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Newton's Third Law

  • B1-24: CENTER OF MASS - CARTS ON BALANCE BOARD

    B1-24
    Show that the center of mass may remain at rest during motion within the system.
    The board is balanced on its fulcrum with the carts touching one another at the center of the board. Releasing the spring causes the carts to push against each other and separate, but the board remains balanced. Questions: What would happen if the two carts had different masses? Would the board become lopsided?

    The experiment can be made more complex by putting an extra weight into one cart, so that the masses of the carts are unequal.

  • B2-01: SUM OF FORCES IN A LINE

    B2-01
    Demonstrate that forces in one dimension add algebraically
    This demonstration consists of two strings connected at one end of a brass spring that is suspended at the other end from one side of a support frame. The strings run over pulleys attached to each side of the frame, and are then connected to weights. Different masses can be hung on the two sides, creating differing forces in each direction. Using the scale provided, the displacement of the spring can be measured, and the forces can be seen to add algebraically.
    B2, FS1

  • B2-02: SUM OF FORCES - SPRING SCALES

    B2-02
    Show that the sum of forces exerted on the mass by the scales is constant
    A weight is set on a platform scale, and the upper spring scale hooked to the weight. As an upward force is applied, the sum of the readings of the two spring scales remains constant, equal to the initial weight on the platform. In these photos, the sum of the forces on the 1kg mass remains slightly less than 10N as more of the lifting is transferred to the upper scale.
    B2, ME1

     

  • 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

  • B2-11: EQUILIBRIUM OF FORCES - OHAUS PROJECTION

    B2-11
    Demonstrate equilibrium of forces
    Forces are applied by hanging arbitrary weights on hangers over pulleys. The ring to which the forces are applied will stabilize symmetrically about the center pin when the forces are in equilibrium. Forces can be applied at arbitrary angles by rotating the pulley arms. The photographs above show details of the center of the force table and the system displayed using an overhead projector.

    Note that there are three arms over which weights may be hung, and thus a maximum of three vectors can be summed.
    B2

     

    Close-up of center of platform, and overhead projector display.

  • B2-12: EQUILIBRIUM OF FORCES - BOARD BACKGROUND

    B2-12
    Demonstrate equilibrium of forces
    Three forces are set up in static equilibrium. The lower spring scale is modified so that it measures its own weight plus the weight hanging on the hook below. Upper spring scales are calibrated to read zero when the connecting arms are in place and the lower scale is removed. A large protractor can be used to measure angles.
    FS1

  • B2-14: SUM OF FORCES - LARGE ROPE VERSION

    B2-14
    Develop a feel for equilibrium of forces on a large scale
    Three or four people can pull ropes in various directions to find combinations of forces and directions that result in equilibrium.
    B2

  • B2-16: VECTOR ADDITION WITH ROPE AND STUDENTS

    B2-16
    Demonstrate vector addition of forces
    Two students pull the ends of the rope. A third student pulls crosswise on the center of the rope. The ends of the rope will be pulled inward by a large force, regardless of the relative size of the third student.

  • B3-03: LEVER - WRECKING BAR

    B3-03
    Demonstrate the mechanical advantage of a lever
    Use the lever to pry a large nail out of a 4"x4" pine wood beam.
    B3, tools

  • B3-14: EQUILIBRIUM PARADOX - SCALES AND PULLEY

    B3-14
    Counterintuitive demonstration involving pulley system
    A frame containing the pulley and the lower scale hangs from the upper scale as photographed. The initial weight of the lower scale, pulley, and frame together is about 5 Newtons, as read on the upper scale; initially the lower scale reads zero. The difference in resultant force due to the pulley can be observed from the difference in the change of the two scales.
    FS2

     

  • C4-11: ACCELEROMETER - BALL IN WATER

    C4-11
    Demonstrate the direction of acceleration for both linear and circular cases.
    When the accelerometer is accelerated linearly, the ping-pong ball, being less dense than the water, moves toward the direction of the acceleration. When the jars are rotated the ping-pong ball moves toward the center of rotation.
    C4

  • C5-01: NEWTON'S THIRD LAW - STATIC DYNAMOMETERS

    C5-01
    Demonstrate action-reaction in the static case
    The dynamometer springs have been arranged so that the upper meter measures the force pulling down on it, while the lower meter measures the force pulling up on it, including its own weight. Pulling down on the hook below the lower meter results in a pair of equal and opposite forces acting on the coupling washer.
    FS2

    c5-01a

  • C5-02 SPRING AND PULLEY PARADOX

    C5-02
    Show that the action-reaction pairs have equal magnitude
    Initially, set this up with the horizontal spring scale facing away from your students. The mass on the hanger pulls down on the vertical spring scale with a force W equal to its weight. Challenge your students to predict what the other scale will read. After discussion, turn it to reveal: The spring scale reads the weight of the mass even thought it is horizontal between the pulleys
    FS2

  • C5-11: AIR TRACK - ACTION-REACTION PAIRS

    C5-11
    Demonstrate Newton's third law of motion
    Two gliders (either M and 2M or M and M) are tied together with a string loop against the force of a compressed spring. Burning the string releases the gliders with no external force. The photogate timer measures the time it takes for each glider tab to move through its respective gate. A reset switch on a cable clears the timer between measurements without the instructor getting in the line of sight.

  • C5-12: BALLISTIC CANNON CARTS

    C5-12
    Demonstrate Newton's third law of motion
    The carts, initially at rest, are placed at the center of a level track as photographed. The cannon fires its projectile into the receptacle on the other cart when a string is burned, releasing a spring. The mass of the projectile plus the mass of the receptacle cart equals the mass of the cannon cart, so if the carts are started in the middle of the track they will move away with equal but opposite velocities and reach their respective ends simultaneously.
    C5, OS0

    c5-12

  • C5-13 WATER ROCKET

    C5-13
    Demonstrate Newton's third law of motion
    Air is compressed in the rocket by means of the pump; when the air is released, the rocket rises by a small amount. If a small amount of water is poured into the air compartment from the squeeze bottle pictured at the right and air compressed in the rocket to the same pressure as before, the rocket will rise very high when released. Due to its greater mass, the water exhaust has more momentum than the air; thus more reaction force is applied to the rocket by the exhausting water.
    C5

  • C5-14 ROCKET TRIKE

    C5-14
    Demonstrate Newton's third law of motion

    Pressing the fire extinguisher handle expels carbon dioxide out a nozzle straight behind the tricycle, causing forward thrust of the tricycle. Be sure the exhaust is not oriented to hit the audience or anything else likely to be adversely affected but a sudden blast of cold air.
    Background
    This is a dramatic illustration of Newton's Third Law of Motion: the principle of action and reaction. The mass of gas being ejected out of the back of the tricycle at a very high velocity imparts an equal and opposite force to the tricycle, which thus moves forward. The tricycle is much more massive, so it does not move as quickly, but the acceleration is still very real - be careful not to run into the wall!
    FS1

  • C5-15: REACTION FORCE ON AIR BLOWER

    C5-15
    Demonstrate Newton's third law of motion
    A fan is mounted on a skateboard. The motor is turned on, blowing air out the back of the fan. The reaction force on the skateboard causes the board to move in the direction opposite to the exhausted air.

    Note that this device must operate unloaded; it cannot support a passenger.
    C5

  • C5-16: HERO'S ENGINE

    C5-16
    Demonstrate action and reaction in a rotational system.

    The boiler is partially filled with water and heated until steam is produced. The steam emerges from right-angle arms on the side of the boiler, causing the boiler to rotate in the direction opposite to that of the emerging steam.

    Danger:Do not tilt burner until it is warm.
    C5, I0

  • C5-17: ROCKET BOTTLE

    C5-17
    Illustrate the rocket principle in a dramatic way
    Pour about 100-200 ml of liquid nitrogen into the bottle and install the stopper. Exhausting nitrogen gas and liquid result in motion of the bottle. An untethered stopper is available for comparison.
    OS6, I0, F2