demonstration on the right, the four adjustable masses are farther from the axis of rotation and therefore the rotational inertia of the system is larger. When both demonstrators are simultaneously released from rest, because the net torque caused by the 100 g masses is approximately the same, the demonstrator with the larger rotational inertia on the right has a smaller angular acceleration. In other words, the demonstrator with the larger rotational inertia speeds up rotationally at a slower rate. Going back to the Rotational Form of Newton’s Second Law of Motion, because the net torque is almost the same, a larger rotational inertia results in a smaller angular acceleration:
Notice we are always keeping the four adjustable masses the same distance from the axle, or axis of rotation. This is to keep the center of mass of the system at the axis of rotation of the system. When the four masses are not equally spaced from the axis of rotation, then the center of mass of the system is offset from the axis of rotation and the force of gravity acting on the system causes a torque on the system. The force of gravity causing a torque on the system makes understanding the demonstration much more complicated. In the examples shown below, the demonstrator on the left with four masses equally spaced from the axle rotates at almost a constant angular velocity. The demonstrator on the right has one mass farther from the axis of rotation and therefore the whole system actually becomes a physical pendulum. The system oscillates back and forth in simple harmonic motion. While this is interesting, it does not provide an obvious way to learn about rotational inertia. In summary, it is much easier to learn about rotational inertia from the demonstrator if all four masses are equally spaced from the axis of rotation.
Let’s look at another set of demonstrations below to learn about rotational inertia. As in the previous demonstration, on the right, we have a 100 g mass hanging from the largest pulley and all four adjustable masses far from the axis of rotation. On the left, all four adjustable masses are still far from the axis of rotation, however, the 100 g mass is hanging from the smallest pulley instead. In other words, both rotational inertia demonstrators have the same rotational inertia and the force of gravity acting on the string is the same, however, the net torque acting on each demonstrator is different. Recall torque equals the “r” vector times the force causing the torque times the angle between the direction of the “r” vector and the direction of the force. The magnitude of the “r” vector is the distance from the axis of rotation to where the force is applied to the object:
Because the 100 g mass is hanging from the small pulley on the left and the large pulley on the right, the “r” vector for the small pulley is smaller and therefore the net torque acting on the demonstrator through the small pulley is less. Therefore, according to the Rotational Form of Newton’s Second Law of Motion, the angular acceleration of the demonstrator on the left is less than the angular acceleration of the demonstrator on the right.
Our last set of demonstrations has both demonstrators with identical rotational inertias and masses hanging from the smallest pulleys. Also, both demonstrators have a 100 g mass hanging over the left side of the pulley. However, the demonstrator on the right has a second mass, a 200 g mass, hanging over the right side of the pulley. This means the demonstrator on the right has two different masses hanging off of the smallest pulley.
In order to determine what is going to happen, remember the Rotational Form of Newton’s Second Law of Motion includes net torque, not just torque.
In this example, the net torque from the two masses on the demonstrator on the right actually has roughly the same magnitude as the net torque acting on the demonstrator on the left, however, the directions are opposite from one another.
Again, both demonstrators have the same rotational inertia, are using the same pulley, and have a 100 g mass hanging over the left side of the pulley. The pulley on the right adds a 200 g mass hanging over the right side of the pulley. For the demonstrator on the right, the 100 g mass hanging over the left side of the pulley essentially cancels out 100 g of the 200 g mass hanging over the right side of the pulley. This effectively means the right demonstrator essentially has a 100 g mass hanging over the right side of the pulley. Therefore, the net torques on both demonstrators have essentially the same magnitude and opposite directions. Therefore, the angular accelerations of both demonstrators should have roughly the same magnitude and opposite directions. You can see that is true in the demonstration.
But why do the two demonstrators have “roughly” the same magnitude angular accelerations? Adding the 200 g mass to the demonstrator on the right increases the total mass of the system. Because inertial mass is resistance to acceleration, increasing the total mass of the system actually decreases the angular acceleration of the system a little bit, even though the net torque should be roughly the same. Proving this requires drawing free body diagrams, summing the torques on the wheel, and summing the forces on each mass hanging, so I am not going to walk all the way thought that solution here.
There are many more ways you can make adjustments to the rotational inertia demonstrator to better help understand rotational inertia. For example, ask yourself what would happen to the angular acceleration of the demonstrator if the only change we make to it is to increase the mass hanging from the demonstrator? Increasing the mass hanging from the demonstrator increases the net torque acting on the demonstrator. The rotational inertia remains the same. Therefore, according to the Rotational Form of Newton’s Second Law of Motion,  , the angular acceleration of the demonstrator will increase.
What if the only change we make is to change the locations of the four adjustable masses from all being at their farthest extreme positions to having two of the adjustable masses near the axis of rotation and two adjustable masses far from the axis of rotation? Bringing two adjustable masses near the axis of rotation decreases the rotational inertia of the system and therefore, according to the Rotational Form of Newton’s Second Law of Motion, the angular acceleration of the demonstrator will increase. Notice, this will only work when the two close adjustable masses are opposite one another and the two far adjustable masses are also opposite one another. If this is not the case, the center of mass of the rotational inertia demonstrator will not be at the axle, or axis of rotation, which is a problem we addressed earlier.
The pulley sizes of the rotational inertia demonstrator are provided by Arbor Scientific. They are 20.22 mm for the small pulley, 28.65 mm for the medium pulley, and 38.52 mm for the large pulley. Given this information, we can even predict which way the rotational inertia demonstrator will rotate if we were to hang 100 g over one side of the large pulley and 200 g over the other side of the small pulley. Before releasing the demonstrator, the angular acceleration of the demonstrator is zero because it is at rest. Therefore the torque caused by the 100 g mass will be 0.3852 meters times 0.100 kilograms times 9.81 m/s2 times the sine of 90 degrees which equals roughly 0.38 N.
The torque caused by the 200-gram mass will be 0.2022 meters times 0.200 kilograms times 9.81 m/s2 times the sine of 90 degrees which equals roughly 0.40 N.
Therefore, the net torque caused by both masses acting on the demonstrator before it starts to accelerate is the difference between these two torques because they act in opposite directions.
Therefore, because the torque caused by the 200 g mass is larger than the torque caused by the 100 g mass, the rotational inertia demonstrator will rotate in the direction caused by the torque of the 200 g mass.
Please realize these torque calculations are only correct while the demonstrator is at rest. Once the demonstrator begins to accelerate, the force of gravity and the force of tension acting on the mass hanging are no longer the same and we would need to draw free body diagrams and sum the forces on each hanging mass.
If you enjoyed watching this video by Jonothan Palmer, the creator of Flipping Physics, please let us know in the comment section below and check out his YouTube page for more videos like this one.

