# Why do forces make an object move?

by sameeralord
Tags: forces, object
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P: 40,261
 Quote by sameeralord How is a normal reaction force generated. I thought it was due to momentum, the box pushes on the table, the table pushes on the box, like a momentum collision.
Nothing's moving, so where does momentum come in?
P: 615
 Quote by Doc Al Nothing's moving, so where does momentum come in?
The box is trying to move but each time it loses energy to the table. Ok then if this is not right, how is normal reaction force generated. Also Russ said earlier that movement is a by product of energy, I mean how can you be certain it is not otherway round, meaning no clear cut answer.
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P: 21,655
 Quote by sameeralord How is a normal reaction force generated. I thought it was due to momentum, the box pushes on the table, the table pushes on the box, like a momentum collision.
A force can be generated due to change in momentum (a=f/m=dp/dt where p=mv), but if there is no change in momentum, then the force isn't generated that way. In a box sitting on a table, the force is generated in a different way: by gravity.
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P: 21,655
 Quote by sameeralord The box is trying to move but each time it loses energy to the table.
No. Again, you need to stop thinking in terms of the words and start thinking in terms of what the math says those words mean. Without the math, the words have no meanings. Math is the language of physics.
 Ok then if this is not right, how is normal reaction force generated.
In the case of a book on a table, the pair of forces is created by gravity.
 Also Russ said earlier that movement is a by product of energy, I mean how can you be certain it is not otherway round, meaning no clear cut answer.
No, you said [implied] movement is a biproduct of (caused by) energy. I corrected you and said you have it backwards, that energy is a biproduct of movement. And how do I know it isn't the other way around? Again, it's the math. You must look at and think in terms of the math!

w=fd
e=.5mv^2

What do these equations say about work/energy?
 P: 615 Ok first of all are most newton's third law action reaction pairs due to momentum collisions. Then why is normal reaction not an action reaction pair. Ok then if it gravity. Since gravity is the attraction between 2 objects. How is gravity creating a normal reaction force? Ok I apoligize for the mistake in the previous, that was unintentional, however how do you know from maths it is the other way round. If a force provides energy for the object, why is energy bi product of movement. How can you be certain like that?
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P: 40,261
 Quote by sameeralord Ok first of all are most newton's third law action reaction pairs due to momentum collisions.
No. You don't need a collision to have an action-reaction pair.
 Then why is normal reaction not an action reaction pair.
The normal force is part of an action-reaction pair, just like all contact forces are.
 P: 615 Then how is normal reaction force created by gravity Doc Al. Also if gravity is attraction between two objects, why am I not attracted to objects near me beside the earth. All this time I thought change in momentum and newton's third law is the same thing. Is it not in the normal reaction force case, is their no change in momentum?
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P: 40,261
 Quote by sameeralord Then how is normal reaction force created by gravity Doc Al.
Imagine a book on a table. Gravity pulls it down. The table exerts an upward force on the book to cancel the pull of gravity to prevent the book from falling through the table. Newton's 3rd law tells us that the book must exert an equal and opposite force on the table.
 Also if gravity is attraction between two objects, why am I not attracted to objects near me beside the earth.
You are! But those objects have tiny mass compared to the earth, so the force is small.
P: 615
 Quote by Doc Al Imagine a book on a table. Gravity pulls it down. The table exerts an upward force on the book to cancel the pull of gravity to prevent the book from falling through the table. Newton's 3rd law tells us that the book must exert an equal and opposite force on the table. You are! But those objects have tiny mass compared to the earth, so the force is small.
Ok thanks for the reply Ok now how is the opposite reaction force exactly equal to gravity, how come it is not greater or smaller. Now all this time I though opposite reaction occurs due to change in momentum. So there is no change in momentum in this situation. Then why does every action has an opposite reaction, if it is not because of change of momentum I can't understand why every reaction has an opposite reaction.
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P: 21,655
 Quote by sameeralord ... how do you know from maths it is the other way round. If a force provides energy for the object, why is energy bi product of movement. How can you be certain like that?
because force doesn't always provide energy. When you know that force MIGHT or might not cause motion....well....you use the word "cause".
A static force pair is f=f
A force pair resulting in motion is f=ma

