# TBR - what can and can't Newton's 2nd law explain?

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#### UIUCstudent

##### Full Member
10+ Year Member
So we got a generic pulley problem like this:

http://www.screencast.com/users/trinhn812/folders/Jing/media/9fe7c5ed-2d5d-4a1e-be22-ca3bbd2b78cc

There is kinetic friction on box A. Pulley is massless.

Which of the following is true:
I. The magnitude of the normal force equals the weight of M1 by Newton's 3rd law
II.The normal force is proportional to the tension in the rope by Newton's 3rd law
III. The normal force is related to friction, by Newton's 2nd law
IV, The normal force is in the direction opposite of weight by Newton's 2nd law

Answer choices only included 2 of the above and I got this right based on that. But I think question really boggled me.

I. True. I agree. basic definition
II. False. BR says the whole thing is untrue, but I think that if the statement said it was according to Newton's 2nd law, it would be true. F(kinetic)=uN=T if acceleration is zero. So shouldn't normal force should be proportional to tension according the 2nd law?
III. False. Hmm Friction = uN so why isn't it related.
IV. True. This seems a bit of a stretched and BR's explanation isn't that strong. But if they stretched it this much, III should be true..

So we got a generic pulley problem like this:

http://www.screencast.com/users/trinhn812/folders/Jing/media/9fe7c5ed-2d5d-4a1e-be22-ca3bbd2b78cc

There is kinetic friction on box A. Pulley is massless.

Which of the following is true:
I. The magnitude of the normal force equals the weight of M1 by Newton's 3rd law
II.The normal force is proportional to the tension in the rope by Newton's 3rd law
III. The normal force is related to friction, by Newton's 2nd law
IV, The normal force is in the direction opposite of weight by Newton's 2nd law

Answer choices only included 2 of the above and I got this right based on that. But I think question really boggled me.

I. True. I agree. basic definition
II. False. BR says the whole thing is untrue, but I think that if the statement said it was according to Newton's 2nd law, it would be true. F(kinetic)=uN=T if acceleration is zero. So shouldn't normal force should be proportional to tension according the 2nd law?
III. False. Hmm Friction = uN so why isn't it related.
IV. True. This seems a bit of a stretched and BR's explanation isn't that strong. But if they stretched it this much, III should be true..

II is false, because normal force is not proportional to tension in the rope. Normal force is simply equal and opposite to weight in this problem. If you multiplied mass of A by 10, then the normal force will also be multiplied by 10, but you won't have any change in tension. Kinetic friction has nothing to do with this. I don't see why you would set kinetic friction equal to tension either.

III is false because Newton's second law has nothing to do with Friction = &#956;N. Newton's second law relates net force to mass and acceleration.

IV isn't a stretch. Vertical acceleration is 0, so the net force vertically must be 0, by Newton's second law. If weight pulls down on the box, the normal force must push up.

So we got a generic pulley problem like this:

http://www.screencast.com/users/trinhn812/folders/Jing/media/9fe7c5ed-2d5d-4a1e-be22-ca3bbd2b78cc

There is kinetic friction on box A. Pulley is massless.

Which of the following is true:
I. The magnitude of the normal force equals the weight of M1 by Newton's 3rd law
II.The normal force is proportional to the tension in the rope by Newton's 3rd law
III. The normal force is related to friction, by Newton's 2nd law
IV, The normal force is in the direction opposite of weight by Newton's 2nd law

Answer choices only included 2 of the above and I got this right based on that. But I think question really boggled me.

I. True. I agree. basic definition
II. False. BR says the whole thing is untrue, but I think that if the statement said it was according to Newton's 2nd law, it would be true. F(kinetic)=uN=T if acceleration is zero. So shouldn't normal force should be proportional to tension according the 2nd law?
III. False. Hmm Friction = uN so why isn't it related.
IV. True. This seems a bit of a stretched and BR's explanation isn't that strong. But if they stretched it this much, III should be true..

I could kick myself over this, but I loaned out my older version of the BR physics books, so I don't have access to the exact question. But the fact that this question doesn't appear in the current book says it all.

I recall not liking a couple of the Newton law questions in this homework set, so I wouldn't worry about it too much. I think it's a case where technically it's correct if you look at it from the author's stretched perspective, but I don't think it's a good question.

The key point to get from this is that although you can calculate the magnitude of the kinetic friction force from the normal force, they are perpendicular to one another and their correlation is not by a Newton's law. If the object were at rest, kinetic friction is 0, but there is still a normal force on the object.

Is wind resistance proportional to buoyant force, because an object floats higher as bouyant force goes up, so it's more exposed to the wind?