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05:26 IWF (Göttingen) German 2003

Trägheit des Magnetfeldes: Induktive Schaltverzögerung

The switching on and off of electric devices is usually characterized by their instantaneous response. The presence of inductors with large self-inductance can, however, cause considerable delays. Iron cores can lead to additional delays resulting from a change of the magnetization. Such delays, extending over minutes, will be shown here. A large copper spool is wound on a heavy closed iron core. It can be connected with a switch to a 2 volt accumulator (lead-acid battery) and a projection ammeter of short response time (less than 1 sec). The time is measured with a large, hand-operated (historic) stopwatch. These experiments always are extremely surprising, considering that we tend to associate electric phenomena with the idea of the instantaneous, the timeless.
  • Published: 2003
  • Publisher: IWF (Göttingen)
  • Language: German
00:40 IWF (Göttingen) German 2003

Langsam ablaufender elastischer Stoß

Elastic collision between two bodies in slow motion through the proper choice of the experimental conditions. It will be shown that during the collision the kinetic energy is briefly converted into potential energy and then back into kinetic energy. The slow collision, achieved through the proper choice of masses and spring constants, demonstrates clearly the transformation from kinetic to potential energy, and back again, as is characteristic for elastic collisions. This experiment also demonstrates the conservation of linear momentum.
  • Published: 2003
  • Publisher: IWF (Göttingen)
  • Language: German
02:03 IWF (Göttingen) German 2003

Zur Vektornatur des Drehimpulses

A ventilator which can rotate freely around a vertical axis can be used to demonstrate that the angular momentum is a vector. A ventilator is mounted so that it can rotate freely around a vertical axis. It can also be tilted so that the air is blown either horizontally or upward. Upon turning on the ventilator, both the propeller and the driven air jet receive an angular momentum. Its vector points in the direction of the jet, parallel to the axis of the propeller. The ventilator receives an equal angular momentum in the opposite direction (opposite sense of rotation). The experiment is carried out for two different orientations of the ventilator.
  • Published: 2003
  • Publisher: IWF (Göttingen)
  • Language: German
00:56 IWF (Göttingen) German 2003

Präzession eines rotierenden Rades

A spinning top (gyroscope) does not follow a torque in the same way as, for example, a top at rest. Rather, it moves sideways, it "precesses". This startling behavior will be shown here. The top is a bicycle wheel with its rim filled with lead wire. It can be suspended on a rope dangling from the ceiling. The wheel is spun while holding its axle horizontal. Then, one end of the axle is attached to the rope. The weight of the wheel and the tension in the rope together cause a torque which, however, does not turn the axle into the vertical position, as would happen if the wheel were not spinning. Instead, the axle, which indicates the direction of the wheels angular momentum, circles on a horizontal plane around the point on which it is suspended. This sideways motion of a top under the influence of a torque is called "precession".
  • Published: 2003
  • Publisher: IWF (Göttingen)
  • Language: German
02:42 IWF (Göttingen) German 2003

Drehstuhlexperimente zur Erhaltung des Drehimpulses

Conservation of angular momentum around the vertical axis of a rotating chair. A man sitting on a rotating chair with very little friction uses weights, a spinning wheel, and a wooden hammer with a long handle to demonstrate the vector nature of the angular momentum as well as its conservation in the absence of external torques. This setup allows to limit the observation to a single (the vertical) component of the angular momentum.
  • Published: 2003
  • Publisher: IWF (Göttingen)
  • Language: German
01:21 IWF (Göttingen) German 2003

Dynamische Stabilität einer Fahrradkette

Belts made of soft material which are flexible while at rest, in this case a chain made of metal links, can become quite rigid, independent of their shape, when their tangential velocity is sufficiently large. This phenomenon is called "dynamic stability". Through the radial forces the chain stiffens, and after it gets thrown off the toothed wheel, it roles along the floor, jumps over a wooden beam, and collapses into its shapeless form only after coming to rest on the rug after colliding with the wall. It is clearly seen how the stiff chain deforms somewhat as it hits the beam. Note also how long the chain continues to rotate on the rug near the end of the experiment, before it finally comes to rest and collapses.
  • Published: 2003
  • Publisher: IWF (Göttingen)
  • Language: German
00:44 IWF (Göttingen) German 2003

