#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).
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.
2 answers
I know this is a late response - yes indeed it really is - but for #1 the auto car would have had to rear-end the soft-drink truck because Newton's third law tells us how "for every action, there is an equal, but opposite reaction". So if the truck exerted a forward force on the car then the car would exert an equal (in magnitude) but opposite force back to the truck; however, this would make the bottles fall backwards, therefore vice versa that and the backwards force from the car would result in a forwards force on the truck and the bottles. For #2 the writer above me is correct (Newton's first law is involved), the simple explanation being that when the tablecloth is pulled, it exerts an applied force to the items on the table, the thing is, those items have inertia that prevents the tablecloth (which is a separate object) from accelerating/forcing them off. And lastly, #3, this question is simply about the frame of reference, the person relative to the log would move forward relative to the log, but while walking you exert a backwards force on the log which causes it to travel the opposite direction with respect to the person. Newton's third law is invoked much like the commenter above said since the backwards force exerted from the person's feet to the log as they walk would have an equal but opposite force causing the person to accelerate forwards. So if those are the only two forces they would equally gain distance from each other with respect to the reference/starting point.