# Newton's 3rd law and two forces

1. Sep 14, 2007

### Vivee=)

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..

Last edited: Sep 14, 2007
2. Sep 14, 2007

### PhanthomJay

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. Sep 14, 2007

### dynamicsolo

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. Sep 14, 2007

### Vivee=)

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. Sep 14, 2007

### dynamicsolo

I think the statement is asking whether one force (in an interaction) is accompanied by just one or some number other than one force, as described by the Third Law. To me, this statement is poorly written. In any case, in an interaction between A and B, if we are talking about the force of, say, A on B, many texts nowadays refer to the force of B on A as the "(third-law) pair force". So consider your choice in that light...

You may need to look again at what the definition of "normal force" is. Specifically, it is a *contact* force acting between two objects; yes, it is treated as acting perpendicularly to the surfaces touching, but it is the "touching" they are concerned with.

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.

If the statement means what I think it means, you're correct here. The "pair forces" can't both be acting on a single object.

There is the same issue here as with your answer to the second statement. The forces on each object in the pair arise from a single interaction, so only one type of force is acting there.

Yes. In fact, this statement is just a variation on the same idea in the third statement.

Careful here: the two *forces* in the interaction pair will have equal magnitudes. But how do you calculate *accelerations* of the two objects?

You're getting there: you've got two correct now...

6. Sep 14, 2007

### learningphysics

Yes, but isn't the statement also saying that same thing... every force has one and only one 3rd law pair force... ie forces occur in pairs...

Hmmm... normal force is not the pair force to the gravitational force... when the earth exerts a gravitational force on you... you exert a gravitational force on the earth.

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.

Yes, you're right... the two forces can't act on the same body.

This is a badly written question... what exactly do they mean by physical origin...

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.

Yes.

But if the forces are equal and opposite, but the masses are different, how are the accelerations?

7. Sep 15, 2007

### BlackWyvern

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. Sep 15, 2007

### Vivee=)

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. Sep 15, 2007

### dynamicsolo

Neither the difference in their individual sizes nor masses matter. The Earth and Moon are linked by the same gravitational interaction, so the strength of that interaction (the magnitude of the force on each of them) has a single value. [That is why, by the way, Newton's Universal Law of Gravitation, F = GMm / (r^2), contains the product Mm -- the order of the product doesn't matter, which is equivalent to saying it doesn't matter which mass, M or m, we call the Earth's or the Moon's.]

Anti-parallel means the direction of the *two* (vector) forces lie along the same line, but point in opposite directions. The "third-law pair forces" will always point anti-parallel. As an example, the gravitational interaction between the Earth and Moon attract them to one another. So the force of the Moon on the Earth points from the Earth toward the Moon, while the force of the Earth on the Moon points from the Moon toward the Earth.

10. Sep 15, 2007

### learningphysics

That last question is about accelerations.

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. Oct 5, 2007