Department of Physics and Astronomy University of Missouri Yun Zhang It is hard to overemphasize the importance of practical demonstrations for a true and deep understanding of physics. While it is of course very important to understand the fundamental laws and relationships of physics in their theoretical aspects, perhaps nothing like a practical demonstration can bring them to life for a student of physics. Practical demonstrations of the laws of physics make clear to students the power of physics to systematize the seemingly random phenomena, and the power of our own mind to make sense of them.
Transcript
Department of Physics and Astronomy
University of Missouri Yun Zhang
It is hard to overemphasize the importance of practical demonstrations for
a true and deep understanding of physics. While it is of course very
important to understand the fundamental laws and relationships of physics
in their theoretical aspects, perhaps nothing like a practical demonstration
can bring them to life for a student of physics. Practical demonstrations of
the laws of physics make clear to students the power of physics to
systematize the seemingly random phenomena, and the power of our own
mind to make sense of them.
i
Part 1: Mechanics ....................................................................................................................... 1 1. Projectile Motion: Shoot the Monkey ............................................................................................ 1 2. Centripetal Force: Pail of Water ..................................................................................................... 2 3. Newton’s Laws and Conservation of Momentum: Skateboard ....................................................... 2 4. Elastic Collision Between Equal Masses: Newton’s Cradle .............................................................. 3 5. Elastic Collision Between Unequal Masses: Astroblaster/Collision between a basketball and a bouncy ball ........................................................................................................................................... 3 6. Center of Mass: Flying Balancing Bird ............................................................................................ 4 7. Center of Gravity: One Bottle Wine Holder .................................................................................... 4 8. Center of gravity: An Arbitrary Shape ............................................................................................ 5 9. Bicycle Wheel Gyroscopic Precession ............................................................................................. 5 10. Toy Gyroscope ............................................................................................................................. 6 11. Gyroscopic Precession: Varying the Magnitude of the Torque ..................................................... 7 12. Moment of inertia and Angular Momentum: Rotating Stool and Dumbbells ............................... 7 13. Angular momentum: Bicycle Wheel and Rotating Stool or platform ............................................ 8 14. Moment of Inertia: Race Between a Solid Cylinder and a Hollow One .......................................... 8 15. Moving Spool .............................................................................................................................. 9 16. Coupled Pendulum .................................................................................................................... 10 17. Coupled Harmonic Oscillators – Air Track .................................................................................. 11
Part 2: Fluids .............................................................................................................................. 12 1. Bed of Nails ................................................................................................................................. 12 2. Water Seeks Its Own Level (Pressure in a Fluid) ........................................................................... 12 3. Bernoulli’s Principle 1: One Sheet of Paper .................................................................................. 13 4. Bernoulli’s Principle 2: Two Sheets of Paper ................................................................................ 13 5. Bernoulli’s Principle 3: Ball in the Air ........................................................................................... 13
Part 3: Waves and Sound ........................................................................................................... 14 1. Long Slinky: Longitudinal and Transverse Waves. ......................................................................... 14 2. Sound pipe .................................................................................................................................. 14 3. Sound Tube ................................................................................................................................. 15 4. Organ Pipes: Pitch vs. Length ....................................................................................................... 15 5. Hearing and Seeing Sound Waves ................................................................................................ 16 6. Tuning Forks: Resonance ............................................................................................................. 16 7. Tuning Forks: Beats ...................................................................................................................... 17 8. Standing Sound Waves in a Pipe .................................................................................................. 17 9. Wave Interference ....................................................................................................................... 18 10. Doppler Effect ........................................................................................................................... 18
Part 4. Other Demos .................................................................................................................. 19 1. Liquid Nitrogen Demonstrations .................................................................................................. 19 2. Liquid Nitrogen Ice Cream ............................................................................................................ 19 3. Falling dollar bills ......................................................................................................................... 20 4. Projectile Motion (Zero Launch Angle) ......................................................................................... 20 5. Inertia .......................................................................................................................................... 20 6. A Large Vertical Force Table ......................................................................................................... 21 7. Friction ........................................................................................................................................ 21 8. Centripetal Force ......................................................................................................................... 21 9. Centrifugal Bulge ......................................................................................................................... 21 10. Stacking Meter Sticks ................................................................................................................ 22 11. Moment of Inertia Rotator ........................................................................................................ 22
ii
12. Race Between Three Identical Balls ........................................................................................... 22 13. Pendulums with Different Masses ............................................................................................. 23 14. Buoyant Force ........................................................................................................................... 23 15. Metal Ball in a Ring .................................................................................................................... 23 16. Thermal Conductivity 1: Conductometer ................................................................................... 24 17. Thermal conductivity 2: Conductivity Spider .............................................................................. 24 18. Radiometer ............................................................................................................................... 24 19. Longitudinal Wave Apparatus .................................................................................................... 25 20. Singing Rods 1: Sound Waves .................................................................................................... 25 21. Singing Rods 2: Standing Waves ................................................................................................ 25 22. Rhythm ..................................................................................................................................... 26
1
Part 1: Mechanics
1. Projectile Motion: Shoot the Monkey
A ball (monkey) is attached to a free fall mechanism, and another ball (bullet) is aimed at the Monkey. The Monkey and the ball are released simultaneously and they collide in air.
