Newton's 3rd Law

[dstebbins]

Newton's 3rd law of motion states that for every force, there is a force equal in magnitude and opposite in direction. For example, when a five pound book lays on a desk, it exerts five pounds downward on the desk. At the same time, the desk pushes upward on the book five pounds.

 

But what about when the net force is not zero? For example, a free-falling object in a vacuum. Where is the equal and opposite force there?

[Klaynos]

Urmmm dissreguard what was here before hand, I am too tired....

 

If there is 0 Net force then all the forces added together would each contribute to the oposite and equal force of of the others.

[drochaid]

WHEN I POST A TOPIC, RESPOND ONE AT A TIME! I CAN'T TALK TO EVERYONE AT ONCE! WHAT DO YOU THINK I AM, A COMPUTER?

I was going for utter prick rather than computer tbh.

 

Sanitise your sig. Muppet.

[Cap'n Refsmmat]

Ordinarily I'd beat drochaid over the head for that comment, but it makes the point so well...

 

The advantage of forums is that you can respond to posts whenever you want. This isn't a chatroom or, God forbid, an actual meeting of people, so you're not obligated to respond to everybody. Please remove your obnoxious signature.

[dstebbins]

Ordinarily I'd beat drochaid over the head for that comment, but it makes the point so well...

 

The advantage of forums is that you can respond to posts whenever you want. This isn't a chatroom or, God forbid, an actual meeting of people, so you're not obligated to respond to everybody. Please remove your obnoxious signature.

 

it's not just that. When twenty people respond, each with their own way of explaining my inquiry, it's a really hard time to figure out what the similarities are between them and who's right with the differences.

[Bignose]

But what about when the net force is not zero? For example, a free-falling object in a vacuum. Where is the equal and opposite force there?

 

If the only force on this object is gravity (since it is in a vacuum, I assume that you wanted to neglect all drag), you have to remember gravity is a force that pulls on all objects to the center of mass. If we assume that the only two obejcts in existance are this object and the earth, yes, the earth's gravity pulls on the object, but the object's gravity also pulls on the earth. It is just that the force of gravity, which is small, does not move a large mass like the earth very far at all. Like 10^-20 or 10^-30 m.

 

But, the force of gravity is equal in both directions.

 

and p.s. your signature at the moment is very obnoxious.

[dstebbins]

If the only force on this object is gravity (since it is in a vacuum, I assume that you wanted to neglect all drag), you have to remember gravity is a force that pulls on all objects to the center of mass. If we assume that the only two obejcts in existance are this object and the earth, yes, the earth's gravity pulls on the object, but the object's gravity also pulls on the earth. It is just that the force of gravity, which is small, does not move a large mass like the earth very far at all. Like 10^-20 or 10^-30 m.

 

But, the force of gravity is equal in both directions.

 

and p.s. your signature at the moment is very obnoxious.

 

but that doesn't make sense to me, and I'll explain why.

 

I understand that the object is pulling on the earth, but by that logic, five pounds in one direction and five pounds in another direction should cancel each other out, and there should be a net force of zero, meaning no acceleration, but the object does accelerate at 9.8m/s/s, so the net force isn't zero.

 

I don't know how to explain my confusion any more clearly. And btw, how exactly should I phrase what my sig is saying and still get my point accross?

[Cap'n Refsmmat]

It's not that it's phrased poorly, it's that it takes up half the screen.

[Klaynos]

An object falling towards earth exets a force of m*9.81 (although the 9.81 does change as you get further away but lets stick with this for now) BUT it also pulls the earth towards it with a force equall to this so

 

m_object9.81 =m_eartha_earth

 

But as the earth is SO much bigger than most things that you come into contact with you don't notice this acceleration.

[swansont]

The acceleration of a body depends on the forces exerted on that body. The reaction force acts on a different object, i.e. it is exerted by the body.

 

So a body that feels a force F on it will accelerate, even thought is exerts a force -F. F_on = -F_by is Newton's third law, but F_by should never show up in the free body diagram.

[Sisyphus]

Maybe it would be good to list a few examples.

 

A ball falls via gravity, and the Earth is pulled towards it with the same force.

 

A rocket flies through space, throwing spent fuel out the back with the same force from which the rocket as a whole accelerates.

 

I swing at a baseball, and during the time of contact, the ball exerts the same force on the bat as I exert on the ball.

 

A car drives down the road, pushing itself forward with the same force that it pushes the entire Earth backwards underneath it.

 

The wind moves a turbine, and is slowed by as much force as is needed to turn the blades.

 

I push a boulder across a field, and the boulder and myself are propelled forwards with the same force that the Earth is propelled backwards.

 

I try to push the same boulder across a frictionless field, but, since I can't exert any sideways force on the Earth, I can only push the boulder forwards if I move backwards.

[johnreed]

Newton's third law. Equal and opposite force. This is based on Descartes earlier work on impacting inertial mass objects. Where we define force in terms of impacting inertial mass objects, an equal and opposite quantitative but nonetheless subjective inertial mass generated force ensues. Impacting inertial mass objects is a special case of the "interaction" between inertial mass objects. The balance scale is another special case of this interaction. Since we define the resistance we work against and call gravity, in terms of our inertial mass, when we lift an inertial mass object, this also qualifies as an interaction between inertial mass objects, which we qualify as and where an equal and opposite quantitative inertial mass force also ensues. This equal and opposite action consists of the resistance we work against in lifting the inertial mass object which is precisely equal and opposite to the product of the inertial mass of the object and the planet attractor action (g). Which planet attractor action is independent of the object's inertial mass. Yet we have assigned this equal and opposite inertial mass interaction property as an action between inertial mass objects and celestial objects, which celestial object, the planet qualifies as. Where inertial mass is independent of the planet attractor action. And equal and opposite applies solely to the quantitative interaction reflecting the resistance we work against and accelerated inertial mass objects, again which we qualify as.

johnreed

[Sisyphus]

I was just about to respond to this thread when I noticed that I already had, 9 months ago. Bumping a long-dead thread to the top bumps every living thread above it one spot farther down, in a kind of "equal but opposite" reaction.

[SteveW]

If the object is free falling in a vaccum then the earth applies gravitational force against the object. The object also applies gravitational force against the earth to pull the earth towards it. These forces are equal and oppositely directed. Since the earth is so much more massive, it resists the force applied to it by the object and effectively doesn't even move where the motion of the object free falling is significant.