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PiP Apr 2014

  • 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

  • C2-22 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

  • C2-41: VECTOR ADDITION OF VELOCITIES

    C2-41
    Illustrate addition of two orthogonal velocity components
    A billiard ball is placed at the corner of the apparatus. When the two mallets are pulled back and released they strike the ball simultaneously, giving it two orthogonal velocity components. The relative amount of each component is determined by how high each mallet is raised. Use the floor tiles to define your coordinates and show that equal forces in two orthogonal directions produce motion at 45 degrees with respect to the axes.
    C2

  • 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

  • C4-33 FREE FALL IN VACUUM - FEATHER AND BALL

    C4-33
    Demonstrate that bodies that fall with unequal accelerations in air fall with the same acceleration in the absence of air.
    The ball falls faster than the feather with air in the tubes. When the air is pumped out, the ball and the feather fall with the same acceleration. The double tube assembly is rotated rapidly on its axis to initiate the free fall.
    FS1

  • 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

  • C5-17: 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

  • C7-18 COLLISIONS OF BALLS - ASTROBLASTER

    C7-18
    Shows velocity multiplication in colliding balls

    This device has four balls of graduated masses on a central shaft. The smallest has a slightly larger opening so that it can come off the shaft, while the others are trapped in place. If the whole assembly is dropped from 50cm or so about the table, the smallest ball on the end will fly off with considerable velocity, potentially rising to significantly greater than the initial height.

    Please be careful not to lose the small ball, and do not launch it into the audience or at anything else breakable.

    Engagement Suggestion
    • When presenting this device, describe it clearly, then encourage students to predict what will happen when you drop it.
    • Afterwards, have them discuss the results.
    Background

    The total energy of the system, of course, cannot increase beyond what it gains from the potential energy of the height from which it is dropped. But the elastic collisions of each ball with a successively smaller and less massive one transfer significant kinetic energy. With the smaller mass of the final ball, it can have a higher velocity than the collection as a whole did.

    C7

  • D1-53 LOOP-THE-LOOP

    D1-53
    Demonstrates centripetal force and conservation of energy in a rotating object

    This track can be described as three segments: the long upright segment, the loop, and the shorter upright segment. If you begin by placing the ball on the long upright segment at a height equal to the height of the loop (2R), the ball will roll down the track, begin to climb the loop, and then fall off and roll away. You can then repeat this at increasingly higher positions until the ball makes it all the way around the loop and begins to climb the shorter upright segment. In either case, be ready to catch it as it falls off afterwards!

    This is a good demonstration to encourage students to make predictions about its behaviour. Invite students to make arguments about what starting height will allow the ball to complete the full loop. A meter stick can be additionally provided upon request to aid in measuring the height.

    Background
    Motion of the ball down the track and around the loop-the-loop can be described in terms of gravitational potential energy, rotational and translational kinetic energy, and centripetal force. A ball of mass m and radius r must be released at some minimum height h above the bottom point of the track so that it will not leave the track while passing around the loop-the-loop.

    In order to stay on the track at the top of the loop the centrifugal reaction of the ball on the track must be equal to or greater than the gravitational force on the ball: mv^2/R = mg, or v^2 = gR, where v is its linear velocity at the top of the loop, R is the radius of the loop, and g is the acceleration of gravity. Conservation of energy dictates that at the top of the loop Iw^2/2 + mv^2/2 +2mgR = mgh, where the moment of inertia of the ball I = 2mr^2/5 and w is the angular velocity of the ball at the top of the loop.

    From these considerations we obtain the minimum starting height for the ball above the bottom of the loop-the-loop in order that the ball remain in contact with the track at all times: h = 2.7 R. In the case of an object sliding along a frictionless loop-the-loop, the height would be h = 2.5 R. Marks have been made at the points 2.5 R and 2.7 R. The ball remains in contact with the track at the top of the loop only when the height 2.7 R is reached, demonstrating the effect of the rotation of the rolling ball.
    FS2

  • D3-03 ROTATING CHAIR AND WEIGHTS

    D3-03
    Illustrates conservation of angular momentum

    A subject, holding the weights with their arms extended, is started into rotation. When the weights are pulled inward to the chest of the subject, the moment of inertia of the system is decreased, leading to significant increase in the angular speed of the rotating chair.

    Please take care when using this device, especially when accelerating. You can gain a significant increase in rotational speed, so hold on! And it is best not to have a person push the chair around very much, as it is very easy to hit them with a weight by accident.

    Engagement Suggestions
    • Consider inviting a participant from the class.
    • Encourage students to predict what will happen before performing the demonstration.
    • Once the demonstration has been performed, discuss the activity both in terms of angular momentum and its conservation, and in terms of kinetic energy.
    • For extended discussion, introduce the idea of friction. How does friction work in this system? How does it affect the angular momentum? Where does the kinetic energy go?
    Background
    This device illustrates the conservation of angular momentum. When the heavy weights are moved closer to or farther from the axis of rotation, the distribution of mass and thus the rotational inertia (or moment of inertia) changes.

    To show this in a different way, a single user with a single weight can move themself in a circle by swinging their arm wide holding the weight from front to back, then drawing it inwards before extending their arm forwards again and repeating the motion. This is essentially a rotational analogue of pumping a swing.
    FS0

  • K2-40: MAGNETIC ACCELERATOR

    K2-40
    Demonstrate magnetic potential energy
    A slightly curved track holds a series of steel balls. One ball, superficially similar to the others, is a magnet. To use: Fill the track with nonmagnetic balls, and release a single ball from one upper end of the track. It rolls down and across the track until is collides with a stationary line of balls. As expected, the last ball in the line moves out with slightly less speed than the incoming ball. Now repeat the demonstration, replacing the first stationary ball in the line with the magnetic ball and the result is quite different. The attractive force of the magnet adds energy to the system, in a quite dramatic fashion.

    A similar effect can be seen with C7-19: Gaussian Gun
    K2