Description: This is one of the classic physics demonstrations, one not to be missed. For best results, the gun should be tightly clamped to the table. Carefully aim the gun (by looking through the small hole on the end of the trigger) directly at the monkey. Tighten the screw that controls the angle of the gun. A practice shot is always a good idea, to make sure the gun is on target; just be careful not to move the gun when firing or re-loading. When loading the ball (bullet) into the barrel of the gun, pull the trigger all the way to the back and insert the bullet ball into the barrel. When the trigger is released, the bullet ball is fired and simultaneously the target ball is released for a free fall motion.
The target ball is held by an electromagnet. If the D cell battery powering the electromagnet is weak, the target will not hold.
Discussion: The bullet is initially aimed exactly at the monkey. Both the monkey and the bullet fall under the influence of gravity, and the vertical motion of the bullet is completely independent from the horizontal motion. When the bullet passes the vertical line of the monkey, its height is below the
2
initial height of the monkey by the same amount that the monkey falls through. Therefore, the bullet hits the monkey.
2. Centripetal Force: Pail of Water
A small pail is partially filled with water and then swung (quickly) around a vertical circle without the water spilling. One can surreptitiously exchange the water with confetti.
Discussion: According to Newton's first law of motion, objects in motion tend to remain in motion unless acted upon by an external force. In this case, Newton's law requires the water to continue moving along a tangent to the circle. Thus a force is required to keep it always turning toward the center of the circle. The motion of the water in this demo is similar to that of the motorcycle in the figures. At the top of the circle, apply Newton’s second law:
rv
mmgFN2
=+ . If the water maintains a sufficient speed at the top of the circle, the normal
force between the water and the pail will be positive, meaning water is in contact with the pail, and not spilling.
3. Newton’s Laws and Conservation of Momentum: Skateboard
Have two students (of unequal masses) sit facing each other on skateboards, approximately 3 m apart. Place a rope in their hands. Tell them to pull on their ends of the rope. Ask the class which person is exerting more force. Both students will accelerate toward each other.
Discussion: The two forces exerted by the two students on each other form an “action-‐reaction” pair, and thus are equal in magnitude and opposite in direction. However, since the two students have unequal masses, the forces of the same magnitude cause accelerations of different magnitudes, with the larger mass having the smaller magnitude of acceleration. As a result the two students have difference speeds, with the larger mass having smaller speed.
3
This demonstration can also be understood from the conservation of momentum. The two students form a system. The initial total momentum of the system is zero, since both students are at rest. The final total momentum of the system must also be zero: . The final velocities of the two students will be in opposite directions and the larger mass will have the smaller magnitude of velocity (speed).
4. Elastic Collision Between Equal Masses: Newton’s Cradle
If one ball is lifted from a certain height (while all the other balls are at rest) and released, it will stop at the vertical position and the ball one the opposite side will rise to the same height.
