Foucault Pendulum Vocabulary
Maureen Dooley, B.S.Ed.
Copyright 2004. Permission is granted forclassroom use and for non-commercial educational purposes.
Pendulum
A system, comprised of an overheadsupport, from which a string hangs, and an object at the bottom end of thestring. The object swings back and forth under the combined influence ofgravity and the string.
Pendulum Bob
Any object that hangs on thependulum string. It can be a toy ball, a bowling ball, or a small car (if thestring is string enough). It may be called a "bob" because its verticalposition "bobs" up and down while its horizontal motion moves side to side.
Pendulum Pivot
The pendulum string is attachedto the overhead support by the pivot. The pivot might simply be a knot thatties the string to a hook in the ceiling. For a Foucault pendulum, great careis taken that the pivot allows the pendulum to swing with the same ease in alldirections of the compass.
Position Position
tells where an object is. Thiscan be done with a picture, with words, or with equations. In any case, youhave to define a reference location, or "origin," because position is givenrelative to that location. In all cases you tell two things: The distance fromthe origin, and the direction that you have to go in order to move from theorigin to the object. The distance is measured in meters. The direction can bedescribed by stating an angle between the line from origin to object and areference direction, such as "east."
Oscillate
For the pendulum, the bob moves backand forth. This back and forth motion is called "oscillation." Its position issaid to oscillate back and forth.
Period
The period is the amount of time it takesthe bob to make one round trip. If the pendulum bob is pulled back andreleased, it returns to the hand that released it after a time interval equalto one period. The period of a 1 meter long pendulum is 2 seconds.
Amplitude
The amplitude is the maximumdisplacement of the bob from its equilibrium position. When the pendulum is atrest, not swinging, it hangs straight down. This position is called the"equilibrium position." It is convenient to take this position as the referenceposition mentioned as the "origin" in the definition of position. With thisorigin, the position of the pendulum varies to the left and to the right of theorigin. The size of the largest distance away from the origin is called the"amplitude." The bob swings to a distance equal to the amplitude on the left,and next swings to a distance equal to the amplitude on the right.
Velocity
Velocity tells the rate of change ofposition. In all cases, you tell two things to specify the velocity: The speed,and the direction of the speed. Speed is measured in meters per second, orm/sec. Direction is described by an arrow pointing in the direction of motion,or by the angle between that arrow and the reference direction used forposition.
Acceleration
Acceleration is the rate of changeof velocity. The units are (meters per second) per second or m/sec2Once again, you specify both the size of the acceleration and its direction. Ifthe direction of the acceleration is the same as the direction of the velocity,then the object speeds up. If the direction of acceleration is opposite to thevelocity then the object slows down. If the acceleration direction isperpendicular to the velocity direction, then the size of the velocitydoes not change, but the direction of the velocity does change.Acceleration is different from velocity in a surprising way, best described inthree steps:
1) If you give an object a position and leave it alone, itkeeps that position.
2) If you give an object a velocity and leave italone, it keeps that velocity. (This experimental fact is known as Newton'sfirst law of motion.)
3) If you give an object an acceleration and leave italone, the acceleration drops the zero at the instant you begin leaving italone.
Force
The only way that an object willaccelerate (change velocity) is if it forced to do so. It is sensible to saythat the force has a direction, and that direction is the same as the directionof the acceleration. It is sensible to say that a bigger force causes a largeracceleration.
Resultant Force
The resultant force is the forcethat results from the combination of two or more forces. The two forces thatact on the pendulum are the force of gravity, pulling straight down, and theforce by the pivot, pulling along the string, towards the pivot. Those twoforces combine to produce a resultant force. Just as an arrow is pushed forwardby the two halves of a bowstring, the pendulum bob is pushed by a resultantforce whose arrow "splits" the two component force arrows.
Gravity
Gravity is the name for a phenomenonthat is at once familiar and mysterious. We are pulled so surely towards theearth that we take it for granted; we use the phenomenon to sit, to walk, torun, and to play catch.
Experimentally, an object allowed to fall freelyunder the influence of gravity is observed to accelerate. Since an object mustbe forced to accelerate, there must be a force associated with gravity; we callit the force of gravity. The direction of the force of gravity is down. In factthe direction of the force of gravity defines what we mean by "down."
Plane of Oscillation
The two forces, gravity andstring define a plane. The same plane is also defined by the pendulum stringand the direction down. The resultant force is directed along a line which liesin this plane. The acceleration is also directed along a line that lies in thisplane.
If the bob is pulled back and released from rest, the velocity isdirected along the same line as the acceleration, and the bob moves along thatsame line. The path of the bob lies along the plane defined by the string andgravity. This path lies in the plane of oscillation.
Because the string andgravity lie in the plane, it is expected that the plane of oscillation willnever change. (The surprise of the Foucault pendulum is that the plane ofoscillation changes direction, clockwise in the northern hemisphere, as the daygoes by.
Rotation
If the position of an object changesalong a circular path, the object is said to rotate along that circle. Thesecond hand of an analog clock rotates clockwise. The plane of oscillation of aFoucault pendulum rotates clockwise in the northern hemisphere.
Work
People get paid more for trucking foodacross country that for holding it in place on a shelf. That seems fair, andwork is defined for physics in a way that seems similarly fair. Work is thedistance that an object is moved, multiplied by the force that pushed it alongthat distance.
Work can be positive or negative. If the object moves in thesame direction as the force (as when a truck accelerates) the work is positive.If the object moves in a direction opposite to the force (as when a truckbrakes and slows down) the work is negative.
When the force of gravitypulls down on an object that has been dropped, the force of gravity doespositive work on the object.
Kinetic Energy
When a pendulum bob is pulledback and released from rest, the force of gravity does positive work on the bobas it swings down. After the bob goes through the low point it swings back up,and during that upswing, the force of gravity does negative work, bringing itto rest at the top of the swing.
In fact, the bob swings back up to thesame height as the release height, so the negative work by gravity on theupswing is the same size as the positive work by gravity on the downswing.
It is as though the work was put into the bob, stored a while, and thentaken back. In this picture, the stored work is associated with the velocity ofthe bob at the bottom of the swing. It turns out that the work is proportionalto the square of the speed of the bob.
When converted to speed, the work issaid to be converted to Kinetic Energy. Work is said to be converted to kineticenergy, when the work is done to increase the speed.
Potential Energy
When the bob is pulled back itis ready to swing down, acquiring kinetic energy. The amount of kinetic energywhich it is capable of acquiring is determined by how high the bob was raisedwhen it was pulled back.
Because the bob has potential to gain that kineticenergy, it is said to have "potential energy." It turns out that the potentialenergy of the bob is proportional to the height of the bob above the lowestpoint of the swing.
