MIT Department of Physics Technical Services Group

A can of spray paint is attached to a spring oscillator. A roll of paper is run past the oscillating can. The result is a sine wave on the paper.

A ball is dropped in front of a meter stick and lit by a strobe light. A long exposure photograph captures the position of the ball at each evenly spaced flash of light. The acceleration of the ball can then be measured from the photo.Note that the frame rate of the video capture (30fps) is quite close to the strobe rate (15fps). This is why the strobe flashes in the slow motion video don't appear to be exactly evenly timed.

A pair of spinning wheels 1.5 meters apart is placed in the path of a bullet. A bullet is fired with the wheels stationary for reference position, and fired again with the wheels spinning at a known speed. The second wheel will rotate more than the first as the bullet crosses the gap between them. When the angles between the reference holes and the second holes are compared, the speed of the bullet can be determined.The balloon is used to show when the bullet has passed through both disks. This video was shot with a high speed video camera at approximately 7000 frames per second. The frame data can be seen at the bottom of the screen.

A stuffed monkey is suspended from a rod at one end of a lecture hall by an electromagnet. A golf ball gun aimed directly at the monkey cuts power to the electromagnet when fired. Thus, the monkey begins falling at the same instant the gun fires the golf ball. The projectile and target meet in mid air.Intuitively one might think that the ball will go over the monkey's head due to its fast speed. However, gravity accelerates all objects downward at the same rate, meaning the monkey and the ball will meet at exactly the same point. If the ball was shot even faster, it would still hit the monkey, but higher above the ground. No animals were harmed in this demo.

Two people on carts pull on a long rope, demonstrating that the center of mass does not move.

A wooden ball is attached to the rim of a spinning wheel. The ball is held in place by a string. When the spring is cut, the ball flies in a straight tangent to the wheel.In the camera's frame of reference, the ball constantly accelerates around in a circle due to the centripetal force pulling it inwards. When the string is cut, the acceleration stops, and the ball flies away in a straight tangential line. When the string is cut in the rotating frame of reference, a ficticious force (centrifugal force) accelerates the ball.

A string carrying two weights is hung over a low friction bearing mounted pulley. The weights have slightly different masses, causing a uniform acceleration. When the time it takes the weights to move 1 meter is timed, we can calculate the acceleration of the system due to gravity. Because of the low amount of friction in the system, this value is very close to the theoretical value.Read more about the Atwood Machine.

A bicycle wheel is suspended from one of its axis by a rope, and spun up by hand. The wheel's axle is then placed horizontally and the free end of the axle processes about the supported end. The gyroscope seems to defy gravity because the torque created by the spinning wheel counteracts the torque due to gravity. Read more about gyroscopes here. Gyroscopes have been used through history for varied uses such as stabilizing spacecraft or for guidance systems on ships and missiles.

A 100 uF oil-filled capacitor is charged to 3 KV. This takes approximately 15 minutes to charge, creating a charge on the capacitor that could be lethal. The capacitor is then discharged through a 12" length of 30 gauge bare iron wire.When the high voltage current flows through though high resistance wire, the bonds between iron molecules are shattered, resulting in a loud bang, a shower of sparks, and a cascade of wispy filaments floating through the air.Not all of the charge on the capacitor is disharged through the wire, so a shorting bar must be used to release the remaining charge.

An cathode ray tube (CRT) television is connected to a video camera. When a strong magnet is brought close to the television screen, the image becomes warped and discolored.While many new televisions use flat screen technology, older CRTs produced images by firing electron guns (one red, one green, one blue) through the television body onto the back of the screen. When a magnet is brought close to the screen, it deflects the paths of the electron beams and distorts the picture. A strong enough magnetic field can even create a hole in the electron beams, causing a black spot on the picture. This TV has been subject to many magnet encounters, which has permanently damaged the picture.

Two large coils of wire ("Helmoholz coils") are connected to 125V DC power, and produce a uniform magnetic field between the coils. A separate coil is suspended with this field. Switching the polarity of the DC current in the inner coil causes it to rotate in opposite directions.This principle is used by devices called galvanometers to measure electric current.

A long length of wire is suspended horizontally between the poles of a magnetron magnet. When a large current from a 12V storage battery is passed through the wire, the wire jumps out of the field. When the direction of the current is switched, the wire jumps the opposite direction.The magnetron magnet in this demonstration was originally used in MIT's groundbreaking research developing radar during and after World War II. Microwave emitting cavity magnetrons need strong magnetic fields, which were often created by powerful permanent magnets like the one seen in this demo.