Discussion: The collision between two steel balls is close to an elastic collision, in which both the total linear momentum and the total kinetic energy are conserved. If the two balls have the same mass, and the target ball is stationary, after the collision the two balls exchange their motions. Such motion exchange occurs in the four successive collisions, and the last ball attains the speed of the first ball right before the first collision. With this speed the last ball rises to the initial height of the first ball.
5. Elastic Collision Between Unequal Masses: Astroblaster/Collision between a basketball and a bouncy ball
When the Astroblaster is dropped so the large ball hits the ground first, with the other balls stacked vertically on top, momentum is transferred along the chain of balls to the small one on top, causing it to rise to a much larger height than the height from which the Astroblaster was originally dropped. The small red ball on top is easy to lose, so please try to keep track of it. Some practice shots should also be carried out
before doing this in front of an audience, too.
A variation of this demo is the collision between a basketball and a bouncy ball: put the bouncy ball at the
02211 =+= fff vmvmp
4
center on the top of the basketball, and drop them from some height. The bouncy ball rises to a height much larger than the initial height.
Discussion: In a one-‐dimensional elastic collision between two objects (of unequal masses) that are initially traveling with equal speeds in opposite directions, the smaller object will have more kinetic energy after the collision than before.
6. Center of Mass: Flying Balancing Bird
The bird can be balanced on the tip of its beak. The center of mass of the bird is located at the beak.
7. Center of Gravity: One Bottle Wine Holder The center of gravity of the system (bottle plus holder) is directly over the support point.
5
8. Center of gravity: An Arbitrary Shape A plastic board of an arbitrary shape can be suspended from several different pivot points. On each suspension, use the included axle to draw a straight line on the board. The intersection of such straight lines is the location of the center of gravity.
Discussion: At each suspension, when the object is in equilibrium, the center of gravity must be directly below the suspension point. To see why, note that when the center of gravity is directly below the suspension point, the torque due to gravity is zero, since the force of gravity extends right through the axis of rotation.
9. Bicycle Wheel Gyroscopic Precession A bicycle wheel fitted with handles is spun with its axle horizontal and is held with a string tied to one end of the handle. The string permits the spinning bicycle wheel to precess.
Discussion: The bottom figure shows the direction of the angular
momentum ! (right-‐hand rule), which is perpendicular to the wheel (along the axle /handle). Gravity produces a torque perpendicular to both the axle of the wheel and the vertical. According to Newton’s second law for rotation, ! = !!
!", this torque
causes the change in angular momentum. Since this torque ! is perpendicular to ! , it changes only the direction of ! , but not the magnitude. This results in the horizontal precession.
On a less abstract level, the precession can be explained in terms of the downward pull of gravity that tries to make the wheel rotate faster at the bottom than at the top. Since the wheel is rigid,
ω
6
this can happen only if the wheel moves horizontally in the direction in which the bottom of the wheel is spinning. Note that the precessional frequency !! is inversely proportional to the frequency at which the wheel is spinning!: !! =
!!" , where !
is the magnitude of the torque produced by gravity, and I is the moment of inertia of the wheel. This fact can be illustrated by observing carefully the precession as the spin of the wheel slows down. The kinetic energy associated with the precession has to come from somewhere. It comes from the gravitational potential energy of the gyroscope itself. When the gyroscope is released from an initial fixed horizontal position, it starts to fall in the usual manner. This falling motion rapidly transforms into precession, with the center-‐of-‐mass slightly lower than it was initially. Actually, as it falls, it overshoots its equilibrium position slightly and oscillates up and down about this equilibrium, resulting in nutation. The nutation usually damps out rather quickly, but it can be excited by a rapid upward or downward jerk on the free end of the axle of the gyroscope. If there is friction acting to retard the precession, the center-‐of-‐mass gradually falls until eventually the wheel hangs straight down.
10. Toy Gyroscope
The toy gyroscope can be balanced on the pedestal provided or on the tip of a finger. The gyroscope precesses and begins to lower from a vertical place to a horizontal plane as it slows down (due to friction).
