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PHYS141

  • 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

  • B1-14: 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

  • B2-22: TORQUE - PHYSICAL DEFINITION

    B2-22
    Introduce the concept of torque

    A torque is produced on the two-meter scale by hanging a weight on the right side of the fulcrum. This torque can be balanced by pulling downward with the dynamometer with different forces at various angles on either side of the plumb line.

    The photographs above shows the torque lever with the protractor in place with its plumb bob and the counterweight on the right, and then show the system balancing the torque created by a weight at the right end of the lever by pulling downward on a spring scale at two angles measured by the protractor.

    Note that this demonstration and demonstrations B2-21 and B2-32 use many of the same components.
    B2, ME1, OS0

      

  • B2-23: TORQUE CONDITION FOR EQUILIBRIUM

    B2-23
    Demonstrate that equilibrium of forces is an insufficient condition for static equilibrium
    The weight of the rod is balanced, with the rod held in a horizontal position, using two equal weights hung over pulleys. While holding the rod fixed, move a small weight from one side to the other. The force condition for equilibrium is still met, but the torque condition is not the same, and the rod will rotate.
    B2, OS0, ME1

  • B2-31: EQUILIBRIUM OF TORQUES - WHEELS AND AXLES

    B2-31
    Demonstrate equilibrium of torques

    Two wheel and axle assemblies are provided. For each, there are cylinders of smaller radius r and larger radius R. A smaller weight m is suspended from a string wrapped around the larger radius R, and a larger weight M suspended from the smaller radius r. For equilibrium of torques:

    mR = Mr.

    On this model, radius ratios of 2:1 and 4:1 are available. A set of additional masses can be provided to vary the load.
    FS2

  • B2-42: ARM MODEL

    B2-42
    Model the forces occurring in the arm

    Photograph at the top shows arm model in neutral force configuration

    Force applied by the biceps (lower left), pulling up with the hand: Apply 2.5 kg to the biceps cable to support the unloaded forearm. The forearm may be kept at equilibrium by the simultaneous addition of masses in the ratio of 10:1 at the biceps and at the hand. The torques are balanced almost independently of the angular position of the arm.

    Force applied to the triceps (lower right), pushing down with the hand: Hang the spring scale between the top hook and the hand hook, and attach the hanger to the triceps cable. Add masses to the hanger to determine how much force in the triceps is necessary to push down with the force read on the scale.
    FS2

     

  • C1-01: CENTER OF MASS MOTION - BARBELL

    C1-01
    Demonstrate motion of the center of mass of an asymmetric object.
    A barbell with unequal ends is thrown through the air. The center of mass (red spot near large end) moves in a smooth parabolic arc despite the gyrations of the two asymmetric ends.

    An animation of the movement of an object of this kind can be viewed at http://www.acs.psu.edu/drussell/Demos/COM/com-a.html

    C1

  • C1-04: CENTER OF MASS - BEAR ON TIGHT ROPE

    C1-04
    Show stability in system where the center of mass is outside of the object.
    As the bear rolls along the tightrope, it remains stable because its center of mass is below the rope. Removing the weights and poles renders the system unstable.

  • C1-21: AIR TABLE - TOPPLING STICK

    C1-21
    Illustrate how a rigid rod topples on a frictionless surface.
    A wooden dowel with its center of mass marked is held vertically with the bottom end supported by a very light air table puck. When it is released and allowed to topple, which point of the stick will be directly above the original support point: the top end, the bottom end, or the center of mass? Note that the stick has a small counterweight on its top end to compensate for the mass of the puck. Click your mouse on the photograph for an mpeg video.
    FS0

  • 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

  • C2-09: FREE FALL WITH STROBE

    C2-09
    Show the position of a dropped ball at a series of equal time intervals
    Drop the ball with the strobe on at the desired flash rate (about 10-13 flashes per second, or 600-800 per minute, seem to work well). The increasing distance the ball falls between successive strobe flashes is readily apparent.
    C2, FS1, LS1

