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PHYS121

  • A1-51: SKATEBOARD

    A1-51
    Show things with a skateboard
    Wooden platform with ball bearing wheels. Can be used in a variety of entertaining ways. Please be careful.
    A1

  • A2-22: MAGNETIC VECTORS - LECTURE HALL BLACKBOARDS

    A2-22
    Demonstration of vector properties
    Various length of magnetic vectors that can be affixed to Lecture Hall boards. The lengths are in the ratio 3:4:5, so that they can form a right triangle. Illustrate vector addition, subtraction, motion of vectors, and commutative properties. Only usable in large lecture halls with steel-backed chalkboards.
    A2

    ve

  • B1-01 CENTER OF MASS - DISC SECTION

    B1-01
    Demonstrate finding center of mass for an irregularly-shapped object
    When the crescent shaped object is suspended from one of the holes along its perimeter, the center of mass will be directly below the suspension point, as defined by the plumb bob. When the object is suspended from the hole at its center of mass, it remains stable in any angular position.

    Questions: How do you find the position of the center of mass of this object? (A: From the intersection of lines) Why does the object remain stable when it is suspended at its center of mass?


    See also demonstration B1-09: CENTER OF MASS - TRIANGLE. These two demonstrations use the same stand and are often taught together.
    B2

  • B1-02: CENTER OF MASS - LEANING TOWER

    B1-02
    Demonstrate the effect of center of mass location
    A model tower is comprised of two sections; an askew bottom component and a top component. The bottom component is in a stable equilibrium when sitting on a horizontal surface since the horizontal position of its center of mass is within the perimeter of the base. As the top section is placed onto the base tower, the horizontal position of the center of mass will shift outside the base perimeter, thereby causing the system to be unstable and topple.

    Questions: Would anything change if the tower was hollow? Would the center of mass change? What should you do in order to keep the tower (consisting of both components) stable?

    B1

  • B1-03: CENTER OF MASS - LEANING TOWER ON INCLINED PLANE

    B1-03
    Demonstrate equilibrium when its center of mass is above its base
    This is a variation of B1-02. On a horizontal surface, the addition of the top component onto the base component of the tower shifts the center of mass outside the base perimeter, causing the configuration to change from stable (without top) to unstable (with top). When the tower is placed on an inclined plane, the horizontal position of the center of mass will be shifted more towards the center of the base thereby making it stable even with the top since the center of mass will still be over the base.

    Invite your students to discuss: What would happen if the inclined plane was removed? Is there any other way to make the complete tower (with both the top and base components) stable?

    B1, FS1

  • B1-04: Center of Mass - Brass Barbell

    B1-04
    Demonstrate stable equilibrium when supported at center-of-mass

    A brass barbell with two different disc-shaped masses, one on each end, is supported by a stand at its center of mass.

    Compare this to lifting up an asymmetrical barbell in the gym with one hand, and finding that it is the easiest to hold it at its center of mass.

    Engagement Suggestion:
    Ask students to make predictions:
    • What would happen if the brass barbell is shifted so that the heavier mass is closer to the middle?
    • Would the barbell rotate and if so, in what direction?
    • Would it remain on the stand?
    B1

  • B1-06: Double Cone - Large

    B1-06
    Demonstrate a center-of-mass paradox
    When a double cone is placed on the narrow end of a V-shaped rail, the cone will roll towards the wider end of the rail when released. The cone appears to be rolling uphill (from the narrow end to the wider end), but in reality the center of mass is moving down. Upon special request, we can also provide a cylindrical rod of the same length as the double cone to roll along the rails to show their actual slope. Challenge students to predict whether the cylindrical rod travel in the same direction as the double cone.

    Note: B1-07 is a smaller, more portable version of this demonstration.
    B1

  • B1-09: CENTER OF MASS - TRIANGLE

    B1-09
    Demonstrate the process of locating the center of mass

    TA triangular shaped object can be suspended from any corner by a nail at the top of the support. The center of mass will be directly below each corner, as defined by the plumb bob. Lines of the vertical position of the bob’s string is drawn on the object. The intersection of all the lines is the position of the center of mass.

    See also demonstration B1-01: CENTER OF MASS - DISC SECTION. These two demonstrations use the same stand and are often taught together.

    Ask students: What would happen if the triangle was suspended at its center of mass?
    B2

  • B1-11: BALANCE MAN

    B1-11
    Illustrate stable equilibrium
    When set on a pedestal, the Balance Man will rock back and forth without falling off no matter the position it is in. The center of mass and the center of gravity is along the line of symmetry of the figure, which is a short distance below the legs. Since the Balance Man’s center of gravity always remains below its pivot point, when it is pulled to one side, gravity will always exert a restoring force which will pull it back to an upwards position. Question: Will the Balance Man be in stable equilibrium if it is set on one leg? (Yes, it will always be in a stable equilibrium since the center of gravity is below the pivot point). Why does the toy rock back and forth instead of falling off the pedestal?
    B1

