Conservation of Linear Momentum
Various sets of dynamics carts are available: all of them are about 20 cm long and 500 grams in weight, with low-friction ball-bearing wheels. In one set the carts have strong horse-shoe magnets attached on top of them. All the others have built-in spring plungers. The carts can be held together by a string loop or hook and loop fasteners. Burning the string loop or activating the trigger button launches each cart in the opposite direction. Different weights can be added to the carts to observe different reactions.
Two large carts can be used to show conservation of momentum. Two volunteers stand on the carts facing each other. When one pushes on the other's hands, the velocities will be inversely proportional to their weights -- choose volunteers with very different weights.
Sitting on a skateboard, a volunteer holds a fire extinguisher and fires up. The fire extinguisher works as an engine moving the cart. It is also a good demonstration of Newton's Third Law.
This demo has five super balls of different masses staked on a rod. Drop it from a height of one meter and the top ball may rebound as high as 10 meters!
The ball at either end is swung at the other balls. Upon impact its momentum is carried through all the others balls up to the first one, which swings out with the original velocity. If two balls are swung, two will swing at the other end and so forth. This illustrates conservation of momentum and kinetic energy as well at Newton's Third Law.
This demonstration consists of two balls identical in shape and color. The "dead ball" has a very small repulsive modulus while the "live ball" rebounds very well. Drop both balls from the same height -- the live ball bounces very high but the other one barely gets off the floor.