  • C2-11 RACING BALLS

    C2-11
    Illustrate linear kinematics

    Two balls are launched by a spring-operated launcher from one end of the track. They depart with the same velocities and the same kinetic energy imparted by the spring. As shown in the picture, one track runs in a straight line; the other dips down, runs straight for a time, then rises back up to the original level.
    Engagement Suggestion:
    Have students make predictions (and justify them):
    • Which ball will reach the end first, or if they will reach the end at the same time?
    • Which one (if either) will be moving faster at the end?
    Background:

    The ball on the straight track retains essentially the same velocity and the same kinetic energy throughout the length of its run, the kinetic energy from the spring. The ball on the dipped track, however, has a more complex path. When it goes downhill, it gains kinetic energy from gravitational potential, accelerating it. It travels along the lower section of track with this increased kinetic energy, and thus greater velocity. The ball then goes uphill again, losing that additional kinetic energy – it has returned to the same height, so the principle of conservation of energy dictates that it must return to the same gravitational potential as before, giving up kinetic energy equal to what it gained. It now has only the same kinetic energy it started with, as imparted by the spring. So its velocity is now the same as its starting velocity, and the same as the velocity of the other ball.

    However, during the time it was on the lowered section track, it had greater kinetic energy and greater velocity, so it traveled that distance faster than the ball on the straight track. And thus the ball on the dipped track reaches the end first, but with the same final velocity and the same final kinetic energy.
    OS0

  • C2-21 PROJECTILES DROPPED AND SHOT

    C2-21
    Demonstrate the independence of horizontal and vertical components of motion

    A latchable spring launching mechanism is mounted at the top of a stand. Two metal cubes are attached to the mechanism. When the latch is released, one cube will be projected horizontally while the other is dropped straight down. They strike the floor at the same time.
    Engagement Suggestion
    • Before showing the experiment, challenge students to predict what will happen. Will the horizontal motion of one pellet make it strike the floor before or after the other?
    • Afterwards, discuss why or why not.
    Background

    The gravitational force on each of the cubes is the same, so they experience the same downward acceleration. So since they started from the same height with zero vertical velocity, they reach the floor at the same time, even though one has traveled some distance horizontally in the meantime.

    This is an example of the independence or separability of the components of motion. We can define the axes along which we measure, and treat vectors as the sum of their components along those axes.
    FS2

  • C2-22 MONKEY AND HUNTER

    C2-22
    Demonstrate the independence of horizontal and vertical components of motion
    A physical example of a classic textbook illustration, this demonstration shows the independence of the components of motion and the equal acceleration of bodies due to gravity.

    The launcher is aimed at the monkey and shot. As the projectile leaves the muzzle of the gun it breaks a circuit producing the magnetic field which holds the monkey in place. The monkey then begins to fall at the same time the projectile is fired directly at the monkey. Due to independence of horizontal and vertical components of motion, the projectile will strike the monkey.

    Note that the angle can be varied to show different horizontal and vertical components.
    FS1

  • C2-25: FUNNEL CART

    C2-25
    Demonstrate the independence of horizontal and vertical components of motion
    A ball is placed in the funnel and the funnel cocked by compressing a spring. The cart is then pushed across the track. At a certain point a bump below the track trips a lever, releasing the spring and ejecting the ball vertically. Because the ball and the cart both move with the same horizontal speed, the ball stays directly above the funnel at all times, and falls back into the funnel. Before doing the experiment, ask your students where the ball will fall: in front, behind, or in the funnel.
    C2, OS0

  • C2-26 FUNNEL CART WITH MASS OVER PULLEY

    C2-26
    Demonstrate the independence of horizontal and vertical components of motion
    A ball is placed in the funnel and the funnel cocked by compressing a spring. A mass on a string passing over a pulley is attached to the funnel cart, and the cart released so that it accelerates across the track. At a certain point a bump below the track trips a lever, releasing the spring and ejecting the ball vertically. Due to the acceleration of the cart, the ball falls behind the funnel.
    C2, OS0

