Ballistocardiograph - Third Law

 

 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.