Why do forces make an object move?

[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?

 

[sameeralord]

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

Yes slowing down first and then accelerating in the other direction. I'm thinking that slowing down at the start is due to loss of energy, where is this energy going to?

 

[Doc Al]

Yes slowing down first and then accelerating in the other direction. I'm thinking that slowing down at the start is due to loss of energy, where is this energy going to?

As the ball goes up, gravitational potential energy increases while kinetic energy decreases. Once it reaches maximum height, the process reverses and it gains kinetic energy while losing potential. The total energy never changes.

But at all points the force on the ball is downward, thus the acceleration acts accordingly.

 

[sameeralord]

As the ball goes up, gravitational potential energy increases while kinetic energy decreases. Once it reaches maximum height, the process reverses and it gains kinetic energy while losing potential. The total energy never changes.

But at all points the force on the ball is downward, thus the acceleration acts accordingly.

Thanks again Doc Al. Newton's third law then seems to be coming from potential energy I get the feeling. When I compress a spring, it stores elastic potential energy and this gives the reaction force. It seems it is trying to restore its previous energy state all the time, it has to act in opposite direction to restore the energy, it doesn't matter which way the object is traveling as long as the previous energy state is restored. This is starting to make sense I think. Let me reread your posts again, and ask the other questions I have.

 

[sameeralord]

Hello Doc Al,

It seems I peaked too early and I don't understand this much yet.

Now in this picture if I find change in momentum of full back it is.

400-640= -240

Now if the full back lost 240 momentum,then the lineman should have gained it. So line man's change of momentum must also be 240 in opposite direction. When I do the equation it is.

400+240=620

Why is it not equal?

 

[Doc Al]

Now in this picture if I find change in momentum of full back it is.

400-640= -240

Nope. 400 kg-m/s is not the final momentum of the full back, but the combined momentum of both players. (+640 -240 = 400)

If you new the masses and initial velocities of the players, you could determine their final speed and the change in their momenta.

 

[Dale]

Newton's third law then seems to be coming from potential energy I get the feeling.

If you want to talk about Newton's 3rd law coming from energy then you will need to learn Lagrangian mechanics and Noether's theorem. Newton's 3rd law can be derived as a consequence of the spatial symmetry of the Lagrangian.

 

[sameeralord]

Ok thanks for all the replies. Now let's say there was an object with initial momentum of 1 (m=1 v=1)and final momentum of -1(v=-1). That means change of momentum is -1-1= -2. Now if I consider the change in kinetic energy of the object 1-1 is zero. This is my biggest question, how come there is a change of momentum without a change in energy.

 

[trojsi]

The people will provide energy to the box only if the box is compressed slightly for example the energy will result in heat on a molecular level.

 

[Doc Al]

Ok thanks for all the replies. Now let's say there was an object with initial momentum of 1 (m=1 v=1)and final momentum of -1(v=-1). That means change of momentum is -1-1= -2. Now if I consider the change in kinetic energy of the object 1-1 is zero. This is my biggest question, how come there is a change of momentum without a change in energy.

For one thing, momentum is a vector while energy is a scalar. The momentum can change direction, which is what happened here. Note that the magnitude of the momentum--and the speed of the object--has not changed. In other words, momentum depends on velocity while kinetic energy depends on speed. The velocity changed but the speed remained the same.

 

[sameeralord]

For one thing, momentum is a vector while energy is a scalar. The momentum can change direction, which is what happened here. Note that the magnitude of the momentum--and the speed of the object--has not changed. In other words, momentum depends on velocity while kinetic energy depends on speed. The velocity changed but the speed remained the same.

Ok the object was traveling at 1m/s at the start. Then a force must have acted on it to make it travel at -1m/s. So first the force would have made the object lose some energy and make it travel at 0m/s and then add some energy making it -1m/s. Now my question is when the force has made the object lose energy, what is this energy transferred to, there is no gravity in this.

 

[Doc Al]

Now my question is when the force has made the object lose energy, what is this energy transferred to, there is no gravity in this.

You tell me. What is the nature of the force accelerating the object?

 

[sameeralord]

You tell me. What is the nature of the force accelerating the object?

A ball traveling towards a bat, the ball is coming toward the bat and then moves in opposite direction after the collision.

 

[Doc Al]

Ok the object was traveling at 1m/s at the start. Then a force must have acted on it to make it travel at -1m/s. So first the force would have made the object lose some energy and make it travel at 0m/s and then add some energy making it -1m/s.

Just to be clear, this is not necessarily true. You can have an object reverse its momentum without at any point coming to rest. Example: Drive a car around a circular track. Your initial momentum is X units North. When you've gone halfway around the track, your momentum will be X units South. Your change in momentum is 2X units south, yet your momentum never was zero at any time. The energy never changed a bit.

