Why do forces make an object move?

[sameeralord]

Hello everyone,

May be the question is when something has energy and there is no resistance why does it move? Thanks!

 

[Mu naught]

A force by definition is a change in velocity of some object with mass m. Therefore by definition a force moves an object.

 

[n.karthick]

Forces can make an object to accelerate only if the resultant force (vector sum of all forces) is non-zero.
If you ask why, it is the effect in order to neutralize the cause. For example, if there is a potential difference between two points, electric current flows from high potential to lower one, provided there is a path, to bring both the points at same potential. It seems that nature favors equality and does action whenever wherever possible to attain that.

 

[Mu naught]

Forces can make an object to accelerate only if the resultant force (vector sum of all forces) is non-zero.
If you ask why, it is the effect in order to neutralize the cause. For example, if there is a potential difference between two points, electric current flows from high potential to lower one, provided there is a path, to bring both the points at same potential. It seems that nature favors equality and does action whenever wherever possible to attain that.

I can take a stone and push it uphill, thereby making things "unequal" if your definition of equality is all things are at an equal potential. Objects move when they experience a "force" because that's what we define a force to be. Forces don't have to be real things.

 

[Dickfore]

Forces can also stop movement. For example, a sliding block on a horizontal surface will eventually stop due to the friction force acting on it.

 

[Jerbearrrrrr]

Well...change in velocity, as already discussed. Stopping and starting is all potato potarto. xD

Sort of going out on a tangent...The concept of a force is kind of weird. I don't know what teachers expect to teach kids when they talk about forces.
Especially since acceleration is generally discussed year(s) later. To be honest, I never really understood anything about forces until I did some F=ma examples. And then the confusion begins again with F=mv^2/r.
I guess it's confusing because forces don't really exist per se, but acceleration does.
I think people get mixed up with all the force/force field stuff without relating them back to their affect on stuff's accelerations.
/o\ just rambling.

 

[sameeralord]

Thanks for all the answers. My question is more however, let's say I push a box on the floor, it gains energy and moves. Why does an object decide to change in position when energy is gained. Is this something we know because of observation. I think this question is more philosophical.

 

[sameeralord]

Why does energy behave the way it does?

Hello everyone,

I don't know I have got really confused. Now if an object has energy, it can move. Is their any scientific explanation in molecular level or something why does energy make an object move. Also how do net forces work. If their is a box at rest and two people are pushing it with equal force in opposite directions. The object stays at rest. My question is what happens to the energy provided by two people. Does it cancel (how?), or does the object keep gaining energy without moving? I don't understand how forces give energy in a certain direction, what determines the direction? Thanks!

 

[n.karthick]

Forces do really exist in nature and they are measurable. As we all know its SI unit is Newton.
I would like to say, when an object is disturbed by a force and with no resistance in the path, it will move (accelerate) in order to show its opposition to the source which is creating force. The source has to impart some energy to it, thereby it is weakened.
Force can be viewed as a disturbance and the objects react to eliminate/de-magnify it.

 

[Doc Al]

Also how do net forces work. If their is a box at rest and two people are pushing it with equal force in opposite directions. The object stays at rest. My question is what happens to the energy provided by two people. Does it cancel (how?), or does the object keep gaining energy without moving?

The people are not providing any energy to the box.

 

[sameeralord]

The people are not providing any energy to the box.

Thanks for the reply When one person pushes on the box, due to momentum collision energy is transferrred to the box. Then the other person who pushes in the opposite direction also transfers energy to the box. I don't understand why no energy is transferred?

 

[russ_watters]

No. Force is not energy. Energy (work) is force times distance, so if there is no disance, there is no energy.

 

[Doc Al]

When one person pushes on the box, due to momentum collision energy is transferrred to the box.

Are you talking about pushing the box or colliding with the box? If you push something and it doesn't move, then you are not transferring any mechanical energy to that something.