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## The physics of a roller coaster loop

Each year millions of people will visit amusement parks in order to ride some of the fastest, highest, most extreme roller coasters. These machines thrill us because of their ability to accelerate us from a standstill to unbelievable speeds in a matter of seconds while changing from one direction to the next in an instant.
There is so much physics going on in the loop of a roller coaster. Angular velocity, centripetal acceleration, conservation of energy, and more! In this Cool Demo, we are going to look at how we can collect the data by using a Hot Wheels track and by placing a PocketLab Voyager on the Hot Wheels car.
By placing magnets at each connecting section of the track you can now generate “gate” times with PocketLab’s magnetometer. Using a 3D printer you can print a new set of connectors that are designed to house a small magnet. ( 3D print files are available to download in the resource section) When the car passes over these sections of the track you will be able to see a change in the magnetic field. Using this change and time we can come up with “timing gates” at each of these sections, and knowing the distance the car has traveled we can calculate the speed of the car.
The most obvious section of a roller coaster, or in this case, the Hot Wheels track is the loop. Although the loop of the Hot Wheel track is a circle, in reality, roller coaster loops have a tear-dropped shape that is geometrically referred to as a clothoid.
As the car passes through the loop, you can see the track bends into a tear-dropped shape. Once the car passes through the loop we are able to measure the angular velocity or the rate of change of the angular rotations, as it’s moving through that loop using the PocketLab’s Gyroscope.
Roller coaster rides are notorious for creating g-forces. The PocketLab also has an accelerometer, so as the car passes through the loop you can also measure the g-forces a person would be experiencing if they were traveling in the car. Traveling around a circle creates a centripetal force that the rider experiences as a g-force. The force is a function of speed and radius.

