posted by Stuck on .
I'm learning about Newton's Laws of Motions and I'm having trouble understanding these questions:
"A rear-end collision between a soft-drink truck and a car occurs. The truck driver claims the car backed into him, while the auto driver claims that the truck hit him from behind. The only evidence is that quite a number of soft-drink bottles fell forward in the truck. From the evidence, can you tell who was at fault?"
I think it's a car's fault, because if the truck moved forward (and hit the car), then the bottles would have fell backwards...
"What is the principle behind a magicians ability to pull a tablecloth from beneath china and glassware without breaking them?"
I'm assuming this has to do with Newton's 1st Law, and how objects that aren't moving don't want to move.
"Explain why when you walk on a log floating in water, the log moves backward as you move forward."
This one I don't know how to explain.
#1 The truck was behind the car. If the bottles in the truck fell forward, it suddenly accelerated backwards. This would happen if the car backed into it or if the truck rear-ended the car. You can't tell which happened.
#2 Yes, Newton's first law is involved.
#3 There is negligible friction between log and water. The center of mass of the man-plus-log system stays in the same place. If the man moves in one direction, the log must move in the opposite direction to keep, the center of mass in the same place. (You could also invoke Newton's third law).