 

[russ_watters]

Thanks for all the answers. My question is more however, let's say I push a box on the floor, it gains energy and moves. Why does an object decide to change in position when energy is gained. Is this something we know because of observation. I think this question is more philosophical.

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.

 

[sameeralord]

Are you talking about pushing the box or colliding with the box? If you push something and it doesn't move, then you are not transferring any mechanical energy to that something.

Thanks again Doc Al. I have lots of misunderstandings in physics. When something is pushes I'm using the momentum theory for that, so it doesn't work? Why does force have a direction? Could you explain it to me when two opposite forces act, why do they cancel each other. When there is a box on the table. Box due to gravity pushes down on the table, then a normal reaction force occurs, isn't this due to momentum transfer.

 

[russ_watters]

I have lots of misunderstandings in physics. When something is pushes I'm using the momentum theory for that, so it doesn't work?

You're not being very clear: When you say "when something pushes" do you mean that the object being pushed moves? If it moves, energy is transferred and momentum is generated. If it doesn't move, no energy is transferred and no momentum is generated.

Why does force have a direction?

It's just a property of force - it is a vector. There is no "why".

Could you explain it to me when two opposite forces act, why do they cancel each other.

It's just math: 1-1=0

When there is a box on the table. Box due to gravity pushes down on the table, then a normal reaction force occurs, isn't this due to momentum transfer.

No, momentum is mv. If v=0, then momentum is zero.

I think you need to stop thinking in terms of the words and start thinking in terms of the mathematical meanings of the words. You aren't using the words correctly. You have the definitions wrong and that's why you are getting confused about how these concepts interact.

 

[sameeralord]

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.

When something is acted upon by a force. It gains energy. So doesn't energy cause movement. Also if something is not acted upon by a force it can still travel at constant speed, that is because it has energy. I know force is something that causes an acceleration. What exactly is a force physically though. Is this going in a circle.

 

[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.

 

[Doc Al]

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?

 

[sameeralord]

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.

 

[russ_watters]

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.

 

[russ_watters]

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?

 

[sameeralord]

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?

 

[Doc Al]

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.

 

[sameeralord]

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?

 

[Doc Al]

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.

 

[sameeralord]

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.

 

[russ_watters]

... 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.

 

[sameeralord]

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 canceled out, inhibiting movement but I was thinking they were still providing energy and energy was getting canceled 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.

 

[Doc Al]

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.

 

[Doc Al]

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?

 

[sameeralord]

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 occurring here.

 

[Doc Al]

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!

 

[Dale]

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.

 

[Frankthought]

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.

 

[Doc Al]

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'?

 

[sameeralord]

No you don't. Why do you think this?

When I'm pushing the wall, I'm not moving.

Not only are your statements not an explanation, they are not even true!

Why am I not moving when the wall is pushing back on me? Then what is your explanation to why does every reaction has an equal and opposite reaction? Change in momentum= Ft. Is this force not related to Newton's third law anyway. If I push a stationary box and the box moves, that means the box must have gained some energy when I pushed it. Where does this energy come from, haven't I lost it meaning change of momentum?

 

[sameeralord]

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.

Hey thanks for the reply Dalespam I'll surely look into it but for now I think I have overthought and got confused with basic physics concepts. If you can help me, post in my other thread. My question is related to why Newton's third law occur and is change of momentum related to Newton's third law.

 

[Doc Al]

Why am I not moving when the wall is pushing back on me?

Because other forces are acting on you to give a net force on you of zero.

If I push a stationary box and the box moves, that means the box must have gained some energy when I pushed it.

Sure. If you push something and it moves, you are transfering energy to it. (And momentum.)

Where does this energy come from, haven't I lost it meaning change of momentum?

You did the work on the box so the energy comes from you. If you imagine that you and the box are on a frictionless surface (to avoid having to deal with other forces), and you give the box a shove (and it shoves you back) then the box gains some energy and momentum. The amount of momentum the box gains is equal and opposite to the amount of momentum that you gain. (That is related to Newton's third law, since the forces you and the box exert on each other must be equal.)

