Physics Demonstrations, Lusk, WY, April 18, 2011

Preliminary list for grades 3-5: There will be a few changes, some items will be dropped due to time constraints. If topics are important to your curriculum let me know.

This activity is in collaboration with:

National Security Technologies, Los Alamos Operations
The Bradbury Science Museum, Los Alamos National Laboratory

with support from:
Casper Fire Extinguisher
AirGas

### 1 Geysers

The model of a Geysers shows that a geyser requires a unique structure as well as a heat source. The restricted pipe upward prevents heat transfer through convection. The weight of the column of water increases the pressure at the bottom of the column increasing the boiling point of water. Once the water starts to boil, the hot steam moving upward lowers the pressure at the bottom since steam has a much lower density than water. With lower pressure, the boiling point falls and the water quickly evaporates creating the eruption. Evaporation cools the bulk of the water and the process starts again.

### 2 Free Fall

The Earth’s atmosphere affects everything around us including the motion of falling objects. We demonstrate that without air friction all objects, independent of mass, accelerate at the same rate under the influence of gravity. This was shown in 1971 by Apollo 15 astronauts dropping a hammer and feather on the Moon (download video clip at: www.schwellenbach.com/casper09). On the surface of the Moon there is no air and the acceleration of gravity is the value at the Earth’s surface.

### 3 Projectile Motion – Vectors

At the instant the paint ball leaves the barrel, a light beam is interrupted and the target is released. The paint ball hits the target because both objects fall at the same rate independent of their initial motion. Similarly, the Moon is orbiting the Earth with an average speed of over 2,000 MPH. Like the paint-ball, gravity pulls the Moon perpendicular to its motion, changing its direction but not its orbital speed.

### 4 ”Newton’s Eggs” – Newton’s 1st Law

Objects at rest remain at rest unless acted on by a net force.
The eggs have enough mass (and therefore inertia) to have no noticeable motion in the horizontal direction. Actually, friction with the moving plastic plate provides a very small horizontal force on the eggs, and they will experience a very small horizontal acceleration that can be neglected.

### 5 Centripetal Force – Newton’s First Law

Objects in motion will continue to move in a straight line unless acted upon by a force. The water in the bucket tends to move in a straight line. We must apply a force (centripetal force) to keep it moving in a circle. In the case of orbits (satellites, the Moon, planets) the centripetal force is provided by gravity.

### 6 Newton’s Second Law (F=ma)

Force = mass x acceleration. The firecrackers apply the same force to both cans but the less massive can has a greater acceleration, reaches a greater speed, and therefore flies to a greater height than the more massive can. The less massive can is not always obvious. Extra metal was added to one can making it about twice the mass of the unaltered can, pointing out the importance of have complete information before making predictions.

### 7 Action/Reaction – Newton’s Third Law

For every action there is an equal and opposite reaction. The pressure in the tank (approx. 800 psi) forces the CO2 out of the modified fire extinguisher, the reaction is an equal but opposite force accelerating the passenger. The same principle explains the thrust provided by rockets for use in the atmosphere or in space. (Conservation of momentum can also be used to describe this demonstration.)

### 8 Conservation of Energy

Conservation of energy is shown using a pendulum made from a bowling ball suspended by a rope. When the bowling ball is pulled back against the demonstrator’s nose all of the energy is Potential Energy. At the bottom of the swing all of the Potential Energy (PE) is converted to Kinetic Energy (KE). The total energy is conserved (remains constant) and can only be transferred from between KE and PE so the demonstrator’s nose is safe; the bowling ball cannot return to a greater height than it started.

### 9 Angular Momentum

Show that the effect of gravity on the motion of a gyroscope (spinning bicycle wheel) depends on the initial rotation. Gyroscopes are used to maintain stable orbits of spacecraft such as the International Space Station.

### 10 Electromagnets

Show how electric current can be used to make a magnet and that the unique structure of iron enhances an applied magnetic field.

### 11 Jacob’s Ladder – Plasma

The spark on Jacob’s Ladder is created by a 15,000 volt transformer. The spark tends to move up as the air is heated, hotter air has a lower density so rises relative to cooler air. The electric current (flow of electrons) through the air adds energy to the air molecules heating the air and creating a plasma. As this plasma loses energy to return to its stable state it gives off energy in the form of light, similar to a lightning bolt or the northern lights.

### 12 Electromagnetic Pinch

The 5,000 volt power supply takes about 30 seconds to charge the 60 μF capacitor to a total energy of nearly 500 Joules. When the spark gap breaks down, all of this energy is released in a fraction of a second causing a large current to flow in the loop creating a strong magnetic field. This magnetic field is also rapidly changing which induces an electric current in the aluminum foil tube. The result of the interaction between the magnetic field and the induced current is a force which collapses the tube. This demonstrates an important distinction between Energy and Power (Power = Energy/time). No damage occurs while charging (adding 500 J of energy) since we charge slowly (low power). Discharging quickly causes damage because of the higher power.

### 13 Polarization of Light

The first polarizing filter passes light with the the waves polarized in one direction. The corns syrup rotates the polarization direction different amounts for each color so each color show up a different angle when viewed with the second polarizer.

### 14 Total Internal Reflection

Demonstrate the concept of fiber optics work by observing a laser beam following a stream of water. Similar to the lens, the beam of light changes direction at the water/air interface. Due to the difference in the speed of light in air relative to water, all of the light energy is reflected back into the water.

### 15 Pressure (= Force ÷ Area)

Pressure and force are related but it is important to understand the distinction. A person can lie on a bed of nails because their weight (the force of gravity pulling them down) is distributed over a large enough area; the pressure is low enough that the skin is not damaged.

### 16 Atmospheric Pressure – “Magdeburg Spheres”

Adding up the contribution of the weight of the entire atmosphere (greater than 50 miles in height) leads to a substantial total weight which we measure as atmospheric pressure. Atmospheric pressure is 14.7 psi at sea level, but only about 12 psi (engineeringtoolbox.com) at the altitude of Casper. The significant force due to atmospheric pressure holds the halves of the Magdeburg Spheres (we are actually using cylinders but the physics is the same) since with a vacuum inside there is an unbalanced force. The 2” diameter cylinder can be pulled apart (F=PA=12 psi x πr2= 38 lbs.) but it is difficult to pull the 4” cylinders apart (F=PA=12 psi x πr2= 150 lbs.). It takes 4 times the force when the radius is doubled because force r2 .

### 17 Bernoulli Effect

Make a screwdriver “float” in front of class. Just like an airplane wing, the pressure is lower above the screwdriver because the air is moving faster.

### 18 Thermal Expansion

As the air in a balloon is cooled in liquid nitrogen it contracts and eventually turns to liquid because of the low temperature. The properties of materials, such as a racquet ball, change greatly at low temperatures. The temperature of liquid nitrogen is similar to the temperature near the surface of Jupiter or Saturn which shows one of the challenges of building spacecraft to withstand the hostile environment of space.

### 19 Ping Pong Bazooka

We usually think of energy being stored in a pressurized container, but energy is also stored when pumping the air out of a container to create a vacuum. Energy = Work =Force x Distance, to pump air out of the cylinder we apply a force acting over the distance the air must travel to be pumped out. The bazooka’s vacuum cylinder is sealed on both ends by a thin membrane of aluminum foil. When the foil on one side fails, the air rushing in to return to equilibrium creates a shock wave and the ping pong ball is carried by this wave to a high speed as shown by the damage it can do.