  • B1-12: CENTER OF MASS - HINGED STICK PARADOX

    B1-12
    Illustrate the center of mass in a surprising way
    To demonstrate center of mass, find how the hammer is supposed to be positioned in order to have it hanging from the loop on the bottom of the board so that the board sticks out horizontally. Question: How can you hang the hammer from the loop on the bottom of the hinged board so that the board will stick straight out horizontally? (A: Insert the handle into the loop so that the head of the hammer is pointing back toward the hinge.)
    B1, tools

    ct

  • B1-13: CENTER OF MASS - FORK AND SPOON ON TOOTHPICK

    B1-13
    Illustrate the center of mass in a surprising way
    Insert a spoon between the tines of a fork and a toothpick through an interstice of the fork. The system will be balanced when the toothpick is set at the edge of the glass. Question: If the toothpick were to be burned from the inside, would the system fail? (Show this). If not, why does the spoon and fork remain balanced?
    B1

  • B1-14: CENTER OF MASS - BOTTLE

    B1-14
    Illustrate the center of mass in a surprising way
    When the neck of a wine bottle is position inside a hole of a plastic support, the system will be in a stable equilibrium therefore the wine bottle will remain at rest in an unlikely position. Question: Why is this system in equilibrium? (A: The center of mass of the system is above the base area of the plastic support).
    B1

  • B2-04: VERTICAL FORCES - FOUR SPRING SCALES

    B2-04
    Show addition of forces along a line
    A set of four spring scales are hung from a stand. Each scale shows the total weight of those hanging below it plus any force exerted downward on the lower scale. The bottom scale measures its own weight, about 3 Newtons. In the picture above, a downward force of 9 Newtons is being exerted by Jack's hand at the bottom of the picture, so the scales read (bottom to top) 12N, 12N, 15N, and 18N.
    ME1, FS2

  • B3-12 PULLEY - MECHANICAL ADVANTAGE

    B3-12
    Illustrate pulley systems
    The system is initially balanced to account for the weight of the pulleys and rope by adding small weights on the free end of the rope, as seen in the photograph at the left. For every two kilograms hanging from the pulley, the system requires one kilogram hanging from the free end of the rope to obtain equilibrium. Show that deviation from the 2:1 ratio destroys the static equilibrium.
    FS1

  • B4-01 HOOKE'S LAW

    B4-01
    Demonstrate the linear relationship between force and stretching for a simple spring.
    Two weights are provided to show linearity over a factor of two in applied force.
    FS2

  • B4-04: SPRING AND STRING THING

    B4-04
    Illustrate series and parallel springs in a counterintuitive way.

    Two springs connected in series support a weight. Strings slightly longer than the springs are connected in parallel with each spring, as photographed. The connecting wire loop between the two springs is then removed, forming two separate parallel routes, each consisting of a spring and a string in series. Comparing the final configuration with the initial configuration, will the weight be higher, lower, or at the same vertical position?

    The pictures above show the system in its initial and final configurations, as well as in detail of how the springs and strings are coupled at the center.

    This demonstration is an analog to paradoxical behavior in complex series/parallel arrangements for other mechanical, hydraulic, and electrical systems. Perhaps the most notable is Braess' paradox for traffic flow. In certain types of congested traffic flow situations, opening an additional new route between two points may actually increase the average time taken to travel between the two points.
    FS2

    b4-04a b4-04b

  • B4-11: ELASTIC LIMIT OF RUBBER BAND

    B4-11
    Demonstrate Hooke's law and elastic limit.
    Load small weights to demonstrate Hooke's law. Hanging a few kilograms from the rubber band exceeds its elastic limit.
    FS2, ME1

    b4-11a

  • B4-12: ELASTIC LIMIT OF SOLDER SPRING

    B4-12
    Demonstrate the elastic limit for an inherently inelastic object.
    Place spring on overhead projector, and stretch and compress it. It will quickly become obvious when the elastic limit is exceeded.
    B4

  • B4-14: ELASTIC LIMIT OF WIRE

    B4-14
    Demonstrate the variation in tensile strength with wire diameter.
    Two wires are used: 24 AWG, 0.474 mm diameter, and 20 AWG, 0.786 mm diameter. The ratio of tension required to break two wires is proportional to the square of their diameters, for this case F2 / F1 = 2.75. The two wires can be broken, the required tensions read off the attached spring scale, and the ratio calculated.
    B4

  • C2-06 BALL DROP ON ROPE - EQUAL AND UNEQUAL INTERVALS

    C2-06
    Illustrate the geometrical effect of free fall
    Two ropes of equal length have steel balls tied at five points along their length. One rope has the balls at equal distances along the rope, while the second has balls positioned geometrically, at distances proportional to the squares of integers: 1, 4, 9, 16, and 25. When the first rope is dropped the equally spaced balls hit the floor at progressively shorter time intervals; when the second rope is dropped, the geometrically positioned balls hit the floor at equal time intervals. NOTE: This demonstration can only be properly done in the lecture halls because it requires 12 feet of height to fully extend the ropes.
    C2