A string is used to spin the gyroscope rapidly. Hold the frame firmly in your hand. Thread the string through the small hole near the top of the spindle. Turning the wheel, carefully let the string wind around the spindle – from hole to hub and back again. Be sure to keep the winding as smooth and tight as possible, and be sure to keep the winding between the hole and the hub. To create the rapid spin, pull the string away from the gyroscope with a quick, strong motion.
7
11. Gyroscopic Precession: Varying the Magnitude of the Torque This demo utilizes a motor driven gyroscope. It illustrates precession by hanging slotted masses on the end of its axle. The precessional requency !! is proportional to the magnitude of the torque ! produced by gravity. Increasing the hanging slotted mass increases the precessional frequency.
12. Moment of inertia and Angular Momentum: Rotating Stool and Dumbbells
A demonstrator sits on a rotating stool with arms stretched and each hand holds a dumbbell. The demonstrator is spun by a volunteer and then brings the dumbbells to chest. This will reduce the moment of inertia and increase the angular velocity.
8
HAZARDS: Dizziness can be induced by the rotating stool. A pause of a few moments to regain equilibrium before getting off is recommended.
Discussion: The person and the stool rotate together, and can be treated as a single object. The conservation of angular momentum for a single-‐object system is expressed as: !!!! = !!!! .
13. Angular momentum: Bicycle Wheel and Rotating Stool or platform
A demonstrator sits on a rotating stool (or stands on a rotating platform) and holds a bicycle wheel fitted with handles. The bicycle wheel is spun by a volunteer and the demonstrator slowly flips the wheel 180 degrees. The demonstrator will then turn on the stool (or the platform), in the same direction as the bicycle wheel’s INITIAL spin. Discussion: The system consists of the demonstrator, the stool and the bicycle wheel. Initially the total angular momentum of the system comes entirely from the spinning wheel. !! = !!,!!!!" .
As the wheel is inverted, !!,!!!!" = −!!,!!!!" . The demonstrator applies a torque to the wheel, but this torque is internal to the system. No external torque is acting on the system about the vertical axis. Therefore, the total angular momentum of the system is conserved. !! = !!.
14. Moment of Inertia: Race Between a Solid Cylinder and a Hollow One
A solid cylinder and a hollow cylinder with the same radius are placed at the same height at the top of a long incline and are released at the same time. The solid cylinder reaches the bottom first.
Use carton boxes to catch the cylinders to avoid the cylinders’ hitting the floor.
Discussion: As the cylinders roll down the incline without slipping, total mechanical energy is conserved (since the frictional force is static, and hence does no work), while gravitational potential energy is converted to kinetic energy. Kinetic energy is the sum of the translational kinetic energy and the rotational kinetic energy. The two cylinders have different moments of inertia, !!!""!# = !!! , !!"#$% = !
!!!!. For the hollow cylinder, a
larger portion of the kinetic energy is rotational since its moment of inertia is larger, and
9
hence less kinetic energy is for the translational motion. As a result, the hollow cylinder rolls down more slowly and loses the race. In a rolling motion,
Apply conservation of mechanical energy:
15. Moving Spool
A large wooden spool (or yo-‐yo) with a string wound around it from below can be made to move either in the direction in which the string is pulled or in the opposite direction depending upon the angle of the string with respect to the horizontal.
The demonstration is effectively introduced by asking the audience to predict whether the spool will move forward or backward when the string is pulled. Whichever way the majority of the audience votes, the spool can be made to go the opposite direction by pulling the string at the appropriate angle. A small angle (theta) between the string and the horizontal will make the spool move in the direction of the pull, and a large (theta) will make the spool move away from the pull. A change in angle so small that the audience does not notice reverses the direction. The behavior of the spool is quite mysterious.
22
21
21 ωImvKE +=
22
21
21
⎟⎠⎞⎜
⎝⎛+=RvImv 2
222 )(
21
21 mv
RvmRmvKEHollow =⎟⎠⎞⎜
⎝⎛+=
22
22
43)
21(
21
21 mv
RvmRmvKEsolid =⎟⎠⎞⎜
⎝⎛+=
KEmgh ＝0
0ghvhollow = 034 ghvsolid =<
10
Discussion: The explanation involves a consideration of the forces and torques on the spool (see the top diagram). It is easiest to consider the case where the string is pulled with a force and at an angle such that the spool is just on the verge of slipping without rolling.