The Flip Flap Railway was built in 1895 and was the first roller coaster to have a loop. It was “famous” for its extreme g-forces that it produced on its riders of approximately 12 gs. The circular nature of the coaster’s loop along with its small diameter of 25 feet caused riders to experience neck injuries from whiplash. There are some interesting accounts where riders are hanging on for dear life in a death grip on the sides of the railcar and surviving a 12g ride which is absolutely nuts! Modern looping roller coasters all use teardrop-shaped loops to reduce the g-forces. The Flip Flap Railway was the last coaster to use a truly circular loop.

Looking at the Data:
1. The time it took the car to travel through the loop = 0.34 seconds.
2. The average angular velocity (gyroscope) through the loop = 1,170 degrees/seconds
3. The average acceleration through the loop = 3.7 g
Data analysis:

Looking at the angular velocity inside the loop can be done in two ways:

1. We can calculate the average loop velocity using our timing gates. (The time we exit the loop – the time we enter the loop and using the circumference of our track. Plugging in the geometry in our time we get 1.9 meters per second as our average velocity.
2. Using the (1.9 m/s) velocity we can calculate the average angular velocity of 18.5 radians per second or 1060 degrees per second.

To get the g-force we need to calculate the following:

1. Taking the timing gate data to calculate the G-Forces that would be felt inside the loop; (18.5 radians per second)²(0.1 meters per second) = 3.9 g.

The PocketLab Voyager has an array of sensors built into a small package. This allows you to measure data in scenarios such as this Hot Wheels Loop track experiment. Simply connect it to your smartphone or tablet through Bluetooth and you will be able to see the data live in the palm of your hands. On-board memory is also included for when you the PocketLab Voyager is out of Bluetooth range. The best thing about PocketLab Voyager is that that it comes packaged with some many features compared to equipment that costs thousands more.

Explore the world around you with the sensors built into the PocketLab Voyager:

• Measure Acceleration
• Angular Velocity
• Magnetic Field
• Range Finder
• Altitude
• Barometric Pressure
• Ambient Temperature
• Humidity
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• Dew Point
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## Demonstration of The Photoelectric Effect!

In 1905 Albert Einstein had his miracle year, publishing 5 papers, including the Special Theory of Relativity, the Mathematical Description of Brownian Motion, and the E=mc2 formula.  One of these papers was titled “On a Heuristic Point of View about the Creation and Conversion of Light.” [Ref 1]  In section 8 of this paper he develops a mathematic model that describes how light creates cathode rays.  What was heuristic was that he described light as “energy quanta” (photons) for the first time but what was worth a Nobel Prize, was that he was correct.
 Figure 1.A –Max Planck presents Einstein with the 1929 Planck Medal for extraordinary achievements in theoretical physics. Figure 1. B – Albert Einstein c1905 at the patent office desk where he worked as a third-class clerk, and occasionally worked on his “miracle” papers.
It is quite easy to demonstrate the photoelectric effect with an electroscope and a short wave UV-C lamp. By placing a negative charge on the electroscope, and shining the short wave UV light on top, it will discharge. Short wave UV is usually blocked by glass, but visible light is not, thus a pane of glass can be used to show that it is not just regular light that is causing the discharge.

Figure 2 – The electroscope is discharged by shining short wave ultraviolet light upon it.

