70. Partial Pie Plate

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.


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.)