There are four forces: the weight (mg), the upward normal force of the table (N), the tension in the string (T) and the friction force (µN). In the vertical direction, net force = 0, !"#$%+! =!" ! =!"− !"#$% (1) If the spool is not yet moving, the net horizontal force is zero, or !"#$!! = !". (2) Only two of the forces produce a torque about the center of the spool (T and µN), and these torques must be equal and opposite if the spool is to slip rather than rotate. Equating the torques gives !!! = !!!". (3)
Dividing equation (3) into equation (2) gives !"#!! =!!!!.
Thus the critical angle that determines which way the spool will rotate depends only of the ratio of the two radii and is independent of the mass of the spool, the tension in the string and the coefficient of friction. With calipers and a protractor, one can verify the predicted critical angle.
If the actual angle ! < !!, it is possible that the net horizontal force is to the right while the net torque is clockwise. Likewise, If the actual angle ! > !!, it is possible that the net horizontal force is to the left while the net torque is counterclockwise.
16. Coupled Pendulum
Two identical pendulums are hanged over the same horizontally stretched string. The coupled pendulums transfer energy to each other through the thin string that couples them. This setup allows introducing resonance and the normal modes of oscillation.
Resonance: Both pendulums are brought to rest. One then starts one pendulum swinging. After a while, the first pendulum will stop swinging, and the other will be swinging with a large amplitude. Then the first will slowly begin swinging again while the second comes to rest and so forth until the energy is damped away through friction.
11
Normal modes of two coupled pendulums:
Instead of using the two individual angular coordinates (!! !"# !!) to describe the motion of the system, introduce two new coordinates: the center of mass !!" (!!" = !!!!!
!), and
the relative coordinate !! (!! = !! − !!). These two new coordinates are in oscillations, and are independent from one another, and are hence called “normal” modes. This property makes it possible to “energize” only one of the two modes, keeping the other one still. To “energize” the center of mass mode, the two pendulums must be released from the same height and from the same side (to keep the relative coordinate constant). To “energize” the relative mode, the two pendulums must be released from the same height but from the opposite sides (to keep the center of mass still).
17. Coupled Harmonic Oscillators – Air Track
Up to 5 air-‐track gliders are coupled with springs and mounted on an air track to investigate normal modes of oscillation.
For two coupled harmonic Oscillators, the discussion is similar to the Coupled Pendulum.
12
Part 2: Fluids
1. Bed of Nails
This demonstration is very useful to introduce the concept of pressure. If a person is to lie down on the nail bed, cautions must be taken: (1) The tips of the nails must be even. (2) The person must be lowered carefully, slowly, and evenly to ensure the person makes
simultaneous contact with a large number of nails, Discussion: pressure is defined as magnitude of the force / the area in which the force is applied. When the person makes simultaneous contact with a large number of nails, his weight is spread over a large area, thus reducing the pressure each nail exerts on his skin.
2. Water Seeks Its Own Level (Pressure in a Fluid)
Several tubes of different shapes and sizes are connected to a common reservoir filled with water. It is observed that the heights of the water are the same in all of the tubes.
Discussion: Pressure in a fluid varies with the depth (vertical height). Pressure at the surface is the same in all the tubes. The tubes are connected at the bottom. In equilibrium the pressure at the bottom must be the same everywhere. Therefore, the vertical height of the fluid in all the tubeds must be the same, regardless of the shape and the cross-‐sectional area of the tubes.
13
3. Bernoulli’s Principle 1: One Sheet of Paper
Hold a piece of paper by its end, it will bend downward. Blowing across the top of the paper reduces the pressure there, resulting in a net upward force which lifts the paper to a nearly horizontal position.
Discussion: In the region where a fluid flows faster, the fluid pressure is reduced. Blowing the air above the sheet of paper makes the air pressure above the sheet lower than that below the sheet. This pressure difference results in a net upward force on the sheet.
