Jump to content

# how does an object actually achieve motion?

## Recommended Posts

newton's 1st law states that acceleration can only be acheived if an external UNBALANCED force acts upon an object

newton's 3rd law states that forces ALWAYS occur in pairs

so how can there ever be an UNBALANCED FORCE?

• Replies 61
• Created
• Last Reply

#### Popular Posts

in newtons third law, the reaction force is not on the same object. so, in the overall syystem the forces are balanced but if you only look at the forces on the object of concern then the forces are unbalanced.

##### Share on other sites

Put another way, you actually can't move anything without moving something else in the opposite direction. If you look at each object individually, they each have an unbalanced force acting on them. For example, if you throw a rock in the air, it exerts the same downward force on you that you exert on it. But you are standing on the Earth, and so it's actually the whole Earth that moves "down" very very very slightly when the rock moves up. (Since the Earth is so much bigger than the rock, the same force that moved the rock only immeasurably moves the Earth.) Then, gravity pulls each back towards the other, again with equal forces on each, the rock falls to the ground, and the Earth is pulled very very very slightly "up" again.

Other examples: a rocket moves forward by throwing stuff (fuel) with tremendous force (generated by chemical explosion) backwards. A propellor or jet aircraft moves forward by sucking up air and throwing that backwards. A car or a walking human moves by pushing laterally against the Earth, therefore rotating it very slightly in order to move along the surface.

Edited by Sisyphus
##### Share on other sites

forces are always balanced but at least one of the fonces is proportional to the rate of change of motion.

##### Share on other sites
so how can there ever be an UNBALANCED FORCE?

Insane_alien and Sisyphus have it right: third law reactions always take place between different objects. Some primary/secondary teachers mistakenly teach that the earth pulling down on you (gravity) and the earth pushing up on you (normal force) as an example of Newton's third law. This is wrong for several reasons.

Third law reactions involve the same force acting on two different bodies in an equal-but-opposite manner. The Earth does pulls you down gravitationally; suppose for argument that this force has a magnitude of 160 pounds-force. The third law reaction: Your body is exerting a 160 pound gravitational force on the Earth. The Earth also does push up on you with a normal force. The third law reaction: Your feet exert an equal-but-opposite normal force on the surface of the Earth.

Another reason that this high school example is wrong is that the normal force is not equal but opposite to the gravitational force. If you are standing at 45 degrees latitude, the normal force is neither equal to (159.7 pounds force, not 160) nor opposite to (179.9 degrees apart, not 180). There are some third law reactions going on here, but the earth pulling on you and pushing on you is not a third law reaction pair.

forces are always balanced but at least one of the fonces is proportional to the rate of change of motion.

No. Look at the above example. Suppose a person located at 45 degrees latitude measures his weight with on an accurate spring scale to be 159.7 pounds. The gravitational force on that person is 160 pounds (force). The vector sum of the normal and gravitational forces is a 0.39 pound force directed about 45 degrees north of down (i.e., toward the Earth's axis of rotation). The resulting motion is circular rotation about the Earth's axis with a period of one revolution per sidereal day. Note well: Neither the gravitational force nor normal force has a magnitude of 0.39 pounds (force).

##### Share on other sites

the sum of all the forces is zero

##### Share on other sites
forces are always balanced but at least one of the fonces is proportional to the rate of change of motion.

Action force and reaction force act on different objects.

##### Share on other sites
the sum of all the forces is zero

The person standing on the surface of the Earth undergoes (nearly) uniform circular motion due to the rotation of the Earth. There is necessarily a net force directed toward the axis of rotation. For a person with a mass of 160 pounds (mass) located at 45 degrees latitude, this force is

$F=mr\omega^2 = 72.57\,\textrm{kg}\,\times\,\cos(45^\circ)\,\times\,6367\,\textrm{km}\,\times\, (2\pi/\textrm{sidereal day})^2 = 1.74\,\textrm{newtons} = 0.39\,\textrm{lbf}$

##### Share on other sites

hello guys!!

it's quite an interesting topic but to granpa I have a question:

what do you mean by the sum of all forces are equal?? In whivh case??

##### Share on other sites

Forget two of the laws, and concentrate on the physical law of $F=Ma$. The ability of moving object, like the earth to a stone, requires that force and acceleration are directly inverse, for a mass to move. A simple manipulation of $a=\frac{F}{M}$, shows us that the acceleration of an object, the ability to have a change in speed, is found with the force over the mass.

From a relativistic picture, energy is in fact more fundamental than the matter that is under a force of movement. In fact, because $M$ normally does not remain constant, because accepting relativistic speeds, we need to differentiate the equation $F=Ma$. But fundamentally speaking, a system to move, requires the energy content of the system, and THIS TIME, not to cause confusion, the energy spoke of here is the relativistic energy, rather than applying to our concepts rest energy.