It helps that on the top of the electroscope there is a zinc plate which has been scrubbed with steel wool.  This removes the zinc oxide layer and makes it more sensitive to the light.  Also note that if the experiment is tried with a positive charge, for example with a piece of glass rubbed with silk, it will fail to discharge.  This is because the photoelectric effect is only for electrons and will only work on negative charges because they are at the surface, but the positive charges are held deep in the nucleus by the strong force. In fact, the photoelectric effect is a great way to identify positive vs. negative charge.

With any form of visible light, you will not get the electroscope to discharge. It does not matter how bright the light is, even lasers will fail.  It takes ultraviolet, specifically the short wave UV-C, because even blacklights are not energetic enough to liberate the electrons.

The thing with light is, the shorter wavelength the more energy it has, and this was already well-known, from Max Planck’s formula  E = h f  but what was worth a Nobel prize, was the idea that the light hits one electron at a time.  Not as waves, but as individual pieces of light.  ONE particle of light hitting ONE electron at a time.  Einstein called this particle a quantum of light, meaning that it is a discrete exchange of energy.  We now call this a PHOTON.

Figure 3 – Albert Einstein took Max Planck’s formula for the quantization of energy in black bodies and extended it to describe light.  This implied that light delivered its energy in bundles of E=hf.  This was a new and heuristic idea, but it explained the photoelectric effect.  KE is the energy of the escaping electrons and W is the energy required to liberate them from the metal.

Figure 4 – The Phet Simulation of the photoelectric effect is a great way to engage students with the details of this modern physics concept. It is based on the experiments of Robert A. Millikan who proved Einstein’s perspective by experiment.

It is also possible to demonstrate the photoelectric effect with a small neon bulb.  These will turn on at about 70 volts.  Hook one to a high voltage source and then dial it back until it is just about to turn on.  Now it will be so sensitive, that just a little extra energy will make the electrons jump and conduct. This will NOT work for red and green light.  It must be blue, or ultraviolet.  That’s right, blue light ACTUALLY DOES have more energy than red and green light.  This is because light travels as photons, and the shorter the wavelength of light, the more energy per photon.

Figure 5 – The energy of blue light can cause electron conduction in a small neon bulb. The bulb stays on because plasma conducts better than rarefied neon.

Reference

http://hermes.ffn.ub.es/luisnavarro/nuevo_maletin/Einstein_1905_heuristic.pdf

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## Reflection, Refraction, Diffraction & Interference…… That’s COOL!

The Mini Ripple Tank is a great way to address the wave-energy standards and to teach about the properties of waves by showing how water waves behave.  This is in keeping with the history of physics and the modern experimental approaches to science instruction.

The Mini Ripple Tank eliminates the clumsiness of the larger ripple tanks of old and gives the opportunity for students and teachers to interact with the wave properties quickly and engagingly.  Because of the competitive price, and variety of available experiments.  It is even reasonable to buy a class set.

Fig 1.  Plane waves are being produced and are readily visible on the built-in screen.

Getting Familiar

The Mini Ripple Tank contains a small pan for water and a vibrating source.  The strobe light below projects waves of various frequencies on a fold-down screen.  Both the strobe and the wave frequencies can be varied, generating many interesting effects.  There is also a synchronizing mode which links the two (this is very helpful when measuring wavelength).

Fig 2. Water is filled up to half of the height.  The adjustable strobe projects from underneath.

The device comes with three distinct wave generating mechanisms: single source, double source, and plane waves.  The single source is the most fundamental and is helpful in instructing on wave basics and Huy gen’s Principle (plane waves are a sum of circular waves).  The double source can be used best at teaching interference experiments (more below) as well as testing out the diffraction formula.  The plane wave source is the one I tend to use the most often because it sets up a standard wave that can readily land upon the other implements which are used to redirect the waves.

Fig 3. The nine components.  Left to right: the two lenses and the prism, the two barriers and the parabolic mirror, and the double and single sources, as well as the plane wave source.

