A spring gun, clamped to the lecture table, fires a steel ball into a pendulum bob, which traps it. The pendulum bob swings away and is held at the highest position reached by a ratchet. The laws of conservation of energy and momentum for the inelastic collision of the steel ball with the pendulum bob are used to calculate the velocity of the ball as fired by the gun from the data of the height of rise of the bob and the mass of the ball and the bob.
The pendulum bob is then moved out of the way so the gun can fire across the lecture hall. Tne laws of ballistics are used to calculate the spot on the floor the ball will hit, and a metal can is placed on the predicted spot to catch the ball.
A toy LGB train has a flat car with a vertically mounted gun, actuated by a flashlight. When the train is at rest, the projectile ball is shot vertically upward and falls back down into the gun muzzle. If the train is moving along a straight section of track, the ball is still caught by the gun, even though the trajectory of the ball is now a parabola. The train can even be sent through a tunnel so the ball is fired up before the flat car enters the tunnel, passes over the tunnel, and is caught as the flat car exists the tunnel.
If the gun is fired as the train is rounding a curved section of track, the ball will not be caught, illustrating the restriction of special relativity to inertial frames.
This is a fairly elaborate demonstration, and several people are required to set it up, the set-up time often running five minutes or so into the beginning of class. Please give extra notice for your planned use.
Instructions
Do not run above 18 V. Train will wreck!
(There is probably a 2 V margin up to 20 V, but a wreck causes serious damage.)
The switch on the battery car turns on the system. Switch off after use to conserve batteries. (The switch on the gun car should be left on.)
llustrates that the vertically downward acceleration is independent of the horizontal velocity. See: Horizontal and Vertical Ball Drop (Gravitational Acceleration) [1].
A monkey hanging from the branch of a tree in Africa is spotted by a small game hunter. But this is no ordinary monkey; this monkey knows some physics. He sees that the barrel of the gun is pointed directly at him and reasons that if he lets go of the branch at the right instant, the bullet will pass over his head. But he knows that the sound from the gun will not reach him much before the bullet, and maybe even after, so he decides to watch for the light flash from the gun, knowing that the light reaches him almost instantly, and to let go the instant he sees the flash. Is his reasoning correct?
Modern technology gives us a 25mm, laser guided, anti simian cannon. The monkey and hunter demonstration is on a wheeled table with the elevation of the gun controlled by a crank. The gun is fired by compressed air from a pressure cooker tank. A laser beams through the gun barrel so all can see where the straight line of the gun-aim is directed.
Besides a monkey target, we also have a zombie target, so if monkey hunter is not politically correct, you can call this, shoot the zombie instead. You can see a video test firing with the zombie here [2]. A picture of the zombie with the laser spot is shown below.
The illustration below is available on a viewgraph for overhead projection to the class.
Zombie head with laser spot.
Mouse over the animation below to see how the monkey hunter works at two different pressures.
A ping-pong ball is placed inside the PVC tube and both ends are sealed with pieces of Mylar after which the tube is evacuated to ~ 10 Torr. When the Mylar piece near the ball end is punctured, the ball accelerates due to the expanding air behind it, leaving the tube at speed close to 300 m/s.
The ping pong ball can tear through two aluminum cans.
It is interesting to note that the Mylar piece used as a barrier at the exit end becomes detached before the ball reaches it. The following photograph is taken from G. Olson, R. Peterson, B. Pulford, M. Seaberg, K. Stein, R. Weber, The Role of Shock Waves in Expansion Tube Accelerators, Am. J. Phys, 74 (12), December 2006, p. 1071-1076.
The explanation requires consideration of nonlinear gas dynamics and shock behavior. Compression waves traveling ahead of the ball quickly develop into a shock wave that reflects off the exit end of the tube and in turn off the ball several times. With each reflection there is a localized pressure and temperature increase so that by the third or fourth one a heated pressure pulse builds up at the exit end and is enough to remove the barrier piece.
Ballistic Motion is studied with a water stream that continually shows the parabolic trajectory of particles in a gravitational field. Measuring rulers hang down near the stream so that you can show, for example, that the stream falls a distance 1/2 gt� below the straight line it would have followed had there been no gravity. You can vary the angle of projection to show the 45° angle of greatest range, equal ranges at equal angles about 45°, etc. This demostration has audience appeal as the water pressure varies, and the instructor struggles to make the measurements while occasionally getting spritzed.
Links:
[1] https://demoweb.physics.ucla.edu/node/386
[2] https://www.youtube.com/watch?v=92PnUomscGU