 

[JDługosz]

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 solid object is just like a spring, only much stiffer. Instead of a table, take a simpler example of a concrete footing. If you put a few tons on top, you can measure how much it is squeezed with a dial gauge. The shortening of the footing (leg) causes a force upwards trying to restore it.

What you normally consider a "spring" is just a solid that's weakened to the point where such changes in length become obvious. But it's always like that.

--John

 

[LostConjugate]

Every action has an equal and opposite reaction, so really nothing is happening in this universe, its all just an illusion :)

 

[QED-Kasper]

people who say that when you push on a box and it doesn't change position you arent giving it energy are wrong. An example of the energy you give to the box is in the form of body heat (if the box is colder than your body). The atoms of your hand have more kinetic energy than the atoms of the box so when they collide when you push the box, the atoms of the box gain kinetic energy. So the box gets energy in the form of heat (which is just kinetic energy, vibrating atoms). Also the box deforms, so in that way there is energy transferred aswell. Also your body heats up etc. energy gets lost in that waytooo.

 

[Doc Al]

people who say that when you push on a box and it doesn't change position you arent giving it energy are wrong. An example of the energy you give to the box is in the form of body heat (if the box is colder than your body). The atoms of your hand have more kinetic energy than the atoms of the box so when they collide when you push the box, the atoms of the box gain kinetic energy. So the box gets energy in the form of heat (which is just kinetic energy, vibrating atoms). Also the box deforms, so in that way there is energy transferred aswell. Also your body heats up etc. energy gets lost in that waytooo.

It's true that if you and the box are not in thermal equilibrium, there will be a transfer of heat. But we were talking about mechanical energy transfer, not thermal.

It's also true that the box will deform a bit when pushes (as mentioned by JDługosz). And there will be an associated (small) amount of work done by you as the box deforms. But once the deformation is complete, no additional work is done.

 

[QED-Kasper]

Okay

 

[russ_watters]

Why am I not moving when the wall is pushing back on me?

The wall is attached to the ground, so it doesn't move and if it doesn't move, it can't make you move!

Then what is your explanation to why does every reaction has an equal and opposite reaction?

There isn't anything to explain: you're making up your own definition here that just isn't correct. Newton's third law says: "Third Law: Whenever a first body exerts a force F on a second body, the second body exerts a force −F on the first body. F and −F are equal in magnitude and opposite in direction." The word "move" does not appear in the definition. It isn't necessary/relevant.

Change in momentum= Ft. Is this force not related to Newton's third law anyway. If I push a stationary box and the box moves, that means the box must have gained some energy when I pushed it.

We're going in circles, repeating ourselves. As said before:
1. Yes, in some cases an exchange of forces involves an exchange of energy and momentum.
2. In other cases, an exchange of forces does not involve an exchange of energy and momentum.

This has been explained several times. I think the issue here is that you aren't paying enough attention/reading carefully, and thinking about what you are reading.

 

[Dale]

My question is related to why Newton's third law occur and is change of momentum related to Newton's third law.

No, change of momentum is related to Newton's 2nd law. Newton's 3rd law is related to conservation of momentum.

 

[sameeralord]

A solid object is just like a spring, only much stiffer. Instead of a table, take a simpler example of a concrete footing. If you put a few tons on top, you can measure how much it is squeezed with a dial gauge. The shortening of the footing (leg) causes a force upwards trying to restore it.

What you normally consider a "spring" is just a solid that's weakened to the point where such changes in length become obvious. But it's always like that.

--John

First of all thanks a lot for all the answers especially this one This actually finally explains the Newton's third law, only if someone can explain why every object has elasticity, what property of atoms make them spring. If I use your analogy to answer about pushing the wall and why it doesn't push me, I think it is because you when you push the wall you can't compress it much, due to the inertia of the particles inside the wall when the force is transmitted, so the opposite force is quite less. I don't understand why Doc Al said net forces acting on you is zero, isn't their still a small force however. I still don't understand negative force or work

Let's say a box was moving forward at constant speed, it must have some energy, then a force in opposite direction acts on it to make the object stop, how has the negative force removed energy from the object, has it been transferred to heat? Also if I keep applying this force, the object would eventually accelerate the opposite side, how come now the negative force is giving energy to object and before it was taking away.