Impulserhaltungssatz

According to the law of the conservation of momentum, which states that in every closed system the sum of the individual momenta stays constant. They can be exchanged inside the system. Three examples will be shown. In all three cases, the total momentum of the closed system remains constant.
  • Published: 2003
  • Publisher: IWF (Göttingen)
  • Language: German
04:49 IWF (Göttingen) German 2003

Freie und erzwungene Schwingungen eines Drehpendels (Pohlsches Rad)

Basic properties of harmonic oscillators on a torsional pendulum, specifically their free and driven oscillations. For the latter, the transient oscillations will be shown, before the steady state has been reached. The torsional pendulum consists of a circular copper ring with an eddy current brake, and a torsional spring. One end of the spring is attached to the copper ring, the other end to a motor via an eccentric-and-rod mechanism. For the experiment, another pendulum, also specifically designed for lecture demonstrations, will be used. It sits on an optical bench and is projected onto the front wall of the lecture hall, together with a meter indicating the exciting frequencies used for the driven oscillations. Two pointers, one attached to the pendulum, the other to the eccentric-and-rod mechanism used for moving the spring, allow observation of the phase difference between the exciting torque and the angular displacement of the pendulum.
  • Published: 2003
  • Publisher: IWF (Göttingen)
  • Language: German
00:55 IWF (Göttingen) German 2003

Plastische Verformung und Zerreißfestigkeit

The measurements performed in the experiment "Elastic deformation, Hooke's law" (C 14833) on a copper wire, which had been limited to elastic deformations at small stresses, will be extended here to larger stresses. When the yield stress is exceeded, plastic, i.e. irreversible deformation is observed. A further increase of the stress beyond the rupture strength, leads to rupture. A 40 cm long piece of wire is suspended on a lab stand, and carries a tray on which weights can be deposited. A pillow under the tray will dampen the fall when the wire ruptures. The loading of the tray occurs in three steps : First, 2 kg cause a clearly visible elongation. The addition of 1 kg results in a slow length change of several centimeters, the wire "flows", until "work hardening" leads to a constant elongation. The addition of only 200 g finally leads to rupture, as the rupture strength is exceeded.
  • Published: 2003
  • Publisher: IWF (Göttingen)
  • Language: German
00:50 IWF (Göttingen) German 2003

Longitudinale Schwingung einer Spiralfeder

Longitudinal standing waves can be excited on helical springs. Through optical projection, nodes and antinodes are made visible. A horizontally mounted small helical spring is projected onto the wall of the lecture hall. A standing longitudinal wave is excited by driving one of its ends with a door bell vibrating at one of the normal mode frequencies of the spring. In the projection, the nodes and antinodes of the longitudinal vibration are easily seen: In the nodes, the sections of the spring are motionless and thus clearly imaged, while in the antinodes they appear fuzzy.
  • Published: 2003
  • Publisher: IWF (Göttingen)
  • Language: German
01:08 IWF (Göttingen) German 2003

Elastische Verformung: Hookesches Gesetz

A demonstration of the proportionality of tension and elongation during elastic deformation of a metal wire (Hooke's law). A copper wire, 4.4 m long, 0.4 mm in diameter, is suspended from the ceiling of the lecture hall. For the measurement of its elongation, a mm scale is attached to its lower end and is projected onto a screen, together with a fixed pointer. The wire is pulled in two steps by attaching consecutively two 300 g weights. The total elongation measured is 1 mm for 300 g, and 2 mm for 600 g, and is shown to be fully reversible when the weights are removed, indicating an elastic deformation. Elongation and force are proportional, in compliance with Hooke's law.
  • Published: 2003
  • Publisher: IWF (Göttingen)
  • Language: German
01:32 IWF (Göttingen) German 2003