##### Share on other sites
hello guys!!

it's quite an interesting topic but to granpa I have a question:

what do you mean by the sum of all forces are equal?? In whivh case??

That was, I believe, referring to a force and the reaction force given by Newton's third law. They are equal in magnitude and opposite in direction, so they add to zero. Since they act on different objects, saying they add to zero and thinking that applies to an object's motion is a misconception. An object's acceleration is dictated by the forces acting on it, not by the forces it exerts on other things.

##### Share on other sites

(And i will show the operation of differentiation, if one requires. It's very straight forward.)

That was, I believe, referring to a force and the reaction force given by Newton's third law. They are equal in magnitude and opposite in direction, so they add to zero. Since they act on different objects, saying they add to zero and thinking that applies to an object's motion is a misconception. An object's acceleration is dictated by the forces acting on it, not by the forces it exerts on other things.

I could be wrong, but i think this person means, or is thinking of is the net force.

##### Share on other sites
I could be wrong, but i think this person means, or is thinking of is the net force.

The original poster asked how there can be any non-zero net force acting on an object since, according to Newton's third law, forces always come in balanced pairs.

This question represents a fairly common misperception of Newton's third law. It certainly doesn't help that primary and secondary teachers do not understand Newton's third law. Teachers often portray the downward gravitational force exerted on a person by the Earth as a whole and upward normal force exerted on a person by the Earth's surface as an example of Newton's third law.

It would be nice if the educational systems required primary and secondary teachers to know something about the subject matter they are teaching. Lacking that, the way to overcome this misperception is to properly describe Newton's third law as being the same force mechanism acting on two different bodies. The above example, gravity and normal forces forces acting on a person, obviously are not third law pairs because (a) these two forces are acting on the same object, and (b) these two forces arise from very different mechanisms.

Tom, you have to gauge your help for the audience. When someone has a misunderstanding of some very basic concepts of physics, delving into more advanced concepts such as energy or relativity or quantum mechanics or Lie algebras is not offering help.

##### Share on other sites

I apologize. I must have misunderstood the meaning of the posters intellect of the subejcts involved.

##### Share on other sites
• 4 weeks later...

newton's 1st law states that acceleration can only be acheived if an external UNBALANCED force acts upon an object

newton's 3rd law states that forces ALWAYS occur in pairs

so how can there ever be an UNBALANCED FORCE?

let me disagree with everyone, passionately!

OP seem to be closest to answering own question, which is actually a good question and all the answers here were not so good, until now...

this is simply about "resistance", that's it. so, it means this:

- if "resisting force" is less than applied force object will accelerate (unbalanced), otherwise "resisting force" will be equal in magnitude and opposite in direction.

Edited by PlayStationX
##### Share on other sites
let me disagree with everyone, passionately!

OP seem to be closest to answering own question, which is actually a good question and all the answers here were not so good, until now...

this is simply about "resistance", that's it. so, it means this:

- if "resisting force" is less than applied force object will accelerate (unbalanced), otherwise "resisting force" will be equal in magnitude and opposite in direction.

What is the "resisting force" before the applied force is applied? Why doesn't the object move in response to this?

The only "resisting force" I can think of is friction, and we can discuss forces without worrying about friction. There is no inherent "resisting force," and it is not the reaction force of Newton's 3rd law, so this is not a good answer to the OP.

##### Share on other sites

What is the "resisting force" before the applied force is applied? Why doesn't the object move in response to this?

The only "resisting force" I can think of is friction, and we can discuss forces without worrying about friction. There is no inherent "resisting force," and it is not the reaction force of Newton's 3rd law, so this is not a good answer to the OP.

at first i meant to be humorous because it seemed this silly "explanation" kind of worked, but i did not really think it makes any sense, i was just blabbering... however, after thinking about it, well now, i think it makes perfect sense!

"resisting force" is like inertia, only better! ...since it also works with 3rd law, 2-in-1.

it can also be a friction and drag... or maybe not, not sure, if they fit description count them in

##### Share on other sites

"Resisting force" is always equal and opposite in direction. You push a rock, it pushes you back with the same force, and you both move.

##### Share on other sites
"Resisting force" is always equal and opposite in direction. You push a rock, it pushes you back with the same force, and you both move.

But in that sense, they act on different objects. The motion of an object is the result of forces acting ON it, as has already been discussed.

##### Share on other sites

"Resisting force" is always equal and opposite in direction. You push a rock, it pushes you back with the same force, and you both move.