That's why it depends on the nature of the force.

 

[Doc Al]

A ball traveling towards a bat, the ball is coming toward the bat and then moves in opposite direction after the collision.

When the ball hits the bat, it compresses storing some of the energy as elastic potential energy. That energy is released when the ball rebounds from the bat. In addition, the bat gives energy to the ball. And some of the total energy is 'lost' to thermal energy and sound and deformation.

 

[sameeralord]

When the ball hits the bat, it compresses storing some of the energy as elastic potential energy. That energy is released when the ball rebounds from the bat. In addition, the bat gives energy to the ball. And some of the total energy is 'lost' to thermal energy and sound and deformation.

Ok thanks for the help Doc Al. It is starting to make sense now. So just because of momentum changed it doesn't always mean a change of energy occured. Ok but these are the final questions I have. If I throw a ball at a wall and the ball got compressed against the wall storing elastic potential energy, now the ball rebounds, does the ball rebound with more force than it hits the wall because as you said the wall provides momentum to the ball. I mean when I the ball rebound it is not exactly the reaction force because extra bit of energy is added to the reaction force. Is that right?

 

[Doc Al]

If I throw a ball at a wall and the ball got compressed against the wall storing elastic potential energy, now the ball rebounds, does the ball rebound with more force than it hits the wall because as you said the wall provides momentum to the ball. I mean when I the ball rebound it is not exactly the reaction force because extra bit of energy is added to the reaction force. Is that right?

No. At every point in the interaction of the ball with the wall, they exert equal and opposite forces on each other. That's the content of Newton's 3rd law.

(And statements like "adding energy to a force" don't make much sense, since energy and force are different things. It's like saying "my weight increased by 2 feet".)

 

[sameeralord]

No. At every point in the interaction of the ball with the wall, they exert equal and opposite forces on each other. That's the content of Newton's 3rd law.

(And statements like "adding energy to a force" don't make much sense, since energy and force are different things. It's like saying "my weight increased by 2 feet".)

Oh ok I see. So when the ball hits the wall can I atleast say 2 reactive forces occured, one from spring compressing back, other from the momentum collision. So I think then I understand. So Newton's third law can basically occur two ways.

1. Momentum conservation
2. Spring and elastic potential energy

In this scenario both occurs. Is that right?

 

[Doc Al]

Oh ok I see. So when the ball hits the wall can I atleast say 2 reactive forces occured, one from spring compressing back, other from the momentum collision. So I think then I understand. So Newton's third law can basically occur two ways.

1. Momentum conservation
2. Spring and elastic potential energy

In this scenario both occurs. Is that right?

Sorry, but your thinking doesn't make sense to me. For some reason, you are trying to get some deeper understanding of Newton's 3rd law before you understand what it says. Better to stick with the latter goal for a while. First understand what it says, then worry about 'how it can be that way'.

Think of a force between two objects as an interaction, not as separate forces. If two things interact--exert forces on each other--those forces must be equal and opposite.

A useful exercise would be to go through a number of scenarios where things interact and forces are involved--bouncing a ball against the wall, hitting a ball with a bat, tossing a ball in the air, and so on--and identify all the 3rd law pairs in the interaction.

 

[sameeralord]

Sorry, but your thinking doesn't make sense to me. For some reason, you are trying to get some deeper understanding of Newton's 3rd law before you understand what it says. Better to stick with the latter goal for a while. First understand what it says, then worry about 'how it can be that way'.

Think of a force between two objects as an interaction, not as separate forces. If two things interact--exert forces on each other--those forces must be equal and opposite.

A useful exercise would be to go through a number of scenarios where things interact and forces are involved--bouncing a ball against the wall, hitting a ball with a bat, tossing a ball in the air, and so on--and identify all the 3rd law pairs in the interaction.

Why is what I said all wrong? If I push a box and it gains momentum and moves. Then I work out the change in momentum of the object I can work out the force of Newton's third law. So why is that what I said wrong. I want to know why Newton's third law occur that's all, I know what it does.

 

[Jerbearrrrrr]

At some point, stuff has to just be observed. We see stuff happening and propose that they obey some laws. Where this point is, however, I'm not sure.

If you take the conservation of momentum for granted, then (at least for this simple scenario) you don't have to think about forces. (force is the d/dt of momentum, whose sum is constant, which gives equal and opposite forces for the you-box pair)

As for Newton himself, did he just observe his three laws and postulate (on empirical evidence, rather than derivation) that they were, indeed, laws?