Kreiselkompass

Principle of a gyrocompass using a bicycle wheel on a rotating chair. A bicycle wheel in a fork is mounted on a chair. The fork is free to rotate around its axis which is inclined 45 degrees away from the vertical. As the chair rotates, the center of the wheel moves on a circle. The motion of its axle is observed by an experimenter on the chair, and also by a stationary observer (the movie camera). With the chair at rest, the wheel is started to spin, its angular momentum vector pointing in an arbitrary direction. When the chair is rotated, the axle of the wheel begins to oscillate. The oscillation is damped, and after a few periods, the axle of the wheel (which indicates the direction of its angular momentum) points towards the axis of rotation of the chair. When the chair is made to rotate in the opposite direction, the wheel flips 180 degrees, and after a few periods of oscillation again points towards the axis of the chair.
  • Published: 2003
  • Publisher: IWF (Göttingen)
  • Language: German
02:15 IWF (Göttingen) German 2003

Stabilisierung mit Hilfe eines Kreisels ("Einschienenbahn")

In an impressive demonstration, the precession effect is used to stabilize a very top-heavy device. A man tries to balance while sitting on a steel bar which can rotate freely around its horizontal axis. A top consisting of a lead-filled bicycle rim driven by an electric motor is attached to this bar with a joint allowing the top to be tilted away and towards the man, but not sideways. The center of mass of man and top are far above the bar. As long as the top is at rest, the man has to keep his feet on the floor to avoid tipping over. With the top spinning, pushing or pulling its axis will make the top precess, i.e. to move sideways. This motion can be used to balance the top-heavy system.
  • Published: 2003
  • Publisher: IWF (Göttingen)
  • Language: German
02:38 IWF (Göttingen) German 2003

Foucaultscher Pendelversuch

A relatively simple demonstration and quantitative determination of the rotation of the earth using a simple pendulum several meters long, the so-called Foucault pendulum. The frequency of rotation can be measured in less than one minute. After carefully damping any spurious vibrations of the pendulum held at its maximum excursion, it is released using a mechanism that avoids shaking it. On the projection screen one can see how the maximum excursion of the pendulum wire shifts from one period to the next. A quantitative evaluation leads to the well known rotational frequency of the earth (1/day). Note that for this evaluation the geographic latitude has to be considered at which the experiment is being performed (f = 51.5 degrees). The actual rotational frequency of the observed rosette trace is here smaller than on the pole. On the equator, it would vanish.
  • Published: 2003
  • Publisher: IWF (Göttingen)
  • Language: German
01:47 IWF (Göttingen) German 2003

Pendelbewegung im rotierenden Bezugssystem

The demonstration of inertial forces in accelerated frames of reference. These forces do not occur in reference frames which are not accelerated (inertial frames). Rotating frames of reference are also accelerated even when the angular velocity is constant. In such a frame, the Coriolis force acts on moving objects in a direction perpendicular to their motion. For freely moving objects, this leads to a curved path. Such paths, in the form of rosettes, will be shown for a simple pendulum swinging on a rotating chair. This experiment is in principle identical to that of the Foucault pendulum, with the earth being the rotating reference frame, the only difference being that the earth rotates about ten thousand times more slowly than the rotating chair in the lecture hall.
  • Published: 2003
  • Publisher: IWF (Göttingen)
  • Language: German
01:04 IWF (Göttingen) German 2003

Kraft = Gegenkraft

A demonstration that forces between two bodies always occur in pairs: They are opposite in direction and equal in magnitude. These facts were summarized by Newton with the statement "action = reaction". Two people are standing on flat carts on a level floor, a few meters apart. The forces are exerted by means of a rope. The experiment shows that in both cases the forces are equal in magnitude and opposite in direction, in short: action = reaction. It also shows that the total momentum of the system is conserved.
  • Published: 2003
  • Publisher: IWF (Göttingen)
  • Language: German
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