
#1
Sep1407, 03:23 PM

P: 15

These are true/false statements, except for the last question, which is multiple choice.
If a statement refers to “two bodies” interacting via some force, you are not to assume that these two bodies have the same mass. Every force has one and only one 3rd law pair force. false because you can't have one force by itself The two forces in each pair act in opposite directions. true The two forces in each pair can act on the same body or on different bodies true The two forces in each pair may have different physical origins (for instance, one of the forces could due to gravity, and its pair force could be a normal contact force). true The two forces of a 3rd law pair always act on different bodies. i thought this was false but i was told that this is true. can someone explain it to me please? Given that two bodies interact via some force, the accelerations of these two bodies have the same magnitude but opposite direction. (Assume no other forces act on either body.) false According to Newton's 3rd law, the force on the (smaller) moon due to the (larger) earth is greater in magnitude and antiparallel to the force on the earth due to the moon. greater in magnitude and parallel to the force on the earth due to the moon. equal in magnitude but antiparallel to the force on the earth due to the moon. equal in magnitude and parallel to the force on the earth due to the moon. smaller in magnitude and antiparallel to the force on the earth due to the moon. smaller in magnitude and parallel to the force on the earth due to the moon. again, i'm very confused with the third law.. 



#2
Sep1407, 03:46 PM

Sci Advisor
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P: 5,963

Yes, sadly, most of your answers are wrong. Here's Newton 3 in a nutshell, if a particle A exerts a force on particle B, then particle B exerts an equal but opposite force on particle A. So if I push against a wall with a force of 100 newtons to the right, then the wall pushes on me with a force of 100 newtons to the left. So give these questions another try, please, and report your results.




#3
Sep1407, 04:13 PM

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P: 1,664

Perhaps it will be helpful to picture the two objects as being connected by a single interaction, such as in the last question about the Earth and the Moon. The interaction involves a single type of force, and that force must act between a pair of objects. Thus the amount (or magnitude) of force acting on A and acting on B through this interaction will be the same. Since the forces are pictured as acting along a line connecting A and B, the direction of the force on A points in the opposite direction from the force on B.




#4
Sep1407, 08:56 PM

P: 15

Am I correct?
Ok, let me try again with the true/false questions first.
"Every force has one and only one 3rd law pair force" false because force always occurs in pairs "The two forces in each pair act in opposite directions" false, normal force may point perpendicularly while gravitational force may point downwards if a body's going down a slope "The two forces in each pair can act on the same body or on different bodies" the third law has to have two bodies involved, so it's true for different bodies but false for same body..? (this statement confuses me) "The two forces in each pair may have different physical origins (for instance, one of the forces could due to gravity, and its pair force could be a normal contact force)" true because, for example, normal force could pull a body perpendicularly from the surface while gravity pulls the body vertically downward. that's two right there. "The two forces of a 3rd law pair always act on different bodies" true because that's basically what the law states "Given that two bodies interact via some force, the accelerations of these two bodies have the same magnitude but opposite direction. (Assume no other forces act on either body.)" true, because if the forces have equal magnitudes and opposite directions, the net force is nonzero and therefore it accelerates. (i read this from my book, but i'm not sure whether my wording is correct) 



#5
Sep1407, 10:20 PM

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P: 1,664

For this question, they are only asking about one specific interaction and the forces involved in it. So you don't want to compare normal forces vs. gravitational forces. You're getting there: you've got two correct now... 



#6
Sep1407, 10:28 PM

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P: 4,125

The normal force is a reaction to the force a body exerts on the surface... Newton's third law says the two forces are equal in magnitude and opposite in direction. So it's a given that this is true. Well the pair force for a gravitational force is another gravitational force... I actually think the answer to this one is false... but I don't think that is a consequence of newton's 3rd law but more a consequences of the fundamental forces... I'm iffy about this one. 



#7
Sep1507, 12:25 AM

P: 105

Good work man, you're learning.
But, it seems to me that you have a greater concept to understand. That greater concept isn't just mechanics. It's that when you're taught something, it doesn't mean that it's correct. It could be correct in some instances, but it's not always. What I'm trying to say is don't believe everything you're taught, understand it, and if something doesn't seem right, go and research it. When I started learning Physics, I had a lot of arguments with seasoned physicists because I simply didn't see the whole picture. I knew what I'd been taught, and that it occurred here, but not that what I'd been taught was a simplification. Just about every formula in Physics is a simplification, or approximation. The "real" formula would be determining each atom's movement and reaction to a certain thing, rather than treating it as one object. What I'm trying to say is keep learning, and never believe something blindly. 



#8
Sep1507, 02:45 AM

P: 15

Thanks for all the help. I'm learning more from this one thread than from my textbook. x]
Ok, the last question! I don't see any reason how the size of the object can affect the magnitude, so the force acting upon the earth and the moon, or their magnitudes, are equal? And how can we tell if it is parallel or antiparallel? I say parallel because it's just the gravitational force acting upon the earth and the moon. 



#9
Sep1507, 02:57 AM

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#10
Sep1507, 03:02 AM

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P: 4,125

That last statement says "Given that two bodies interact via some force, the accelerations of these two bodies have the same magnitude but opposite direction. (Assume no other forces act on either body.)" If two bodies are experiencing a force of the same magnitude... but they have different masses... will they experience the same magnitude of acceleration? 



#11
Oct507, 04:03 PM

P: 3

answers: 1.True
2.True 3.False 4.False 5.True 6. False 7.equal in magnitude but antiparallel to the force on the earth due to the moon 


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