Newton III q?

[Guest noinipo]

in Newton 3, what does it mean by equal and opposite force? i mean, the force I exert on the earth is far less than what the earth exerts on me;so how can they be equal?

[swansont]

in Newton 3, what does it mean by equal and opposite force? i mean, the force I exert on the earth is far less than what the earth exerts on me;so how can they be equal?

 

They are the same magnitude.

[Johnny5]

Swanson answered you correctly.

 

A lot of times, physicists use the word 'force' when what they really mean is "magnitude of force." I am as guilty of this as the next fella. But you can always clear things up.

 

Force is a vector quantity, not a scalar quantity, but I am not aware of any special name for "magnitude of force."

 

So in Newton's third law you have two things interacting. Each exerts a force on the other, during the interaction.

 

Let

[math] \vec F_{12} [/math]

 

denote the force that object one exerts on object two.

 

Let

[math] \vec F_{21} [/math]

 

denote the force that object two exerts on object one.

 

Here is Newton's third law in mathematical form:

 

[math] \vec F_{12} + \vec F_{21} = 0 [/math]

 

From here, you are one mathematical step away from this:

 

[math] \vec F_{12} = - \vec F_{21} [/math]

 

The minus sign tells you that the forces have antiparallel directions.

 

Since the directions are along the same line of action, you can divide both sides of the equation above by the direction, to obtain a vector independent statement, which is this:

 

[math] | \vec F_{12} | = | \vec F_{21} | [/math]

 

This states that the magnitude of the forces are equal.

 

Regards

[J.C.MacSwell]

in Newton 3, what does it mean by equal and opposite force? i mean, the force I exert on the earth is far less than what the earth exerts on me;[/b']so how can they be equal?

 

This is not correct.

[Guest noinipo]

They are the same magnitude.

so is the force i exert on the earth negligible, because of the earth's size/mass?

[swansont]

so is the force i exert on the earth negligible, because of the earth's size/mass?

 

Well, the force is the magnitude of your weight. The acceleration you would cause, if you jumped, would be negligible.

[Flareon]

This reminds me of a query on Straight Dope:

 

"If all one billion Chinese people jumped up at once, would the earth wobble?"

[swansont]

This reminds me of a query on Straight Dope:

 

"If all one billion Chinese people jumped up at once' date=' would the earth wobble?"[/quote']

 

Of course the answer is no. But it turns out that if everyone got in their cars or on their bikes and drove west (or east) then you could get a measurable change in the earth's rotation rate.

[Johnny5]

Of course the answer is no. But it turns out that if everyone got in their cars or on their bikes and drove west (or east) then you could get a measurable change in the earth's rotation rate.

 

 

Certainly.

[sciencedummy]

in Newton 3, what does it mean by equal and opposite force? i mean, the force I exert on the earth is far less than what the earth exerts on me;so how can they be equal?

 

you exert force on the earth right, but then there is a normal force that is equal to the force that you exert on the earth.

 

it has to be equal or else you would've bounced into space or you would've sunk into the earth right?

 

i hope this helps... im not sure if im making any sense here...

[swansont]

you exert force on the earth right' date=' but then there is a normal force that is equal to the force that you exert on the earth.

 

it has to be equal or else you would've bounced into space or you would've sunk into the earth right?

 

i hope this helps... im not sure if im making any sense here...[/quote']

 

But you can bounce up or sink in. If you jump up, the normal force exerted by/on you will be greater than the gravitational force, which allows you to accelerate. If you are not accelerating, then those two forces are equal by virtue of Newton's first and second laws: if F=0, then a=0. But action-reaction forces are always equal.

 

If the action force is gravity, the normal force is not the reaction force of Newton's third law.