As for general tips, it is helpful to use a document camera for larger classes, also adding blue dye can sometimes improve visibility, and try to not overload the tank with water – either fill halfway or just enough to barely cover the lenses and prism.  Experiment a lot with wave and strobe speeds to improve the visibility of the desired effects.

Refraction by the Lenses and the Prism

The bending of light waves by glass is well-known, but is this a property of all waves?  Yes! Demonstrate this dramatically by bending water waves with lenses and prisms.  The shallower the water, the slower the waves.  This is analogous to the denser the medium, the slower the light waves (with few exceptions).

Fig 4. The prism can bend the waves by slowing their propagation.

Again, remember to keep the water shallow.  Some experiments can include measuring the focal lengths of the two lenses (positive for convex, negative for concave), measuring the index of refraction for the prism (by wavelength change, speed change, or Snell’s Law), and measuring how water depth affects refractive index.

Fig 5. The refraction formulas that can be used for quantitative experiments.  The first formula might be the least familiar – wavelength changes with index of refraction.  The second formula compares a standard speed c with the new slower one v to define the index n.  The third is the famous Snell’s Law.

Somehow it is very satisfying to see the focusing of water waves when using lenses.  The ray approach to drawing images known as geometric optics does not provide a hypothesis as to the wave nature of light, but this experiment convincingly demonstrates that refraction and focusing is something that waves do!  Refraction and lens effects are a powerful piece of evidence that demonstrates the wave nature of light.

Fig 6. The convex and concave lenses demonstrate convergence and divergence of waves respectively.

Reflection by Barriers and the Parabolic Mirror

The law of reflection can be readily demonstrated by the Mini Ripple Tanks (by stacking the barrier pieces) however, the best demonstration is the focusing of waves by the parabolic mirror.

When a plane wave enters parallel to the axis of a parabolic mirror, it will be reflected to the focus of that mirror.  This is the basis for Newtonian Reflector telescopes that remain the standard style in modern times.  A reversal can also be achieved by placing the single source at the focal point and reflecting out plane waves.

Fig 7.  Reflection of plane waves off a parabolic mirror will focus them to a point.

Diffraction by Barriers

le-slit diffraction of waves is easily demonstrated with this simple device.  Just place the barriers in the path of the plane wave source and the effect is immediately present.  Manipulating the opening and wavelength can help illustrate the variables: more diffraction occurs the smaller the opening is allowed to be.

Fig 8.  Single slit diffraction shows the bending of a plane wave source as it passes through an opening, illustrating Huygen’s Principle that plane waves are a sum of circular waves.

The diffraction formula for quantitative experiments is best applied to the two-source case, however, and while this is only a case of interference and not diffraction, it does provide an opportunity to apply the formula experimentally.    Here, we see both versions of the formula, the angular version, and the small angle approximation.  I prefer the second one because lengths are usually easier to measure than angles.

Fig 9. The diffraction formulas:  The symbol d represents the distance between the sources, and lambda as always is wavelength (which is the dependent variable in this experiment).  Theta is the angular distance to an interference fringe as measured from the spot half-way between the sources.  X is the linear distance between the interference fringes, these are the locations of constructive interference.  L is the linear distance from the point between the sources to the point of interest, and because there is more than one location of constructive interference, m is the index number which labels these points as m=1,0,-1,2, etc, (any integer).

Fig 10. An interference pattern is easily generated with these two sources.  The diffraction formulas above will apply to this double source interference pattern, even though no diffraction is occurring.  This demonstration can be converted into a quantitative experiment.