4. Bernoulli’s Principle 2: Two Sheets of Paper
Hold two sheets of paper facing each other vertically. Blowing in the middle between the two sheets reduces the pressure there, causing the two sheets moving toward each other.
5. Bernoulli’s Principle 3: Ball in the Air
A large beach ball filled with air is suspended in the output stream of a leaf blower. A ping pong ball floats above the nozzle of a hair dryer.
Discussion: The viscous force of air balances the weight, and the lower pressure in the jet keeps the object trapped in the air-‐stream. If the ball makes a lateral displacement, the higher pressure at the outside due to stationary air will push the ball back toward the jet stream.
Lower pressure
higher pressure
14
Part 3: Waves and Sound
1. Long Slinky: Longitudinal and Transverse Waves. A longitudinal wave on a slinky:
A transverse wave on a slinky:
2. Sound pipe
A metal pipe is used as resonant acoustical open tube that generates its sound from the noise generated by convection currents. A piece of metal gauze is inserted into the pipe near one end. The metal gauze is heated to red hot using a burner and the burner is then removed. If the pipe is held vertically, the convection currents created in the pipe by the air heated by the hot gauze create a noise spectrum that leads to a sound resonance in the pipe. The pipe is then tipped to a horizontal orientation, causing the convection currents to stop and the pipe will no longer make any sound.
15
3. Sound Tube
A long corrugated plastic tube, open on both ends, is twirled around in a circle. The fundamental and several harmonics may be excited by twirling it at various speeds.
Discussion: A tube open at both ends resonates at certain frequencies. These frequencies are the ones where an integer number of half-‐wavelengths fit inside the tube. At the fundamental frequency, one half of the wavelength fits the tube.
A tube with a length L = 0.66 m has a 1.32m wavelength at resonance (! = 2!). Frequency = !
!= !
!! , where v is the speed of sound in air, v = 343 m/s in regular air conditions. The
calculated fundamental frequency is 260 Hz. This tube resonates to integer multiples of 260 Hz.
Note the fundamental frequency is not heard (explained below). The notes with medium frequencies are easily obtainable. Higher frequencies require faster twirling.
The frequency of the twirling tube is proportional to the speed of the air flowing through the tube, which is proportional to the speed of the rotating end (in a rotation, tangential speed of a point = !" = !(2!")). The air flowing over the corrugations causes vortices which cause oscillating pressures in the air, heard as a whistle. When the frequency of the vortex matches one of the natural frequencies of the tube, the volume of the sound is greatly amplified. At low speeds, the air flowing through the tube is a smooth flow causing no vortices, and thus the fundamental frequency does not sound.
4. Organ Pipes: Pitch vs. Length
Wooden organ pipes of different lengths are used to demonstrate the relationship of pitch to length. The fundamental frequencies of pipes open at both ends ! = !
!= !
!!. The longer the pipe, the lower the pitch.
16
5. Hearing and Seeing Sound Waves A microphone is connected to an oscilloscope. The microphone picks up sound signals, converts them into electric signals and the waveforms are displayed on the oscilloscope. Sound sources: human voices (shown in the bottom picture), or two speakers driven by a function generator.
6. Tuning Forks: Resonance
This demonstration utilizes two identical tuning forks that are mounted on hollow wooden boxes that are open at both ends. The boxed are lined up with the open ends facing each other and one of the tuning forks is struck. The other tuning fork will then begin vibrating in resonance, and can be heard by stopping the vibration of the struck tuning fork. Two identical tuning forks are mounted on wooden resonance boxes. One of the tuning forks is excited and immediately dampened. The second tuning fork will resonate in sympathy with the first.
17
7. Tuning Forks: Beats
In this demonstration two identical tuning forks, one with tines, are excited using a mallet. The tines on one are adjusted so as to produce beats.
8. Standing Sound Waves in a Pipe
This demonstration utilizes a metal pipe, about 1 m in length, and 6 cm in diameter, on top of which are a line of tiny holes. One end of the pipe is connected to a natural gas source, and the other end is connected to a function generator. The gas is sent through the pipe and lit above the tiny holes, and the frequency of the function generator is changed continuously so as to find resonance frequencies. The nodes (low flames in the regions of rarefaction) and antinodes (high flames in the regions of condensation) are clearly visible to a large audience. The pitch of the sound waves is also clearly heard by the audience.