  • C3-02 INERTIA - TABLE CLOTH TRICK

    C3-02
    Dramatically demonstrate inertia
    The table setting rests on a silk tablecloth. Rapidly yanking the tablecloth out from under the setting pieces leaves the table setting unchanged.
    C3

  • C3-05 INERTIA - PEN IN BOTTLE

    C3-05
    Dramatically demonstrate inertia

    A large-tip felt pen is balanced on a 12" embroidery hoop, which in turn is balanced on a wide-mouth bottle. Yanking the hoop out from under the pen (by striking inside the leading side horizontally) allows the pen to fall straight downward into the bottle. Note that this does take a bit of practice; try it out before class.
    Engagement Suggestion:
    Ask your students: • Why does it matter if the hoop moves up or down while you are moving it?
    • Does it make a difference if you grab the hoop from the outside or the inside?
    Background:

    Newton’s First Law of Motion states that an object’s velocity is constant unless there is a net force acting on it. What this means is that if an object is not moving (at rest), it will not start moving until there is a force pushing or pulling on it. If an object is moving at a constant speed and direction, it will keep going with that same speed and direction unless a force pushes or pulls on it to change that. When the pen is sitting on top of the hoop, the force of gravity is pulling it down, but the normal force of the hoop is exactly equal to the gravitational force and holds it up. If another force suddenly affects the pen (such as if you walk up and tap on its side, or jiggle the hoop up and down), that force could cause it to move, and probably fall.

    But if the hoop is snatched sideways quickly and smoothly, it does not give any force to the pen. Now the only force acting on the pen is gravity, and the pen falls straight down into the bottle.

    C3

  • C3-12 PENCIL AND PLYWOOD

    C3-12
    Dramatically demonstrate inertia

    A pencil is accelerated to almost the speed of sound by blasting it through a four-foot tube using a carbon dioxide fire extinguisher. The pencil will readily impale itself through a piece of 3/8" plywood. With a little bit of luck the pencil point will be virtually intact, although sometimes you need to re-sharpen it after the demonstration.

    CAUTION: Be sure that the hose fitting is securely attached to the tube and that the plastic shield is in place before firing. The shield should be latched in place, with no debris blocking its edge from meeting the baseplate

    Engagement Suggestions
    • • Before using, encourage your students to predict what will happen to the pencil.
    • • For advanced students, discuss the energy involved in the problem and where the kinetic energy of the pencil went after the collision.
        • Background

        This demonstration can be presented in multiple ways. It has been offered classically as an illustration of the principle of inertia – the pencil is in motion at a high velocity, and continues in motion despite the intervening wood until arrested by a greater force. Alternatively, consider the high velocity and high momentum of the pencil. The abrupt deceleration at the plywood means a high impulse. The pointed pencil has a very small cross-sectional area, resulting in force applied over a small area leading to a high momentary pressure.

        Linked below is a slow-motion video of the collision, shot at 600 frames per second. A fun class activity could be to use the video to measure the motion of the pencil and estimate its momentum and kinetic energy, based on what you see in the video and by measuring typical lengths and masses for wooden pencils.

    FS1

  • C4-22: HORIZONTAL ATWOOD MACHINE

    C4-22
    Demonstrate the Horizontal Atwood Machine quantitatively.
    A mass m connected to a string passes over a pulley and is attached to the Horizontal Atwood Machine, of mass M, as photographed. When the system is released, and begins to accelerate, the tension in the string is reduced, as can be read from the spring scale. The tension T in the string while the system is accelerating can be calculated and compared with the tension observed on the scale: T = Mmg/(M+m), or T/g = Mm/(M+m) in mass units indicated by the scale mounted on the cart. In this case M=875g and m=200g, so the scale reads 200g (photograph above) when the system is held at rest and 163g (previous mark on scale is 150g) shortly after it is released. This can be seen in a half-speed mpeg video by clicking the link below.