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## Powering Imagination and Creativity OneCar at a Time

 The OneCar is Arbor Scientific’s answer to the various needs of STEM Educators. It is an open-ended creativity-driven approach to science teaching that addresses cross-cutting concepts and offers an opportunity for tinkering and design. Pictured here is one of the eight OneCar Packets which comes in every kit. Each can be used to construct eight different cars. Students can be creative. The kit itself is jam-packed with 8 sets of experiment options, enough for large classes and extras for spare parts. A typical starter lab would be building the battery-powered motor car. A motor in its housing slides into the chassis. These cars can be assembled and disassembled each class period. This might be used for speed and acceleration experiments. But the options expand rapidly as more options are introduced. The OneCar offers 8 ready to go experiment options. These can be extended and combined in creative ways to allow for the open-ended labs that STEM teachers have been searching for. For example, the motor can be used to drive a fan or be powered by a rechargeable capacitor. The lessons can go beyond physics or include chemistry lessons such as air pressure and electrochemistry or even acid-base reactions. Perfect for Physical Sciences courses. There is more too. The Potential energy in a rubber band lab or the classic mousetrap car can be readily created using this kit. The rubber band-powered car can be used to investigate potential energy. The mousetrap car is a classic lab in physics used to teach energy and simple machines. Adding a lever arm and CDs for wheels is a common innovation. You can also build a solar-powered car. Challenge your students to discover what can be done to optimize its traveling speed? Face it south? Angle the collar panel? Use a mirror to reflect the sunlight? A solar powered car can be created and manipulated to optimize its efficiency. The solar panel can be either connected directly to the motor or, in this case, used to charge up one of the supercapacitors. Note how the solar panel is angled to be perpendicular to the sun, just like solar panels on rooftops. All of these options can be mixed and matched. That is the whole idea of open-ended inquiry education. The OneCar gives students many opportunities to be creative in solving engineering challenges. Many of the above images come from videos on our website: ArborSci.com/OneCar.   Take a look and see how fun and easy it is to build these designs.

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The Lab4Physics App is a helpful tool for teaching physics and physical science. It is a lab app for smartphones and tablets, and because of the familiar controls and friendly, easy-to-use interface, all your students can use it

 The App works by using the built-in features of cell phones and tablets that convert easily to probeware, such as the accelerometer, which we will explore first. Fig 1.  The Lab4Physics home screen.  When you open the app, there are lots of experiments you can try (which are categorized on the left) or you can go straight to the tools (right) and perform your own experiments.

ACCELEROMETER

 If you shake the phone up and down, the accelerometer records this motion in 3D. Deleting the X and Z axis, we will now graph only the Y-vertical motion. Fig 2.  It is easy to use the accelerometer to measure the earth’s gravity field strength.  Here the phone was held vertical then slowly turned to lay flat.  The gravity constant 9.8 m/s2 is measured. The app allows you to zoom in, both vertically and horizontally, and slide the image around, just like a picture or map.  Because this interface is so familiar, students will already know how to do this. Fig 3.  The phone’s Acceleration is measured in 3 dimensions, but typically you only need one. Because the accelerometer is so easy to use, you will find yourself using it in many different applications, such as spring and pendulum experiments.  Note that when facing the phone, X is right and left, Y is up and down, and Z is toward and away.  The positive axes are right, up, and toward, which you can remember with thumb X, open fingers Y, palm-slap Z. Fig 4.  Zoomed-in on the image of the above data.  Vertical zoom for precise amplitude measurements and horizontal zoom for precise time (period) measurements. Fig 5.  A plastic bag is a convenient container for the phone when performing spring and pendulum experiments.  The touchscreen still works fine through the plastic.

SONOMETER

 Using the microphone, Lab4Physics can analyze the intensity and frequency of a sound that the phone records. With this device, you can see the waveform of the frequencies that the phone picked up. Use this to compare the amplitudes of loud and quiet sounds or the frequencies of a high and low pitch.  This works as an instant oscilloscope. It is also possible to measure the period as the time between peaks, it helps to zoom in for this. Fig 6.  The Sonometer makes a measurement of the author’s whistling ability.  The period can be measured as the peak to peak time, or the Highest peak frequency can be displayed automatically by using the Intensity vs. Frequency feature. The waveform displayed looks transverse, but the sound is a longitudinal wave.  Therefore, it is important to explain how this wave was generated.  It was the motion of the vibrating microphone that moved a small magnet that generated the electricity that became the signal displayed. The device also can calculate the frequency of the loudest part of the signal it is detecting.  This can be used to test who sings with the highest or lowest frequency or just to check the frequencies of musical instruments. Fig 7.  A tuning fork, which is supposed to be the musical tuning standard A 440Hz, is revealed to be very nearly correct by the Lab4Physics App’s Sonometer feature.

