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  • F2-22: BUOYANCY PARADOX - ACCELERATED FRAME

    F2-22
    Illustrate dramatically the concept of buoyancy.
    A float is at rest in a water vessel suspended by a spring from a fixed point. The vessel is lifted up and released from rest, so that it oscillates vertically on the spring. In the picture above a band around the floater lies between the two bands around the larger vessel when the system is at rest.

    Q: How will the float move inside the water vessel as the vessel executes simple harmonic motion?

    A: Surprisingly, the float will remain at rest in the water vessel as it oscillates. It will even remain at rest when the vessel is stopped suddenly with your hand! See video 2 below. As the vessel oscillates, the weight density of both the floater and the water bath vary together, as the acceleration of the vessel varies, so the ratio of their densities remains the same, and they will continue to float with the same geometrical relationship!!

  • F2-23: BUOYANCY PARADOX - INVERTED BLOCK

    F2-23
    Illustrate buoyancy in a paradoxical way.
    A large styrofoam block with a smaller aluminum block mounted on top as in the picture at the left, floats with the center of the styrofoam block, marked by the black line, at the water level. Q: When this object is inverted, with the aluminum block in the water, will the water level on the styrofoam block be (a) above, (b) below, or (c) at the black line?

    A: The water level will be below the black line, as seen by clicking your mouse on either of the pictures above. Notice also that the water level in the tank remains the same, at the level of the top of the black tape.

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  • F2-24: ACCELERATED BUOYANT BALL

    F2-24
    Illustrate buoyancy in a paradoxical way.
    A ping pong ball is tethered by a spring to the bottom of a water container, which in turn hangs from a spring attached to a fixed point. At rest, the ping pong ball floats near the center of the water tank. Q: How does the ping pong ball move, if at all, when the water tank is raised vertically and released from rest, so that it executes simple harmonic motion? A: The ping pong ball moves out of phase with the motion of the water tank. The system functions as an accelerometer, where the ball moves (with respect to its equilibrium position) in the direction of the acceleration of the tank. The magnitude of the displacement is roughly proportional to the magnitude of the acceleration of the tank.

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  • F2-25: BALANCE PARADOX - BUOYANCY WITH CROSSOVER

    F2-25
    Present buoyancy in a paradoxical way.
    The balance is initially at equilibrium with a mass hanging from an arm on the left pan in balance with the water beaker on the right pan, as seen in the photograph at the left above. Q: If the mass is allowed to hang into the beaker of water, how does this effect the balance? In particular, what, if anything must be done to restore equilibrium? (Note that there are a 100 gram weight and two 50 gram weights available at the lower left of th picture, and these weights can be added to either side of the balance to restore equilibrium.) A: Because the volume of the block is 50 cm^3, the weight on the left side is reduced by 50 grams when the block is submerged in the water. Conversely, the weight on the right side is increased by 50 grams, the reaction force on that pan. To restore equilibrium, 100 grams must be added to the left pan, as seen in the photograph at the right above.

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  • F2-26: BUOYANCY PARADOX - BALL IN TWO LIQUIDS

    F2-26
    Demonstrate buoyancy with a counterintuitive element.
    A ball floats in a beaker of water (at left in picture above). The ball sinks in a beaker of mineral spirit paint thinner (at right above). The mineral spirit paint thinner is immiscible with water, and will float above the water when poured slowly on top of water. The ball then floats at a higher level because the mineral spirits in which the upper part of the ball is floating provide an additional buoyant force on the ball. If the ball is floating on water alone, the top part of the ball is immersed in air, which has a much smaller density.
    F2

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  • F2-27: Buoyancy Paradox - Two Spheres

    f2-27
    To illustrate an interesting brainteaser about bouyancy
    Each pan of a balance holds a beaker of water, filled to the same level. In one beaker, a ping-pong ball floats, tethered by a string to the bottom of the beaker. In the other beaker, a steel ball of equal volume hangs suspended from an outside support. The balance can be clamped to hold it level. Invite students to predict what will happen when the clamp is removed: Will the balance remain level, will the side with the ping-pong ball go down (that side is heaver), or will the side with the steel ball go down (that side is heavier)? Encourage students to explain their reasoning and discuss amongst themselves.

  • F2-32: FLOATING SQUARE BAR

    F2-32
    Illustrate buoyancy and the orientation in which a long square bar floats in a very dramatic way.
    A long square bar floats in a bath of methyl alcohol, with its sides horizontal and vertical, as shown in the center photograph. If water is added to the bath until the tank is almost full, the bar will rotate so that the sides are diagonal, as shown in the photograph at the right, due to the effective decrease in the density of the bar relative to the bath.

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  • F2-41: DENSITY - SLOPE OF MASS VS VOLUME GRAPH

    F2-41
    Determine the density of water.
    Starting with the container empty, pour in water a small amount at a time and plot the total mass vs. the volume of water in the container. The slope of the graph is the mass density of water.

