Why do forces make an object move?

[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?

 

[sameeralord]

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?

Hey thanks for the reply. So at least for this question I can consider momentum as the cause of Newton's third law, right? For moving the box I mean.

Doc Al can you give me an example where Newton's third law occur and there is no conservation of momentum or elastic potential energy is used?

 

[Jerbearrrrrr]

There's no real causal relationship. Stuff happens, and we draw up laws to explain it.

If you assume the conservation of momentum, you can derive Newton's third law in this case.

I don't know how the laws (/theorems) interact in general, but often you can solve a simple system (as in, determine how stuff will move) by either starting from forces and accelerations; or by starting from conservation laws (cons energy, momentum).
The solutions agree invariably.
The descriptions are consistent. Whether one "causes" the other...I'm not sure what you mean.
Perhaps you mean "if you assume this fact, the other fact can be derived"?

I've only ever attended a handful of lectures on dynamics, but usually Newton's third is just assumed, and it helps in deriving other stuff.

Maybe it might be more useful to ask which laws can be derived from other laws and assumptions.

 

[sameeralord]

There's no real causal relationship. Stuff happens, and we draw up laws to explain it.

If you assume the conservation of momentum, you can derive Newton's third law in this case.

I don't know how the laws (/theorems) interact in general, but often you can solve a simple system (as in, determine how stuff will move) by either starting from forces and accelerations; or by starting from conservation laws (cons energy, momentum).
The solutions agree invariably.
The descriptions are consistent. Whether one "causes" the other...I'm not sure what you mean.
Perhaps you mean "if you assume this fact, the other fact can be derived"?

I've only ever attended a handful of lectures on dynamics, but usually Newton's third is just assumed, and it helps in deriving other stuff.

Maybe it might be more useful to ask which laws can be derived from other laws and assumptions.

Ok thanks I'm not going to question this law anymore. As you said it is that I believe energy must be conserved, I have no question about that, so I like to derive everything from that. Thanks anway

 

[JDługosz]

First of all thanks a lot for all the answers especially this one

I'm glad it helps. Thanks for saying so.

...only if someone can explain why every object has elasticity, what property of atoms make them spring.

Now that is another story all together.

Atoms behave, to a first approximation, something like the consistency of an American Softball. It has a hard contact with a thin amount of "give", and then deforms as you apply a great deal of force.

Bonds hold atoms in fixed positions relative to each other, and bending that bond by pushing one one of the atoms will act in a spring-like manner. A single atom, like a softball, will have its own shape distort due to similar principals.

The reason objects act in a "matter-like" way is due to a fundamental property of electrons and other matter particles. Fermions (as they are called) that are identical will not overlap. The presence of one electron near by will change the energy level of another, so putting them close together takes energy.

No matter how non-rigorous any of that was, or how it combines different issues, that's the bottom line that is capital Truth: it's all about energy. The configuration of atoms and their electrons that are spaced differently (e.g. compressed closer together than in the relaxed mineral grain's natural size) requires energy to accomplish, and so holds potential energy in that new configuration, and will push back to the relaxed form.

--John

 

[jakksincorpse]

Hertz. everything from light, infared, its all electromagnetic radiation. even sound. i read that energy in a vacuum does not bend or have any resistence so that means no friction, which the energy makes its own polarity and begins to move...i think if i read that correctly. energy outside of a vacuum that has friction and resistence moves in waves.

 

[JDługosz]

Jakksincorpse: That doesn't make any sense. And sound is not electromagnetic radiation. The rest of it seems like a bunch of words strung together -- I know the individual words, but it makes my head hurt trying to put them together that way.

 

[Frankthought]

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.

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'?

If the active and reactive force always was equal nothing would ever accelerate or decelerate.

 

[D H]

If the active and reactive force always was equal nothing would ever accelerate or decelerate.

You are misinterpreting Newton's third law. The active/reaction pair are always equal but opposite per Newton's third law. The point Doc Al was making, and the point that many people miss, is that the forces act on different bodies.

Suppose two objects, call them A and B, are interacting with one another. A exerts a force on B, and B exerts a force on A. The two objects are far removed from any other objects; the only forces acting on A and B are the forces that arise from their interaction. At any point in time, object A's acceleration is determined solely by the force that object B exerts on object A. The force that object A exerts on object B does not come into play here. The opposite applies for object B: Object B's acceleration is determined solely by the force that object A exerts on object B.

Newton's third law says that these two forces are equal but opposite. This does not mean nothing can accelerate. Example: The gravitational force exerted by the Earth on the Moon is equal but opposite to the gravitational force exerted by the Moon on the Earth. The two are constantly accelerating toward one another.

 

[Frankthought]

Btw. If you have two blocks on top of each other just resting on the ground. What do you call the force with which the bottom block acts upon the top block? Is it called resistive force?

 

[Doc Al]

Btw. If you have two blocks on top of each other just resting on the ground. What do you call the force with which the bottom block acts upon the top block? Is it called resistive force?

It doesn't have a special name. You can call it the normal force or the contact force between the blocks.