Newton's 2nd, 3rd laws (TBR physics question)

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Jun 1, 2010
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Hello all,

This is about The Berkeley Review Physics Chapter 2 Passage I Question 7. The question has a set-up with mass 1 sitting on a long table with a pulley at the end and mass 2 hanging off the edge of the table, the masses connected by a rope that goes over the pulley.

Question 7 asks "Which of the following statements is true about the normal force acting on M1 (mass 1)?"

The fourth statement is "The normal force is in the direction opposite of weight by Newton's Second Law."

When I read this statement, I thought it was false because it is Newton's THIRD Law that says that says if an object exerts a force on a second object, that object exerts and equal and opposite force on the first object...

But having thought about it, I guess Newton's Second Law F=ma is related too since the mass on the table is not accelerating vertically, so the normal force must equal the weight so as to keep the vertical acceleration zero.
Is that reasoning why the statement is correct??

So, would it be correct to say that the normal force is in the direction opposite of weight by both Newton's Second and Third Laws?


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10+ Year Member
May 16, 2010
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what does newton's 3rd law say. F 1 on 2 = F 2 on 1 note that these forces are acting on 2 DIFFERENT bodies. with the typical book example, the book is being "dragged" down by the force of gravity while the normal forces pushes up on the book so it out so it does not "fall" through the table in some way. So these 2 forces are actually BOTH acting on the book, therefore it is not a 3rd law problem.

the answer is 2nd law is because in order for this book to be in translational equilibrium. The normal force would have to exert F = ma opposite of the force exerted by gravity which is also F = ma.

when you see problems like these don't assume the force by the book is an intrinsic to the book, rather it is caused by gravity on the book.
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