A person lies down on a flat board set on rollers. A laser beam is directed at a tiny mirror positioned on one of the rollers. The laser beam is projected onto the ceiling or wall. The beating of the person's heart causes a slight movement in the body as indicated by the laser. This upward movement of the body is due to the 3rd Law reaction force of the blood being pumped to the lower body. The left ventricle of the heart squeezes blood upward into the aorta shown below. At the peak of the contraction, about 80 grams of blood is moving upward at 30 cm/s. The aorta does a U-turn forcing most of the blood to flow down to the lower body. The aorta and body force the blood down and in turn the body is forced up. The amount is too small to be seen by eye but can be seen when "amplified" by the laser-mirror arrangement used in the demonstration. It can also be seen when standing quietly on a weight scale if the scale is sensitive enough and the vibration is not damped by the scale mechanism. Your weight decreases slightly when the blood slams into the top of the aorta.
A video of the movement of the body as detected by the laser-mirror setup as well as a graph tracing the motion are provided below.
A 1 kg mass weighs 9.8 N as checked by a spring scale. Two 1 kg masses are hooked on the ends of a string which is passed over a pair of pulleys, and the spring scale is placed in the center of the string to measure the tension as shown. The scale is covered with a cloth to hide its reading, and the class is then asked to predict whether the scale will read 0, 9.8, or 19.6 N.
Water Rocket — A toy rocket is loaded with water and compressed air and shot across the room. It can also be fired with air only (no water in it) to show the difference.
C02 rocket — A rod mounted on the end of a shaft is free to turn about the shaft's axis. At the end of the rod is a C02 cartridge. When the cartridge is punctured, the escaping gasses propel the rod with considerable speed.
Hero's engine — spins by the reaction force of escaping steam.
The dynamics track cart has a sail and a battery operated propeller; both are removable. If the propeller is removed and held so it blows against the sail, the cart will roll to the left. If the sail is removed and the propeller mounted alone, the cart will accelerate to the right. If both the sail and the propeller are mounted, the instructor can ask the students which way the cart will roll.
From S.R. Smith and J.D. Wilson, Phys. Teacher, Apr. 72, pp 208.
Note: The above demos may also be demonstrated with the air track at the instructor's request. The air track has an advantage over the dynamics track in that there is less friction associated with it. However, the air track has a draw-back in that it is much nosier than the dynamics track and in a lecture setting it is difficult for the instructor to be heard.