  • F3-01: SURFACE TENSION - JOLLY BALANCE

    F3-01
    Demonstrate surface tension and to determine the value of the surface tension of water.
    This is the classic experiment for determination of the surface tension of water. Weights can be attached to the bottom to determine the spring constant. Measure the equilibrium position of the rectangular frame when not immersed, and when the bottom wire is completely immersed. The average should be taken with the wire just making surface contact. Slowly and smoothly lower the platform until separation occurs, and measure the displacement. The surface tension T is given by T=F/2L, where F is the force of the spring and L is the length of the wire (which is 8 cm for this apparatus). The spring constant k is 7.27 g/cm, so the force is F=kgx, where x is the extension of the spring and g is the acceleration of gravity. The photographs at the center and rightabove show the effect of the surface tension pulling down the spring. Be careful. This apparatus is sensitive.

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  • F3-03: SURFACE TENSION - SOAP BUBBLES

    F3-03
    Demonstrate surface tension in a perhaps counterintuitive way using soap bubbles.
    Pour some soap solution into the small glass lid of the soap solution container. Turn the valve so that only one side is connected to the hose, dip that end into the solution and blow a bubble. Rotate the nozzle so that the other side is connected and blow a bubble of a different size. Rotate the valve so that air can flow from one to the other.

    Q: What will happen to the two bubbles when they are connected?

    A: The smaller bubble will get even smaller and the bigger bubble will get even bigger, as seen in the photograph at the right.

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  • F3-06: SURFACE TENSION - NEEDLE ON WATER

    F3-06
    Illustrate surface tension by floating a needle on water.
    The water bath is allowed to become quiescent and the needle is very carefully placed on the surface of the water. The surface tension of the water supports the needle. Practice! This demonstration requires a delicate touch.

  • F3-21: SURFACE TENSION - ALCOHOL AND WATER IN SAND

    F3-21
    Illustrate the difference between the surface tension in water and in alcohol.
    One jar contains sand with water. The bottle has been tamped so that the sand grains have become aligned with very small cracks between them. The surface tension of the water will not allow the water to flow into these very small volumes. When the jar is squeezed, the cracks open up sufficiently to allow the water to flow into them, thus causing the water in the tube to fall.

    A second jar contains sand with alcohol. Because alcohol has a much smaller surface tension than water, the alcohol will flow into the small cracks between sand grains, filling the space. When the alcohol bottle is squeezed, the volume is already filled with sand and liquid, so the alcohol level in the tube must rise.

  • F3-31: WATER BELL

    F3-3
    Demonstrate surface tension in an artistic manner.
    A stream of water hits a circular horizontal surface and projects out radially. Surface tension in the water pulls the water together, creating a bell or heart-shaped water surface as shown in the photograph.
    F3

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  • F4-01: VISCOSITY OF LIQUIDS

    F4-01
    Compare the viscosities of water and mineral oil.
    Two tubes contain heavy balls in water and mineral oil, respectively. Invert the tubes and compare terminal velocities to compare viscosities.
    OS4

  • F4-02: VISCOSITY OF AIR

    F4-02
    Demonstrate that air is viscous.
    A large thin disc is suspended above a copper plate mounted on a rotator. As the plate is rotated, the thin disc begins to rotate slowly in the same direction, because of drag caused by the viscosity of the air.

  • F4-05: PARACHUTE TOY

    F4-05
    Demonstrate how a parachute quickly reaches its terminal velocity.
    A toy rocket has an attached plastic parachute. The parachute is carefully, loosely folded against the rocket and they are tossed into the air together. The parachute opens and fills with air. The system soon reaches its slow terminal velocity and falls gently to the floor. Be careful not to allow the strings to become tangled or detached, as this will prevent the parachute from operating properly.

  • F4-14: WIND TUNNEL

    F4-14
    Show air flow around different objects.
    Dry ice is dropped into a container of water to produce visible air flow in the wind tunnel, which is then projected using an overhead projector. Various objects can be placed in the wind tunnel, and the air flow past the objects observed.
    F4

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  • F4-21: LIQUID IN SPINNING SPHERE

    F4-21
    Show the behavior of a liquid when subjected to a centripetal force.
    When the sphere is rotated, the water leaves the bottom of the sphere and forms a band in the middle of the sphere, due to the reaction to the centripetal force. Rotate sphere slowly to achieve this effect.
    F4, F1, D1

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  • F4-22: SPINNING WATER BUCKET

    F4-22
    Illustrate the reaction force on spinning water and the shape of the water surface.
    As the glass bucket rotates, the water surface assumes a parabolic shape. Use about 600 ml of water; rotate slowly
    F4, F1, D1

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  • F4-23: WATER PENDULUM

    F4-23
    Show the surface of a container of water in a swinging pendulum.
    A container with water is suspended as a pendulum. When the container is held to one side, the water moves to its lowest point and the surface remains horizontal. When it is released and swings as a pendulum, the water spreads out uniformly on the bottom of the container and stays at rest at all times.
    F4, F1, FS1