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Motion Creates Force ?:

  • When you move your hand (to push a box for example), it creates a force.
  • When electron moves through a wire it creates electrical force.

Or Force Creates Motion ?:

  • Gravity force caused an apple to fall.
  • We pushed a ball and it started to move, but friction force made it stop.

So which one is the correct one?

Actually it feels weird to me if you say : "the earth is static and nothing is moving, but it attract everything to itself (gravity).".

I think like: maybe there is some moving thing on the earth which causes the gravity, or maybe the movement of the earth itself causes the gravity. so when I see a force, I search for a motion that causes it. Is it correct?

Ps: My major is not physics, so please feel free to edit the question.

Ps2: I know the earth is not static. just wanted to simplify the question.

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  • $\begingroup$ A force causes a change in velocity. However, velocity does not generate a force. Only if an object is accelerating, it always has a force acting on it. An object undergoing steady motion does not need external influences, such as a force, to carry on at that motion due to inertia. However, for constant acceleration, there needs to be an external influence to carry on that motion. There is no "inertia" for acceleration. $\endgroup$ – Joshua Pasa Jul 19 '20 at 22:44
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    $\begingroup$ However, when studying more fundamental physics, the idea of forces begin to have no real meaning. So thinking in terms of interactions is a more natural concept. $\endgroup$ – Joshua Pasa Jul 19 '20 at 23:07
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    $\begingroup$ The force is just a way of measuring how "strong" the interaction is. Because it takes more energy to move something more massive. That's why F = ma, is a good starter in measuring interactions. However, the interaction always comes first, the fact that the object is accelerating due to the interaction, does not influence the interaction. $\endgroup$ – Joshua Pasa Jul 19 '20 at 23:34
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    $\begingroup$ Movent doesn't explicitly make other particles move. The particles may move and interact with the field that changes the interactions of other particles. Its the field (interaction) that causes the motion. Different configurations of the particles positions and momentum may change how the field interacts with the particles. However it is always the field that interacts that causes the motion. With gravity John Weehler once said "Space-time tells matter how to move; matter tells space-time how to curve". The same concept can be applied to every interaction, with a few changes to the statement. $\endgroup$ – Joshua Pasa Jul 20 '20 at 0:04
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    $\begingroup$ Pretty much. Except, fields do not create forces, the field is the force that causes the motion. $\endgroup$ – Joshua Pasa Jul 20 '20 at 2:11
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If you want the short answer, then motion does not create a force. The force is what changes the velocity of a object.

The longer answer: The classical equation to calculate a force acting on a object (excluding special and general relativity) is: $$F = m \frac{d^2x}{dt^2}$$ I think the confusion lies in this equation. If you substitute the acceleration of the object into this equation then it gives you a force. However, this does not mean that the acceleration creates a force, all it means is that if we know the acceleration then we know how strong the force has to be to create that acceleration.

However, forces are not fundamental. It is a way of measuring the strength of a interaction. The reason we need interactions in physics is because, even though objects moving at a constant velocity will always move at a constant velocity, the same does not apply to objects that are accelerating. Acceleration is the result of an external influence, unlike inertia. Fundamentally, interactions can be described as a field that fill up all of space. These fields cause objects with certain properties to change their motion. These fields are how attractive and repulsive forces mediate their influence, such as the electric field.

With electric fields, the strength of the force is determined not only by the charge of the particle attracting the second particle, but is also determined by the charge of the second particle as shown in the equation below:

$$F = \frac{1}{4 \pi \epsilon_0} \frac{q_1 q_2}{r^2}$$

Solving for acceleration we can find that:

$$a = \frac{1}{4 \pi \epsilon_0 m_2} \frac{q_1 q_2}{r^2}$$

With this we can deduce that the acceleration (or change in steady motion) is determined by both of the charges. This is where the rule for like charges repel and opposite charges attract. Because of the fact that $1 \times 1 = -1 \times -1 = 1 $ and $ -1 \times 1 = 1 \times -1 = -1$ with positive accelleration being repulsive and negative being attractive.

With gravity, it's a bit different. The equation for gravity is:

$$F = - G \frac{m_1 m_2}{r^2}$$

solving for accelleration we find that:

$$a = - G \frac{m_1}{r^2}$$

This means that all objects attracted by gravity get accelerated by the same amount and is completely independent from the particle getting attracted. This is why things attracted by gravity fall at the same rate

There is a way to seem that motion of one object can cause another object to change its motion, however, the way to describe this is through a field. When the first object moves, it interacts with the field and is in a different configuration than it was before, which changes how the second object moves. If the first object did not move, then the second object would be affected differently, which makes it seem that motion is what generated these forces. Objects are constantly interacting with these fields, which means the field tells objects how to move; objects tells the field how it should change. Which sounds suspiciously like John Wheeler's quote about general relativity.

Even in electromagnetism, where the magnetic field is dependent on the velocity of the object generating the field, the same concept can be applied. The velocity of the object changes the field configuration of the magnetic field which changes how charged objects will act in its vicinity.

There are four fundamental fields that permeate space: Gravity, Electromagnetism, Strong force, Weak force. These fields are the reason anything happens in the universe, without them there would be no change in motion. So in short: matter interacts with fields and fields interact with the matter, however movement is not the reason for why things have a force, the force is why things move.

Hope this helps :)

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  • $\begingroup$ Thanks for answer. can you update your answer with how "Attractive and Repulsive Forces of particles" is related to fields? $\endgroup$ – yaya Jul 21 '20 at 11:29
  • $\begingroup$ Thanks for such a detailed answer. $\endgroup$ – yaya Jul 21 '20 at 15:11
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    $\begingroup$ @yaya no problem. :) $\endgroup$ – Joshua Pasa Jul 21 '20 at 15:44
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In Physics, we break down every force to four fundamental interactions,and they are Electromagnetic, Gravitational, Strong Nuclear Force and Weak Nuclear Force. Let's consider the gravitational and the electric interactions, both of which play a major role in our day to day life. They only depend on the distance between two bodies and their masses/charges, and these forces cause a change in momentum. So you could say that force causes motion (although for some forces, such as magnetism, motion is necessary for any resultant force to show up). Even if two bodies are static, they will feel gravitational attraction between them.

So, when you mention that the movement of one's hand might apply a force upon an object, what's actually happening is that the distance between the object's electrons and the person's hand electrons is getting smaller, so the electric repulsion increases and the object is pushed away.

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  • $\begingroup$ Are we sure in physics that Attractive and Repulsive Forces of particles are not caused by their movements? for example most of electrons are moving, right? can't this movements be the cause for the particles' forces? $\endgroup$ – yaya Jul 20 '20 at 0:30
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    $\begingroup$ @yaya Mainstream physics is very sure that motion is not required for gravitational and electric force. In the case of magnetic force, movement is important. $\endgroup$ – G. Smith Jul 20 '20 at 1:53

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