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Applications of Newton's Laws

  • C6-13: FRICTION BETWEEN GLASS SURFACES

    C6-13
    Demonstrate the increase of friction between two glass surfaces when lubricating surface films are removed.
    Initially, the drinking glass can be pulled across the glass plate easily and quietly. If water with detergent is applied to the plate, the greasy surface films are removed and the glass-to-glass contact is improved. As a result, friction is increased and loud squeaking noises are produced as the drinking glass is pulled across the plate.
    C6

  • C6-14: SOCIAL CLIMBER

    C6-14
    Illustrate frictional forces in a weird way.
    The wooden figure has holes in its arms which are drilled at an angle with respect to the vertical. Strings are passed through the holes to the ends of the wooden bar at the top. When the strings are pulled alternately (like milking a cow) while being kept reasonably taut, the frictional force is greater for the string that is at the greater angle. The string at the lesser angle slips. The alternating frictional force causes the social climber to slowly "climb" up the strings to the top.
    FS2

  • C6-15: SOCIAL DESCENDER - FRICTION TOY

    C6-15
    Illustrate frictional forces in a weird way.
    As the bird springs up and down, friction with the post changes and it slips down the post.

    c6-15a

  • C6-16: SLIDING AND STATIC FRICTION

    C6-16
    Show that the friction force depends linearly on the normal component of the applied force.
    1. Put 5-kg mass on left pan of balance and the blue block on the right pan. Add to the right pan 1 kg plus additional masses from the weight set to achieve balance.
    2. Place block on lecture table with the additional right-pan weights on top of it. (Together they have a mass of 5 kg.)
    3. Determine the force of friction by pulling the box with its weights across the lecture table using twine and the spring scale.
    Place additional 5 kg mass on block and repeat. Repeat with block on edge. Typically about 13 Newtons is required to pull the 5 kg combination. This gives a coefficient of friction mu=13/(5X9.8)=0.26, which is roughly what is expected for wood on wood. The difference between static an sliding friction is readily visible.
    C6, ME1

  • C6-17: FRICTION VS. PULLING ANGLE

    C6-17
    Demonstrate how pulling angle affects the frictional force

    A wooden board of weight w, lying on a sandpaper surface, is pulled at an angle a by a string connected to a spring scale. The force F required to move the board is given by: F = w / [ (1/u)cosa + sina]. Differentiating F with respect to a, the minimum force is seen to occur at the angle of repose, u=tana. Pulling horizontally is definitely not the most efficient angle!

    Pull horizontally to determine u, then check the angle of repose by tilting the sandpaper surface. Then pull at a variety of angles to demonstrate that pulling at the angle of repose requires the least force.
    C6, ME1

  • C6-21: SUSPENDED ROD ON WATER

    C6-21
    An elementary problem involving horizontal friction.
    The top end of a rod is supported by a string connected to a fixed point, with the bottom end floating freely on a small piece of styrofoam on the surface of the water. Which of the three drawings, depicted above, best portrays the equilibrium position of the system: (a), (b), or (c)?
    C6

    c6-21

  • D1-51 BANKED CURVE MODEL

    D1-51
    Aid in explaining banked turns
    The model of the curved road is banked such that at the suggested maximum rate of speed the horizontal component of the normal force provides the centripetal force required to keep the car moving in its circular path, independent of the friction of the car wheels with the road.
    D1

  • D1-52: FAIRGROUND ROTOR

    D1-52
    Illustrate the application of rotational forces
    Place the little people onto their platform against the wall of the rotor and start the rotor in motion. A trip switch (just to the left of the peole in the close-up photo) removes the floor, but the people remain pinned against the wall due to centrifugal reaction, and do not fall due to friction with the wall of the rotor. When the rotor slows down, the frictional force of the people against the wall becomes less than component of the gravitational force down the wall and the people fall off the rotor. The entire assembly may be tilted while the rotor is in motion without causing the people to fall off.
    OS1

  • D1-82: ROLLING FRICTION

    D1-82
    Show the direction of the frictional force when a rolling object is accelerated.
    One of the three cylindrically symmetric rollers is positioned on the base as shown in the photograph. The base has a rubber top surface and rolls almost without friction on small bearings on the horizontal surface below. The cloth web is held horizontally and yanked, causing the roller to move in the direction of the applied force. The frictional reaction force on the cart is indicated by the direction the cart will move when the roller is yanked off. For a solid disc (moment of inertia less than mr^2) the reaction force is in the direction opposite to the pull, and the base will move backward. For a spool rolling on its smaller radius r (moment of inertia greater than mr^2) the reaction force is in the direction of the applied force. For a thin ring (moment of inertia equal to mr^2) there is no reaction force on the base and it will remain motionless. Because the cylindrical shell has a finite thickness there is some reaction force and a slight amount of motion occurs. However, compared with the solid cylinder and the spool the reaction force is minimal.