CAMERA / MOTION TRACKING

 One of the most useful features is the ability to track an object’s motion.  Utilizing the phone’s camera, film an object (usually with a ruler in the picture), and by tracking at a specific point on the object, you can follow its motion through the frames of footage. Fig 8.  An accelerating toy car has its motion tracked through ten frames of footage generating the expected parabola of an accelerating object. Because the frames are equally separated by time intervals the app can turn this data into a distance vs. time graph.  From this data, it further generates the acceleration and velocity graphs.  Even a Data Table is provided so you can sort out anomalous data or analyze further.

SPEEDOMETER

 Lab4Physics also has a speedometer which is a streamlined alternative to stopwatches.  Students can, for example, set up a series of positions and click the split button to get the individual times for when the object is at that position.  Using this, graphs are generated for position and velocity. Fig 9. A typical Speedometer experiment. Tracking the position of a toy car through space. Changing it from going slow to fast can show up on a position vs. time graph.

EXPERIMENTS AND LABS

 Lab4Physics has lots of ready to go labs to instruct your students, or you can use them to give you ideas.  Here we explore some of the labs on waves. Fig 10.  Left, a screenshot from the app shows the four labs on waves.  Choosing Do-Re-Mi takes us eventually to this screen, right, which shows how we will be exploring the frequency of a musical instrument. The labs take the students through the experiment in five or six steps.  They are self-contained and complete and let you know how much time the activity should take.

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## Cool Stuff Demos with Violet Lasers

If you are talking about optics in the classroom and students are learning about how light waves behave, take a look at the blue-violet laser, which produces wavelengths at 405 nm.

“I definitely want one of those blue LASERS! Ahhhh… to write my name on a phosphorescent board the FIRST day of class in the dark from the back of the room. That WILL get their attention!”   -Buzz Putnam

Sure, it is a different color and that is always cool, but why use this over any other Laser?

• Cover most of the visible spectrum – If you only have the red (650nm) and green (532nm) Lasers, you are still missing a large part of the visible spectrum. At 405nm, the blue-violet Laser provides a good representation of the shorter wavelengths present on the opposite end of the visible spectrum from red.
• Diffraction grating differences – You can compare the red, green, and blue-violet Laser colors by pointing them through a diffraction grating to observe where the different wavelengths end up.
• More Fluorescence – Unlike the green Laser, the blue-violet Laser can produce fluorescence on a wide variety of materials. In other words, the blue-violet wavelength of 405 nm excites the electrons of most materials to a higher energy level than the green Laser.
• More phosphorescence – For your next trick, we recommend shining the Laser on something with “glow-in-the-dark” properties, such as a sheet of glow-in-the-dark paper. The effect, called phosphorescence, is due to the same characteristics of excited electrons that we saw in florescence. Only with phosphorescence, it takes longer for the material to transition back to its ground state, and therefore you see it longer with those types of materials. The green Laser does not produce these same effects. Show your students both situations and ask them why!

#### Why pay \$79 for a violet LASER

Arbor Scientific has carefully screened all of our Lasers to make sure they offer a higher level of safety and peace of mind. There are low-cost versions available on the market today that could pose serious risk to your students due to a lack of infrared (IR) filters. Even pointers that use IR filtering could still be harmful, due to shoddy manufacturing that provides poor conversion efficiency (when converting from infra red to visible light). While all Lasers should be handled carefully to prevent users from harm and should never be pointed at unprotected eyeballs, these lower cost Lasers are particularly problematic in the academic atmosphere where there are many people in close proximity. For the safety of your students, please always make sure you have taken all the proper precautions possible, including the use of effective IR filtering.

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### Phosphor Glow Paper

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### Standard Red Laser Pointer

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