Also if you follow a scenario over time you can see force being constant but energy increasing.
P: 615
 Quote by russ_watters because force doesn't always provide energy. When you know that force MIGHT or might not cause motion....well....you use the word "cause". A static force pair is f=f A force pair resulting in motion is f=ma Also if you follow a scenario over time you can see force being constant but energy increasing.
Oh I think you are correct. As you rightly said I think maybe this is why I didn't understand the question about net forces. When a stationary box was pushed equally in opposite reactions. The forces there cancelled out, inhibiting movement but I was thinking they were still providing energy and energy was getting cancelled or something else happening to it. Thanks for correcting me However I still have the question why newton's third law occur, is it because of change of momentum or not.
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P: 40,261
 Quote by sameeralord Ok now how is the opposite reaction force exactly equal to gravity, how come it is not greater or smaller.
Depending upon the situation, the normal force could be greater or smaller than the weight of the object. In this case, the object is not accelerating, so we know the normal force must equal the weight.
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P: 40,261
 Quote by sameeralord However I still have the question why newton's third law occur, is it because of change of momentum or not.
I don't know what you mean by "change of momentum" in this context. You push on a wall; the wall pushes back on you. That's an example of Newton's 3rd law. Where do you see any change in momentum?
P: 615
 Quote by Doc Al I don't know what you mean by "change of momentum" in this context. You push on a wall; the wall pushes back on you. That's an example of Newton's 3rd law. Where do you see any change in momentum?
You push on the wall, you lose energy to the wall and hence slow down(which can also be considered as the wall acting on you). If that is not the explanation, why does every reaction have an equal and opposite reaction? Is their any form of energy transfer occuring here.
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P: 40,261
 Quote by sameeralord You push on the wall, you lose energy to the wall
No you don't. Why do you think this?
 and hence slow down(which can also be considered as the wall acting on you).
When I'm pushing the wall, I'm not moving.
 If that is not the explanation
Not only are your statements not an explanation, they are not even true!
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P: 15,589
 Quote by russ_watters The question isn't philosophical, it is just a misunderstanding of what energy is: you're looking at the issue backwards. Energy is the biproduct of the movement, not the cause. The force is the cause of the motion.
I don't know. In the Lagrangian formalism you certainly can look at it the other way. The energy is the cause of the motion and the force is simply the change in the energy wrt some coordinate.

sameeralord, are you familiar with Lagrangian mechanics? If not, perhaps you should look into it. At least for conservative forces it provides an approach where forces play a very secondary role and energy is the primary thing.
 P: 3 A moving object doesn't have or need a force. You only need a force to accelerate or decelerate an object. It's never just a force acting on an object. Whenever a force acts upon an object the object will in turn always react with a force in the opposite direction. A reactive force. An object will only accelerate or decelerate when the active force is stronger than the reactive force from the object be it due to inertia, friction, mass, structural integrity or whatever. There's always an active force AND a reactive force. And of course if there's any type of friction and you want to have a constant speed you need to constantly apply a force equal to the frictional force. But if there isn't it will continue forever without any forces. So I think there might be some kind of misunderstanding that a moving object has a force or need a force to keep going. It doesn't. It has energy, it has momentum, and sure it can impact something with a force. But for there to be a force there must also be a reactive force, in other words it must hit/act on something.
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 Quote by Frankthought It's never just a force acting on an object. Whenever a force acts upon an object the object will in turn always react with a force in the opposite direction. A reactive force.
If object A exerts a force on object B, then object B will exert an equal and opposite force on object A. Is this the 'reactive force' you are describing? Note that those two forces act on different bodies.
 An object will only accelerate or decelerate when the active force is stronger than the reactive force from the object be it due to inertia, friction, mass, structural integrity or whatever. There's always an active force AND a reactive force.
This is confusing. Per my comment above, the reactive force is always equal to the active force. (And, more importantly, they act on different bodies.) Or do you mean something else by 'reactive force'?

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