- #1
Wormaldson
- 21
- 0
So I was reading through my textbook (specifically, Physics for Scientists and Engineers, Eighth Edition, Volume 1 by Raymond A. Serway and John W. Jewett Jr.) and I noticed that, in one of the "Pitfall Prevention" sections (which are usually quite helpful - not this time, evidently), it says "Newton's first law does not say what happens for an object with zero net force, that is, multiple forces that cancel; it says what happens in the absence of external forces."
The example given to illustrate Newton's first law was a hockey puck floating on an air hockey table. This confused me, as isn't the hockey puck in static equilibrium in the vertical direction because the force exerted upon it by the air streams from the table "cancels" with the force due to gravity?
Seeking clarity, I looked to almighty Wikipedia for guidance, and was puzzled further when it stated that Newton's first law is applicable in the case where the object in question has zero net force acting upon it ("This law states that if the net force (the vector sum of all forces acting on an object) is zero, then the velocity of the object is constant.")
Naturally I found this puzzling. Then, I though, "well, perhaps the textbook means to say that Newton's first law is only applicable in the absence of external forces in a certain direction. This made sense in the context of the hockey puck example - obviously, there wouldn't be any reason to discuss the vertical motion of an object that is assumed to only be moving horizontally, right?" But then that lead to another issue: wouldn't that mean that the statement from the Wikipedia article is wrong? Or, at least, an insufficient explanation? And shouldn't this kind of thing be clarified in the textbook anyway? So I'm a bit confused here. Any help would be much appreciated.
P.S. As a not-so-serious aside, isn't the term "textbook" a bit redundant? I mean, what else would one be expecting to find in a university-level physics book?
The example given to illustrate Newton's first law was a hockey puck floating on an air hockey table. This confused me, as isn't the hockey puck in static equilibrium in the vertical direction because the force exerted upon it by the air streams from the table "cancels" with the force due to gravity?
Seeking clarity, I looked to almighty Wikipedia for guidance, and was puzzled further when it stated that Newton's first law is applicable in the case where the object in question has zero net force acting upon it ("This law states that if the net force (the vector sum of all forces acting on an object) is zero, then the velocity of the object is constant.")
Naturally I found this puzzling. Then, I though, "well, perhaps the textbook means to say that Newton's first law is only applicable in the absence of external forces in a certain direction. This made sense in the context of the hockey puck example - obviously, there wouldn't be any reason to discuss the vertical motion of an object that is assumed to only be moving horizontally, right?" But then that lead to another issue: wouldn't that mean that the statement from the Wikipedia article is wrong? Or, at least, an insufficient explanation? And shouldn't this kind of thing be clarified in the textbook anyway? So I'm a bit confused here. Any help would be much appreciated.
P.S. As a not-so-serious aside, isn't the term "textbook" a bit redundant? I mean, what else would one be expecting to find in a university-level physics book?