Why do forces make an object move?

[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.