  • D1-84: SPINNING CYLINDRICAL SHELL

    D1-84
    A counterintuitive demonstration of rotational dynamics.
    A six-inch diameter thin-walled aluminum tube with O-rings tightly wound around each end lies on a long plastic sheet. The plastic sheet is then quickly pulled horizontally out from under the tube. Q: What will the tube do after the plastic sheet has been removed? (a) move to the left, (b) move to the right, or (c) stop and remain where it was when it left the plastic sheet. A: The aluminum tube moves to the left and spins clockwise as the plastic sheet is removed, then very quickly ceases both its spinning motion and its translation after the plastic sheet is gone! One explanation for this uses a conservation of angular momentum argument. As it is yanked out from under the aluminum tube, the plastic sheet applies no net torque to the tube around the point of contact with the plastic sheet, because the distance between the sheet and the tube is zero. Two components of angular momentum around the contact point can be identified: that due to the clockwise rotation of the tube, and that due to the linear velocity of the tube to the left. The net angular momentum of the tube around the point of contact of the tube with the plastic sheet, however, is zero just before the tube leaves the plastic sheet. Likewise, because the plastic sheet is very thin, there is no net angular momentum around the point of contact of the tube with the table just after the tube leaves the plastic sheet. Sliding friction of the o-rings on the tube with the table causes both the rotation and the translation to quickly cease.
    D1

  • D2-21: CENTER OF PERCUSSION - BAT AND MALLET

    D2-21
    Demonstrate the center of percussion using a baseball bat.

    The bat is held at the small end and struck soundly with the mallet at the bigger end. A yellow marker marks the location of the center of percussion. When the bat is struck below the center of percussion it will spin out of the holder's fingers moving in the opposite direction to that of the incoming mallet. When the bat is struck above the center of percussion it will spin out of the holder's fingers moving the same direction as the incoming mallet. When the bat is struck at the center of percussion it will rotate about the holder's fingers, but will not spin out of the fingers.

    This illustrates how a ball player wants to hit the baseball to get the greatest momentum transfer to the ball with the least reaction force on the batter's hands and arms. A tennnis stroke works the same way. You can minimize "tennis elbow" by hitting the ball at the center of percussion of the racket so that the rotational reaction on your wrist and elbow is minimized.

    Click your mouse here to see the collision of a baseball and a softball with composite bats, taken by a slow motion camera.

    Click your mouse here to see the vibrational modes of a baseball bat.

  • D4-21: SHIP STABILIZER

    D4-21
    Demonstrate how a gyroscope can stabilize the rocking of a ship.
    Frictional torque on the precessing gyroscope stabilizes the rocking frame. The two ends of this frame represent the two transverse bulkheads of the ship. The gyro mounted between these two bulkheads is suspended vertically on a pivot which permits the gyro to swing fore and aft in the ship. The friction in the pivot can be adjusted to give the proper amount of damping against a rolling motion. When undamped and started to rocking, the boat will rock 7-10 times. When properly damped it will rock 1-2 times. This models how such technology is used to stabilize large watercraft.
    FS1

  • D4-22: MONORAIL CAR

    D4-22
    Demonstrate gyroscopic stability
    The gyroscope on the car is driven to a high rotational speed using a motor. The car will then remain balanced on the wire for over thirty seconds.
    D4, OS0

  • D5-21: BALL ROLLING ON ROTATING DISC

    D5-21
    Show that a sphere rolling on a rotating disc will move in circles.
    The ball can be placed at the center of the rotating disc and tapped to initiate the motion or it can be loosely trapped with your fingers at about half the radius of the disc, and then released. The frequency of the circular motion does not depend on either the radius or the mass of the ball, or on the radius of the orbit, but only on the frequency of the rotating disc.
    OS12