18
9. Wave Interference
In this demonstration a model is used to show how two sine waves combine. The model consists of a combination of colored beads and tubes, is supported on steel rods in a free-‐standing wood frame. The model shows a sine wave (basic wave) extending over 2 -‐ ½ wavelengths. Two separate acrylic sine wave profiles with different characteristics slide into the model and combine with the basic wave. The larger profile has a wavelength and amplitude identical to the basic wave, whereas the smaller profile has twice the frequency and half the amplitude of the basic wave.
10. Doppler Effect
Five inch foam ball has a battery-‐powered buzzer inside. Start the buzzer and play catch, Throw it fast enough and student can hear the frequency shift. Attach a string to swing the ball in a circle.
A racket ball, a banana, a balloon are put into liquid nitrogen. The racket ball becomes brittle and it shatters if dropped on the floor. The banana is hardened and can be used as a hammer to drive a nail into a wooden board. The balloon shrinks when cooled and expands after being taken out of the liquid nitrogen.
2. Liquid Nitrogen Ice Cream If you have access to liquid nitrogen and the proper safety equipment and training, try this
in place of your normal cryogenics demonstration!
Ingredients: -‐ 5 or more liters of liquid nitrogen and associated safety gear -‐ 2 quarts (1.9 liters) of Half and Half -‐ 1 cup (237 ml) of sugar -‐ 4 teaspoons (20 ml) of vanilla (optional) -‐ 2 cups (473 ml) of strawberries (or whatever flavor you like) -‐ wooden spoon -‐ wire wisk -‐ large plastic punch bowl
Directions: 1. Mix the Half and Half, sugar and vanilla in a large plastic punch bowl with a wire wisk.
2. Add the flavoring. Wire wisk the mixture further if needed.
3. Pour a small amount (about 250 ml) of liquid nitrogen directly into the plastic punch bowl.
4. Stir the mixture with a wooden spoon. Be careful not to splash! (Helpers should be wearing gloves and goggles!!)
5. Keep adding small amounts of liquid nitrogen until the mixture becomes too thick to stir.
6. Allow any excess liquid nitrogen to boil off before serving.
It has been our experience that 5 liters of liquid nitrogen is more than enough to 'cook up' two batches of ice cream!
From Thomas Jefferson National Accelerator Facility – Office of Science Education
20
3. Falling dollar bills
A flat and a crumpled dollar bill are dropped simultaneously. The crumpled bill should have an acceleration close to g while the flat bill encounters a significant air resistance.
4. Projectile Motion (Zero Launch Angle) Two steel balls are set on this apparatus. When the trigger is pressed, one ball is projected horizontally, while the other ball falls freely. The two balls hit the floor at the same time.
5. Inertia
A large beaker is filled with water and placed on cloth. The cloth is then pulled from under the beaker without spilling any water.
21
6. A Large Vertical Force Table
This force table can illustrate force addition, resolution of forces into components, and static equilibrium.
7. Friction
An inclined plane is slowly raised until a block resting on it just begins to slide.
8. Centripetal Force
This demo utilizes a centripetal force apparatus which consists of two marbles enclosed in a case and free to roll on a wooden semicircular base. The base has two holes of larger diameter than the marbles drilled on the ends of it. When the apparatus is rotated the marble slide over to the holes and remain there.
9. Centrifugal Bulge
Two bronze straps are mounted at right angles on a vertical support. The bottom ends are fixed but the top ends slide freely on the vertical rod. When the support is spun rapidly the two hoops flatten at the top and bottom, assuming the shape of an oblate spheroid, similar to the Earth.
22
10. Stacking Meter Sticks
10 meter sticks are stacked on top of each other so that the top one overhangs by 49cm. The meter sticks are stacked as follows: mark one meter stick at 4.6 cm to serve as the bottom of the stack. The next stick is marked at 5.3 cm, the next 6.1, then 7.3, then 9.0, 11.5, 15.7, 24.0, 49.0, with the last one overhanging 49cm.