Two flexible wires are suspended vertically. The wires are conected in series or parallel to a 12V storage battery. When the wires are connected in series and power is applied they will repel each other; when they are connected in parallel they weill attract one another.This effect is due to the magnetic fields created by the charge flowing through the wires. When the wires are in parallel, the currents in each are going in the same direction and thus attract. In series the currents are going in opposite directions and repel.

A magnet with a very strong magnetic field is held in place on an aluminum disk. The disk is attached to a motor powered by variable AC current. When the disk rotates, the magnet will levitate above it due to eddy currents generated in the disk. With the disk spinning, these eddy currents form to oppose the magnetic field of the magnet, making it levitate. When the motor is turned off, the magnet falls back to the disk.

A light bulb is placed in series with two copper plates immersed in de-ionized water. Touching the plates closes the circuit, lighting the bulb.When kosher salt is dropped into the de-ionized water, the salt dissolves, causing ions to be dispersed throughout the liquid. The free ions allow current to flow through the water, which completes the circuit and lights the bulb.Most water we encounter in everyday life is not de-ionized and contains impurities with dissolved ions. This is why we know water as a good conductor, and why we shouldn't use electronic devices around a bathtub, for example.

A small glass tube, held by copper wire, is placed in series with a light bulb. The glass acts as an insulator at room temperature, meaning the current cannot flow between the copper wires. This leaves an open circuit and the light bulb does not light up. Touching a conductor across the copper wires (with a metal screwdriver for instance) does complete the circuit because it allows current to flow.However, when glass is sufficiently heated by a torch it becomes an ionic conductor. Ionic bonds in the glass are broken, allowing the charge carrying ions to move freely. Thus, when the glass is melted the current can flow, which closes the circuit and lights the bulb.

An RF transmitter is connected to a long antenna, emitting radio waves. A dipole antenna with a light bulb between its elements acts as the receiver. When the receiving antenna is parallel to the transmitter, the radio waves are absorbed, creating a current in the antenna and causing the bulb to glow. When perpendicular, no current is created, and the bulb does not glow.

A polarized microwave emitter and a polarized microwave receiver face each other on a table. At first, the emitter and receiver are polarized in the same direction (up-down), and all the emitted signal gets received. When a metal comb in inserted between them, with the teeth pointed down, the signal is blocked. This is because the microwaves are polarized in the same direction that the teeth are pointing, creating small currents in the metal that reflect the microwaves backwards. When the comb teeth are turned horizontally, the signal passes through undisturbed.When the receiver is turned 90 degrees, no signal is received. However, when the comb is inserted at a 45 degree angle, some signal passes through. This is because light waves are made up of two perpendicular components that add up to form the polarization of the wave. When one component is knocked out, the wave changes polarization. Read more about polarizers here.

A double-horn microwave emitter faces a microwave receiver. The receiver is also connected to a speaker, which displays the received signal as audio, and an oscilloscope, which displays the signal visually. When the receiver is moved perpendicularly to the emitter, constructive and destructive interference can be both seen and heard.When one of the emitter horns is covered, the interference pattern disappears.

A mass on a spring is driven by a large geared motor apparatus, and exhibits resonance at the appropriate frequency. Below resonance, the driving motion is in phase with the motion of the mass. At resonance the mass is 90 degrees out of phase with the driving motion, and above resonance it is 180 degrees out of phase. Note the amplitude of motion difference when the system is at resonance.Driven mechanical oscillation is directly analagous to the oscillation of current in an RLC circuit, where the capacitor is like the spring, the inductor is like the mass, and the resistor is like air friction. Read more about harmonic oscillation here.

A variable capacitor (C), large inductor or solenoid (L), and 200W light bulb (R) are connected in series to 120 VAC. The inductance of the inductor can be varied by inserting an iron core, and the capacitance can be varied by a row of switches. By varying the inductance and capacitance, we can achieve the resonance of the circuit, where maximum current flows through the resistor (light bulb).Read more about RLC circuits here.

A solid copper pendulum is mounted between the poles of an electromagnet (solenoid). The pendulum is set into motion, and then the magnets are turned on. The magnets induce eddy currents in the copper which oppose the motion of the pendulum. The pendulum quickly slows to a stop, demonstrating an effect called eddy current braking. Eddy current brakes are widely used in trains and roller coasters.When a copper pendulum with strips cut into it is swung between the same magnets, it is not slowed nearly as much as the solid pendulum. This is because the cuts in the copper prevent large eddy currents from forming. Only eddy currents smaller than the strips of copper can be formed.

One wire coil is connected to a radio, and another is connected to a speaker. The two coils are not connected to each other. The radio signal is transmitted through the induced magnetic field caused by the current in the wires. The signal is only received when there is a magnetic flux through the receiving coil, and thus no sound is heard when the coils are perpendicular to each other.