A mass (the swing) hangs from a rope that passes over a pulley and is connected to the support post. A second shorter rope hangs freely from the horizontal section of the main rope.

Start the pendulum mass oscillating with a small amplitude. When the pendulum gets to its lowest position, pull gently down on the shorter rope, shortening the pendulum and thereby increasing its velocity. Release the rope as the pendulum nears its high point.

According to a possibly oversimplified analysis, conservation of angular momentum at the low point, before and after the pull is applied, explains why this procedure causes the amplitude of the swing to increase with time. See also discussion of parametric resonance.

When water is squirted out of a sprinkler, the sprinkler head rotates in the direction opposite to the water flow (called the normal direction) due to the reaction force on the head. This is like a Hero's engine, the rotational equivalent of a rocket. Q: If water is sucked into the sprinkler head (actually pushed by a larger ambient pressure in the water vessel) will the sprinkler head (a) rotate in the normal direction, (b) rotate in the inverse direction, or (c) remain motionless? A: When water is sucked into the sprinkler head, the sprinkler head will rotate in the direction opposite the water flow, or inverse, direction.

The cylindrical vessel with four nozzles pictured above floats in a larger container of water. When pressure is applied by squeezing the bulb, water is forced into the nozzles. During the transient time, while the water speed is increasing in the nozzles, a reaction force on the nozzles causes the sprinkler to begin rotating slowly in the inverse direction, so as to conserve angular momentum (at zero) with the water going into the nozzle. While the water flow is at a steady state the sprinkler head rotates with a constant angular speed. When the flow ceases, the sprinkler head stops and remains motionless. When the bulb is released and water flows out of the nozzles, the sprinkler undergoes large continuous acceleration in the forward, or normal, direction.

When water is squirted out of a sprinkler, the sprinkler head rotates in the direction opposite to the water flow (called the normal direction) due to the reaction force on the head. This is like a Hero's engine, the rotational equivalent of a rocket. Q: If water is sucked into the sprinkler head (actually pushed by a larger ambient pressure in the water vessel) will the sprinkler head (a) rotate in the normal direction, (b) rotate in the inverse direction, or (c) remain motionless? A: When water is sucked into the sprinkler head, the sprinkler head will rotate in the direction opposite the water flow, or inverse, direction.

Filling the tank, tube, and bucket with water, the system then functions as a siphon. Raising the bucket above the tank, the water flows out of the sprinkler, creating the normal sprinkler mode. In the normal sprinkler mode, the sprinkler head experiences a reaction force like a rocket and rotates in the "normal" direction. Its angular speed increases until it is limited by the viscosity of the water bath, and the rotation continues for a while after the water flow ceases. Lowering the bucket below the tank, the water flows into the sprinkler, creating the inverse sprinkler mode. In the inverse sprinkler mode, the sprinkler head starts into rotation in the inverse direction, opposite to the direction of the water flow, during the transient period while the water flow is starting. The angular speed of the head remains constant while the water flows at a constant rate, then ceases immediately when the water flow ceases.

A problem with analyzing this in terms of conservation of angular momentum involves the following observation: While water is flowing into the nozzle, reach in and stop the nozzle with your hand. Then release it. If conservation of angular momentum is the important concept, then the nozzle should remain at rest after it is released. However, it starts to rotate at the same angular speed it had attained before being stopped. Go figure it out and let us know what is happening.

The Edgerton Laboratory at MIT has an interesting hallway version of an idea related to the inverse sprinkler. Lamentably, it uses a complicated model that has too much friction and is probably too rigid, and therefore draws an incorrect conclusion.

The original paper on this demonstration is: Richard E. Berg and Michael R. Collier, The Feynman inverse sprinkler problem: A demonstration and quantitative analysis, AJP 57, 654-657, (1989). A video, Inverse Sprinkler Models, by Berg and Collier, produced at the University of Maryland to accompany the paper, can be viewed using the link below. The video demonstrates the experimental features of three different models of inverse sprinkler systems.

The article Alejandro Jenkins: An elementary treatment of the reverse sprinkler, presents more recent data and is probably more complete in its analysis than the earlier article by Berg and Collier using the apparatus in this demonstration and listed in the references linked below (Richard E. Berg and Michael R. Collier, The Feynman inverse sprinkler problem: A demonstration and quantitative analysis, AJP 57, 654-657, (1989)).

http://edgerton.mit.edu/feynman-sprinkler

http://www.youtube.com/watch?v=pkKwSEseJXk

The speed of the rotating puck increases as the radius of the path decreases. Angular momentum is not conserved because the force is not central.

A string about one meter long has a (relatively heavy) set of keys on one end and a (very light) match box on the other end. The string passes over a pencil with the keys hanging down and the matchbox held horizontal to the pencil with about two/thirds of the string between the pencil and the matchbox.

Q: What will happen when the match box is released?

A: Surprisingly, the keys will not fall to the floor. When the matchbox falls it develops angular momentum. Conservation of angular momentum of the matchbox causes it to rotate very rapidly about the pencil as the string pulls it in. Before the string is used up, the matchbox string actually wraps around the pencil, preventing the keys from falling onto the floor!

A rope is tied to a tennis ball at one end, and to a brick at the other. The rope runs loosely through a plastic handle, allowing it to slide freely.

Spin the ball on its rope around in a circle above your head. As the radius of rotation increases, the centripetal acceleration does as well, increasing the force acting on the brick. At a certain point, the force will be sufficient to lift the brick off the table!

Check out further discussion of this demonstration, and a full mathematical treatment, at the link below!

Show how linear and angular momentum are transferred between a small puck and the rectangular bar when the puck strikes the bar at various points along its length and/or at varying angles. Inelastic collisions can also be demonstrated using a puck with a velcro collar.

Spin the wheel, then hang it from the end of the rope with the rope vertical and the wheel axle in a horizontal orientation. It will precess with the rope remaining vertical.

Click the link below to see a video of the action.

A variety of effects can be illustrated with this device, including precession, nutation, effect of forces, etc.

Note: Be gentle with this device, especially the metal ball, as it can be easily damaged. Place cushion under the ball before moving the device. Set the compressor on the floor on a rubber pad to isolate its vibrations.