But in that sense, they act on different objects. The motion of an object is the result of forces acting ON it, as has already been discussed.

i do not think rock pushes you back equally, in fact the difference in how much you push it and it pushes you will be the net force deciding initial acceleration. it is not "active" force, its not real force, it is "passive". it does not act, it does not push - it resists.

the definition of "resistance/inertia" in its meaning almost implies "opposite direction", and it most certainly implies passivity. in this sense, i think it can be understood as both "acting" on same object, while only one is actually 'acting'.

anyway,

-"To every action there is always opposed an equal reaction.", is false. the correct statement is more like:

if you cant push it, then it resists your applied force - obviously with the same amount of resistance(force), but NEVER MORE. however, if you managed to push it, then you just won over "resisting force" - it was unable to balance your force and it got accelerated in the direction you applied the force.

Explanation of Newton's third law by NASA employee:

-"...if thrust is greater than the drag, plane will accelerate."

http://en.wikipedia.org/wiki/Newton_third_law

actually, NASA does not seem to get it either, what they talk about is hardly -"To every action there is always opposed an equal reaction." - they do talk about OPPOSITE direction, but they never say it is ALWAYS equal. in fact, for acceleration to happen there has to be UNBALANCE, which is 1st law. what im trying to say is that 1st and 3rd law really talk about the same thing, but from slightly different perspective.

to sum it all up, 1st, 2nd and 3rd law should be put in one, like this:

- only force accelerates and unless opposed by equal and opposite force, it always does so. F= m*a

this practically means that if you see something has changed its velocity or direction, there is some force acting there. but that doesn't mean there ain't some forces if it is not moving or accelerating, it just means forces are at balance when we observe uniform motion.

Edited by PlayStationX
##### Share on other sites

The correct thing to say is not, "I think Newton's Third Law is wrong." You should say, "I do not understand Newton's Third Law." The law itself is not up for debate.

Yes, the rock pushes back on you with as much force as you push on it. Your confusion is in mixing up what the forces are acting on. You are acting on the rock, and the rock is acting on you. Each of you individually therefore has an unbalanced force acting on you, since the equal and opposite forces are acting on different objects.

Thrust and drag are not the equal and opposite forces. Again, your confusion seems to arise from thinking the opposite paired forces have to act on the same object. They don't, and if they did, then nothing could move. The thrust pushes the air backwards with a certain amount of force, thereby pushing the plane forwards with an equal force. That is thrust. As the plane moves through the air, it has to push the air in front of it out of the way. The air then pushes back on it with an equal force. That is drag. There are four forces in total, in two opposite and equal pairs. One force from each pair is acting on the plane itself.

##### Share on other sites

Explanation of Newton's third law by NASA employee:

-"...if thrust is greater than the drag, plane will accelerate."

http://en.wikipedia.org/wiki/Newton_third_law

That quote is not found in that link. Where does it come from? I don't trust ellipses.

Merged post follows:

Consecutive posts merged

anyway,

-"To every action there is always opposed an equal reaction.", is false. the correct statement is more like:

if you cant push it, then it resists your applied force - obviously with the same amount of resistance(force), but NEVER MORE. however, if you managed to push it, then you just won over "resisting force" - it was unable to balance your force and it got accelerated in the direction you applied the force.

Case 1 is about action/reaction force pairs, and this does not deal with unbalanced forces causing an item to move.

Case 2: If the object is free to move, then any force at all will cause an acceleration, proportional to the mass. The limiting case is zero acceleration for zero force, ergo, there is no such thing as a resisting force. You appear to be assuming that friction is a natural circumstance rather than an external force which must be accounted for separately.

##### Share on other sites

The correct thing to say is not, "I think Newton's Third Law is wrong." You should say, "I do not understand Newton's Third Law." The law itself is not up for debate.

correct thing for you is to not embarrass yourself. im sorry that you were unable to understand, i do not debate laws, but wording and meaning. to avoid misinterpreting me please quote what are you referring to.

Yes, the rock pushes back on you with as much force as you push on it. Your confusion is in mixing up what the forces are acting on. You are acting on the rock, and the rock is acting on you. Each of you individually therefore has an unbalanced force acting on you, since the equal and opposite forces are acting on different objects.

your sentence does not make any sense, id say you are confused.

"Yes, the rock pushes back on you with as much force as you push on it. ",

- only if you are talking about some other rock that does not move when you push it. if it moves, then you obviously pushed it stronger: F(you)=m*a - F(stone)=m*a; if there was any movement, you can see from the formula that resisting force had to be less than applied force for it to happen.

3rd law forgets to mention one important thing - it works only for the system in equilibrium, the one without any acceleration. forces do not act in pairs, that is very silly and literal understanding, which comes as no surprise since it was formulated like that.

but i can explain it all to you, are you ready to learn?

