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#1 2007-09-27 07:27:11

Daniel123
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Registered: 2007-05-23
Posts: 663

Newton's Third Law

Could someone please fully explain Newton's Third Law please?

I'm really confused.

Thanks.

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#2 2007-09-27 08:43:58

JaneFairfax
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Registered: 2007-02-23
Posts: 6,868

Re: Newton's Third Law

When an object is resting on a surface, the weight of the object causes it to exert a force on the surface. The surface, in turn, exerts a force, called the (normal) reaction, on the object. The weight and the reaction are equal in magnitude, have the same line of action and are oppositely directed. That’s Newton’s third law in operation.

When you try and pull a heavy object, you will feel some resistance from the object; it is as if the object is trying to pull you in the opposite direction. The harder you pull, the greater is this resistive force on you. Indeed, the resistive force has the same magnitude and line of action as the force you apply on the object, and is oppositely directed to your pulling force. Again, that’s Newton’s third law.

Newton’s third law is often expressed in the following words: “To every action there is an equal an opposite reaction.” (“Action” here means “force”.) In other words, if X exerts a force on Y, Y will always exert another force on X (and the two forces are always equal in magnitude and opposite in direction, and have the same line of action). wink

Last edited by JaneFairfax (2007-09-27 08:48:59)

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#3 2007-09-27 08:50:18

John E. Franklin
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Registered: 2005-08-29
Posts: 3,588

Re: Newton's Third Law

Last edited by John E. Franklin (2007-09-27 09:13:17)


igloo myrtilles fourmis

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#4 2007-09-27 08:59:31

mathsyperson
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Registered: 2005-06-22
Posts: 4,900

Re: Newton's Third Law

I like this law because it sometimes has some silly consequences.

For example, if you drop a pen, then it falls because the Earth is pulling it, but it also contributes a little bit because it is pulling the Earth. tongue


Why did the vector cross the road?
It wanted to be normal.

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#5 2007-09-27 09:05:22

Daniel123
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Registered: 2007-05-23
Posts: 663

Re: Newton's Third Law

Ok I think I'm starting to understand it.

But if the object you are pulling is "pulling back" with an equal and opposite force, then how can we move things?

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#6 2007-09-27 09:28:19

John E. Franklin
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Registered: 2005-08-29
Posts: 3,588

Re: Newton's Third Law

Object A is pushed leftward.
Object B feels the force that it exerts on A.
So the two forces don't cancel because they are attached to
two different objects.
(At least in the case of pushing.)

Last edited by John E. Franklin (2007-09-27 09:34:26)


igloo myrtilles fourmis

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#7 2007-09-27 10:03:31

mathsyperson
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Registered: 2005-06-22
Posts: 4,900

Re: Newton's Third Law

Daniel123 wrote:

Ok I think I'm starting to understand it.

But if the object you are pulling is "pulling back" with an equal and opposite force, then how can we move things?

Because the object is pulling back on you. The object's force doesn't affect itself, and so it doesn't cancel the force moving it.

If the object is heavy, the reaction force might be enough to move you though.


Why did the vector cross the road?
It wanted to be normal.

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#8 2007-09-27 10:11:40

Daniel123
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Registered: 2007-05-23
Posts: 663

Re: Newton's Third Law

Ohhhh righttt!

Erm.. so why don't you move with lighter objects? what happens to balance the force that is pulling you?

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#9 2007-09-27 10:17:44

mathsyperson
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Registered: 2005-06-22
Posts: 4,900

Re: Newton's Third Law

They exert the same force on you as you do on them, but as f=ma and you're heavier, that means that they accelerate you less.

However, they do still accelerate you, so you should move a bit (albeit slowly).
This doesn't happen because as the object tries to push you, you then push the floor in the same direction. The floor then pushes you back and so the two forces on you cancel. The Earth does move a tiny little bit though.


Why did the vector cross the road?
It wanted to be normal.

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#10 2007-09-27 12:03:04

Identity
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Registered: 2007-04-18
Posts: 934

Re: Newton's Third Law

Several massive explosions could push earth out of its orbit (or so I heard)

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#11 2007-09-27 22:04:10

mathsyperson
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Registered: 2005-06-22
Posts: 4,900

Re: Newton's Third Law

The Earth's orbit is fairly stable though. Even if we did get pushed out of it, we'd most likely just end up in a similar orbit that's possibly a bit more elliptical.


Why did the vector cross the road?
It wanted to be normal.

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#12 2007-09-29 05:14:56

Daniel123
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Registered: 2007-05-23
Posts: 663

Re: Newton's Third Law

If everyone on one side of the world (the more populated side) jumped and hit the ground at the same time, would the earth be knocked out of its orbit?

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#13 2007-09-29 05:37:03

Daniel123
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Registered: 2007-05-23
Posts: 663

Re: Newton's Third Law

OK I think I understand Newton's Third Law.

1. If you push a heavy object, say with a force of 100N, then it will 'push you back' with a force of 100N, in the exactly opposite direction, along the same line of action. You will not move, as you then push the floor with the same 100N in the same direction as the force that is pushing you, in order to cancel out the force. The Earth, however, has nothing that it can push against in the way that you pushed the floor, so the earth will accelerate, but by a very very small amount, due to its huge mass. Although the object you are pushing is pushing you back with the same magnitude of force, it still has a resultant force and still moves. This is because the 100N it is pushing you with does not cancel the 100N you are pushing it with, as the two forces are acting on different bodies. Therefore, the object will have a resultant force (less than 100N due to friction from the floor) and will hence move.

The Third Law Pairs mentioned are:
- the push of person on the object, and the push of the object on the person
- the push of the person's foot on the floor and the push of the floor on the person's foot

2. If I am standing on the ground, and am exerting a force of say 600N on the ground (due to my weight), the earth exerts an equal and opposite reaction force on me. The pull of the earth on me will be opposed by the pull of me on the earth. So the push of me on earth and the pull of me on the earth will cancel (as the push of me on the earth is equal to the pull of the earth on me due to W=mg, and the pull of the earth on me is equal to pull of me on the earth due to Newton's Third Law), and i will stay still.  The push of the earth on me and the pull of the earth on me will also cancel, and the earth will stay still.

Third Law Pairs:
- push of me on earth and push of earth on me
- pull of me on earth and pull of earth on me

Am I right? Please tell me I'm right.

Last edited by Daniel123 (2007-09-29 05:48:48)

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#14 2007-09-29 10:43:46

JaneFairfax
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Registered: 2007-02-23
Posts: 6,868

Re: Newton's Third Law

In the second case, you stay still because the gravitation pull of the Earth on you – in other words, your weight – is exactly balanced by the normal reaction the ground exerts on you. There is no resultant external force on you.

However, if you push the ground with your legs hard enough, and the upward reaction of ground on you exceeds your downward weight, there will be a resultant upward force on you. As a result, you will move upwards: an action commonly known as jumping. tongue The harder you push against the ground, the greater ground exerts a reaction on you – and the higher you can jump.

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#15 2007-09-29 21:58:57

Daniel123
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Registered: 2007-05-23
Posts: 663

Re: Newton's Third Law

You are most helpful Jane smile

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#16 2007-09-29 23:01:05

Daniel123
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Registered: 2007-05-23
Posts: 663

Re: Newton's Third Law

In the second case, is the push of me on the earth balanced by the pull of me on the earth, or does the earth move?

If all action forces have equal and opposite reaction forcs, then wouldn't the action force of the pull of the earth on me be reacted by the pull of me on the earth?

Last edited by Daniel123 (2007-09-29 23:02:11)

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#17 2007-09-29 23:17:17

luca-deltodesco
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Registered: 2006-05-05
Posts: 1,470

Re: Newton's Third Law

the earth pulls you with the same force that you pull onto it, so in the grandscheme of things, everything is balanced, but looking at things seperately, both you and the earth are accelerating towards eachover.


The Beginning Of All Things To End.
The End Of All Things To Come.

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#18 2007-09-29 23:19:04

JaneFairfax
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Registered: 2007-02-23
Posts: 6,868

Re: Newton's Third Law

Yes, you do gravitationally exert a pull (equal in magnitude to your own weight) on the Earth. But your feet in contact with the ground is exerting a force at the same time the other way; the two forces cancel and hence the Earth doesn’t move.

I’m glad to be of help. smile

Last edited by JaneFairfax (2007-09-29 23:20:47)

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#19 2007-09-30 02:42:50

Daniel123
Member
Registered: 2007-05-23
Posts: 663

Re: Newton's Third Law

One last question on Newton's Third Law! I have to label a free body diagram of the earth, that is orbiting the sun. The only force I can think to put is the gravitational pull from the sun. Is that all that goes on there? If so why doesn't the earth move into the sun?

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#20 2007-09-30 02:48:26

Jai Ganesh
Administrator
Registered: 2005-06-28
Posts: 45,966

Re: Newton's Third Law

There are several forces acting, gravitational force of the moon and the other planets too. Just as electrons don't fall into the nucleus, earth doesn't fall into the sun. A combination of centripetal and centrifugal forces at work.


It appears to me that if one wants to make progress in mathematics, one should study the masters and not the pupils. - Niels Henrik Abel.

Nothing is better than reading and gaining more and more knowledge - Stephen William Hawking.

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#21 2007-09-30 03:20:47

Ricky
Moderator
Registered: 2005-12-04
Posts: 3,791

Re: Newton's Third Law

centrifugal_force.png


"In the real world, this would be a problem.  But in mathematics, we can just define a place where this problem doesn't exist.  So we'll go ahead and do that now..."

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#22 2007-09-30 07:26:33

luca-deltodesco
Member
Registered: 2006-05-05
Posts: 1,470

Re: Newton's Third Law

haha, nice cartoon, and yes there is no such thing as centrifugal force, on that cartoon, the actual force that would crush mr bond would be the normal contact force from the inner surface of the wheel as mr bond's body attempts to not move in a circle, this normal contact force supplying the centripetal force required to make mr bond move in a circle


The Beginning Of All Things To End.
The End Of All Things To Come.

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#23 2007-09-30 22:25:25

Identity
Member
Registered: 2007-04-18
Posts: 934

Re: Newton's Third Law

I still don't quite get it... how does the machine work?

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#24 2007-09-30 22:29:15

JaneFairfax
Member
Registered: 2007-02-23
Posts: 6,868

Re: Newton's Third Law

It simply spins poor Mr Bond round and round. lol

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#25 2007-10-02 10:22:11

OMG
Guest

Re: Newton's Third Law

ANd since your body cant bend into a circle backwards  ) face facing this way --> your spine/back will break

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