11. Moment of Inertia Rotator
This apparatus consists of two heavy iron balls mounted on a center axis that is free to spin. When spun and the handle of the apparatus compressed, the balls moves in closer to the center and spin more rapidly.
12. Race Between Three Identical Balls
On the wooden incline, there are three trenches of different widths. Three identical balls roll down the incline along the three trenches. The one along the narrowest trench reaches the bottom first. Discussion: The center of gravity of the ball on the narrowest trench is the highest above the incline. The gravitational force generates the largest torque, and resulting in the largest angular acceleration. The linear acceleration of the center of the mass is therefore the largest.
23
13. Pendulums with Different Masses
One pendulum with a steel bob with mass 63 g and another with a plastic bob with mass 19.2 g are released at the same angle initially. The two pendulums have the same length. It is observed that the periods of the two pendulums are the same. Note: without a projector, students cannot see the pendulums. The demo works fine until about 5 periods. After that time the air resistance is significant for the plastic bob.
14. Buoyant Force
A large coke bottle filled with sand is hung from a large spring scale. When the bottle is immersed in water, it is observed that the reading of the spring scale is reduced.
15. Metal Ball in a Ring
This demo utilizes a metal ball attached to a metal rod with an insulated handle, and a metal ring attached to a metal rod with an insulated handle. The ring and the ball are made from different metals. At room temperature, the ball does not go through the ring. When both are heated using a burner, the ball pass easily through the ring due to the difference in the temperature coefficients of the thermal expansion between the two metals.
24
16. Thermal Conductivity 1: Conductometer
A brass disk is attached to a brass rod with an insulated handle. Five metal rods composed of nickel, copper, iron, zinc, and aluminum of the same diameter are attached to the disk. On each metal rod, including the brass one, a notch is engraved for holding a piece of wax. The disk is then heated using a Bunsen burner and the order in which wax melts on each of the rods is monitored.
17. Thermal conductivity 2: Conductivity Spider
On a plexiglass base the names of four metals, namely, aluminum, iron, brass, and copper, are marked. The metal rods are inserted into a central metal disk and the disk is placed onto the rim of a glass empty reservoir. On each rod a notch is engraved for holding a piece of wax. The entire setup is placed on an over-‐head projector and boiling water poured into the glass reservoir. The order in which wax melts on each of the rods is monitored.
18. Radiometer Four vanes each having one side black and one side white, are mounted on a pivot which is enclosed in a partially evacuated glass bulb. Shining a light on the vanes causes it to spin in a direction that depends on how much air is in the bulb. Tentative explanation: The vanes are heated unequally on both sides with the air near the dark surface heated more than the air near the bright side. A gas-‐stream pattern called thermal creep is then initiated, which exerts a force on the vane. The force is such that the dark side moves away from the radiation. The radiometer action reaches a maximum at about 1 Torr pressure, where the mean free path of the molecules is much less than the dimensions of the enclosing bulb.
25
19. Longitudinal Wave Apparatus
When the apparatus is cranked, the balls move in the horizontal direction, forming a longitudinal wave.
20. Singing Rods 1: Sound Waves
Hold a rod between the thumb and forefinger at one of the three marked nodes. Place a little of the included special rosin on your other hand. Then rub that hand along the rod from middle to the end. As the hand strokes the rod, friction creates a standing wave pattern which emits a tone that grows louder with each stroke.
21. Singing Rods 2: Standing Waves A singing rod is marked at the quarter and halfway points. Holding it at halfway point makes that spot a nodal point (while the two ends are anti-‐nodal points) and creates the first harmonic. Moving the hand to a quarter point forces the second harmonic and a higher pitch.
26
22. Rhythm
This demonstration utilizes the Beyer Metronome and is used to illustrate the concept of rhythm in musical compositions. The metronome consists of an upright pendulum which can oscillate from left to right. A small weight is attached to the pendulum which can be slid up and down to set the tempo. The pitch can be adjusted by using the bell tone selector attached to the side of the device.