Also more about the spring. If I push a spring on a stationary wall and release, the spring would fly in the opposite direction right? I understand that there is elastic potential energy stored and this causes the movement but when I normally compress a spring it springs back to its orginal shape, how does it fly away in other direction. For example when I compress a spring against a wall, why doesn't it regain its original shape and fall down, why does it fly away instead. I'm thinking the elastic potential energy is used to make the spring go back to its orginal shape. Thanks

I'm trying to understand Newton's third law using this. If I push a book, it will push back on me. How does this occur using spring example. When I push the book does my hand compress a bit and then I move back in the other direction, ok but isn't the book compressed a little bit as well, wouldn't the make the book spring back and move back. How does the book move foward, if I compress it, shouldn't it spring back.

Thanks

 

[Doc Al]

If I use your analogy to answer about pushing the wall and why it doesn't push me, I think it is because you when you push the wall you can't compress it much, due to the inertia of the particles inside the wall when the force is transmitted, so the opposite force is quite less.

But it does push you, just as hard as you push it.

I don't understand why Doc Al said net forces acting on you is zero, isn't their still a small force however.

The net force is the vector sum of all forces acting on an object. Just because the net force is zero, doesn't mean there are no forces acting. Imagine yourself sitting quietly in a chair. The chair pushes up on you; gravity pulls down. Forces act on you, yet the net force is zero.

I still don't understand negative force or work

Let's say a box was moving forward at constant speed, it must have some energy, then a force in opposite direction acts on it to make the object stop, how has the negative force removed energy from the object, has it been transferred to heat? Also if I keep applying this force, the object would eventually accelerate the opposite side, how come now the negative force is giving energy to object and before it was taking away.

I recommend that you contemplate Newton's 2nd law. If there's a net force on an object, then the acceleration of the object will be in the direction of that net force. Toss a ball straight up in the air. Why does it lose energy on the way up, then gain it on the way down? The simplest thing to realize is that its acceleration is always downward.

Also more about the spring. If I push a spring on a stationary wall and release, the spring would fly in the opposite direction right? I understand that there is elastic potential energy stored and this causes the movement but when I normally compress a spring it springs back to its orginal shape, how does it fly away in other direction. For example when I compress a spring against a wall, why doesn't it regain its original shape and fall down, why does it fly away instead. I'm thinking the elastic potential energy is used to make the spring go back to its orginal shape. Thanks

It "flies away" because there is an unbalanced force on it. Squeeze a ball between both of your hands, hard. Then quickly remove one hand. Since the other hand is still pushing, the ball goes flying.

I'm trying to understand Newton's third law using this. If I push a book, it will push back on me. How does this occur using spring example. When I push the book does my hand compress a bit and then I move back in the other direction, ok but isn't the book compressed a little bit as well, wouldn't the make the book spring back and move back. How does the book move foward, if I compress it, shouldn't it spring back.

When you push on something, it compresses a bit. The resulting motion of the object depends on all the forces acting on it. You push the book, it compresses; if that's the only force on the book, it accelerates forward.

 

[sameeralord]

But it does push you, just as hard as you push it.

I recommend that you contemplate Newton's 2nd law. If there's a net force on an object, then the acceleration of the object will be in the direction of that net force. Toss a ball straight up in the air. Why does it lose energy on the way up, then gain it on the way down? The simplest thing to realize is that its acceleration is always downward.

Ok thanks for all the replied Doc Al I think I'll try to learn this step by step. When I throw a ball up in the air initially I lose energy. Where does this energy go to, which form is it converted to?

 

[Doc Al]

When I throw a ball up in the air initially I lose energy. Where does this energy go to, which form is it converted to?

Are you talking about the ball losing kinetic energy as it rises?