A "volunteer" removes glasses and jewelry and lies on a bed of 4000 nails. A male volunteer can take off his shirt so that his bare skin is against the nails. A plywood sheet is placed over him and several concrete blocks piled on top. The demonstrator then smashes the blocks with a sledge hammer.
Several physics principles are involved here. The force from any one nail is reduced by spreading the weight over many nails. The inertia of the blocks partially protects the person below from the force of impact. The smashing of the blocks absorbs much of the energy of the blow.
Using too few nails in this demo was a form of torture. In the illustration below Roman consul Marcus Atilius Regulus is tortured to death by Carthaginians in about 255 BC. The illustration was painted in about 1415 in Paris.
A PVC tube goes through the center of a wooden block, which is free to move along the tube after overcoming some amount of friction. As you hammer down on the tube while holding on to it with one hand the block moves up along the tube.
This is a dramatic demonstration of inertia. A metal hoop of spring steel is balanced on the mouth of a flask, and a piece of chalk is balanced on the hoop, directly over the mouth of the flask. The object is to snatch the hoop away so the chalk falls into the flask. Tell your audience that this is a difficult demonstration and you need a couple of practice shots first. In your "practice shots" grab the hoop by its leading edge so the top is compressed upward ejecting the chalk wildly into the air. After telling the audience you are now ready to try it for real, grab the hoop by its trailing edge so its top is pulled out from under the chalk, dropping the chalk neatly into the flask! (From The Physics Teacher, Feb. 1982).
Which way will the Moon go if the earth's gravity were suddenly switched off? The Pie Plate demo gives a good analogy. Spin a ball around the inside rim of the plate. The inward force of the rim keeps the ball in circular motion. But when the rim ends, the ball flies off in a straight line, obeying Newton’s First Law.
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A Scientific American article ("Intuitive Physics" by Michael McCloskey, Scientific American, April 1983) discusses how students, when asked which way the ball will go upon leaving the pie plate before having taken a course in physics, will usually answer that the ball will continue to curve around. Of course, right after taking physics the students (at least the ones that pass the course!) know that the ball moves off in a straight line. But a few years after the course, the ball starts to curve again! The students were enlightened; they knew the truth, but then they fell back into darkness! (The same article discusses several other misconceptions of motion you may wish to discuss with your students.) |