Thrust and drag are not the equal and opposite forces.

what are you blabbering about? who said that? i have no idea what are arguing about.

this is my statement:

- only force accelerates and unless opposed by equal force, it always does so. F= m*a

if you want to show my confusion, then please argument it based on my statement. can you show that it does not work? can you show that it does not contain full information of all three laws?

what say you?

Merged post follows:

Consecutive posts merged

Explanation of Newton's third law by NASA employee:

-"...if thrust is greater than the drag, plane will accelerate."

That quote is not found in that link. Where does it come from? I don't trust ellipses.

are you saying you disagree with it?

Case 1 is about action/reaction force pairs, and this does not deal with unbalanced forces causing an item to move.

Case 2: If the object is free to move, then any force at all will cause an acceleration, proportional to the mass. The limiting case is zero acceleration for zero force, ergo, there is no such thing as a resisting force. You appear to be assuming that friction is a natural circumstance rather than an external force which must be accounted for separately.

of course there is such thing as "resisting force".

-"Inertia is the resistance of an object to a change in its state of motion."

http://en.wikipedia.org/wiki/Inertia

anyway, im not sure what are you talking about. i thought you have objection to my rephrasing the laws?

all im saying this can substitute all three law, it has the same meaning and applicability, but i suppose not everyone can see it...

- only force accelerates and unless opposed by equal force, it always does so. F= m*a

sure, i may be wrong, but then please point it out, where is the false logic? is there something that does not equal to truth?

Edited by PlayStationX
Consecutive post/s merged.
##### Share on other sites
The correct thing to say is not, "I think Newton's Third Law is wrong." You should say, "I do not understand Newton's Third Law." The law itself is not up for debate./QUOTE']correct thing for you is to not embarrass yourself.

PlayStationX, you are saying Newton's Third Law is wrong, and in doing so you are the one embarrassing yourself.

Yes, the rock pushes back on you with as much force as you push on it. Your confusion is in mixing up what the forces are acting on. You are acting on the rock, and the rock is acting on you.

your sentence does not make any sense, id say you are confused.

"Yes, the rock pushes back on you with as much force as you push on it. ",

- only if you are talking about some other rock that does not move when you push it. if it moves, then you obviously pushed it stronger: F(you)=m*a - F(stone)=m*a; if there was any movement, you can see from the formula that resisting force had to be less than applied force for it to happen.

Suppose you push on a rock with some force. The rock will push back on you with a force that is equal in magnitude but opposite in direction to the force you exert on the rock. Whether or not the rock moves is irrelevant.

3rd law forgets to mention one important thing - it works only for the system in equilibrium, the one without any acceleration.

This is completely wrong.

forces do not act in pairs, that is very silly and literal understanding, which comes as no surprise since it was formulated like that.

You have a very basic and very common misunderstanding of Newton's Third Law. Third law force pairs always pertain to a pair of objects. Forces always come in pairs.

but i can explain it all to you, are you ready to learn?

Now you are sounding very much like a crackpot.

You need to drop your condescending attitude, your arrogance, your misunderstandings, and your crackpot notions.

Edited by Sayonara³
Quote tags fixed
##### Share on other sites

PlayStationX, you are saying Newton's Third Law is wrong, and in doing so you are the one embarrassing yourself.

no, you're failing to understand, sorry. im saying your interpretation is wrong.

can you give me some equation that illustrates 3rd law?

Suppose you push on a rock with some force. The rock will push back on you with a force that is equal in magnitude but opposite in direction to the force you exert on the rock. Whether or not the rock moves is irrelevant.

i actually agree, but the way it is understood is wrong. the way you illustrate it does not reveal what is this all about. when we come to that equation which illustrate this law i hope to be more clear about it.

You have a very basic and very common misunderstanding of Newton's Third Law. Third law force pairs always pertain to a pair of objects. Forces always come in pairs.

you have very simplistic way of thinking, sometimes referred to as blindness.

force ALWAYS comes in pairs?

what in the world? can you document that statement?

this is just ridiculous,

- the reason we have two forces here is because the problem is set up that way, sheesh!! it is about INTERACTION of TWO objects. if it was about interaction of three objects, then perhaps you would say forces ALWAYS come in triplets?

do you know at all what is it we are talking about here? give me equation that illustrates that law?

Now you are sounding very much like a crackpot.

don't hate me just because i'm cool

Edited by PlayStationX
Consecutive post/s merged.

## Create an account or sign in to comment

You need to be a member in order to leave a comment

## Create an account

Sign up for a new account in our community. It's easy!

Register a new account

## Sign in

Already have an account? Sign in here.

Sign In Now

×

• #### Activity

• Leaderboard
×
• Create New...

## Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue.