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.)
EmoticonEmoticon