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B Why do things accelerate?

  1. Nov 10, 2016 #1
    Why do things accelerate? Let's take a falling object for example, why does it accelerate towards the Earth? I know that when a net force acts on an object, it accelerates but THAT is my question, why does this happen? Why wouldn't the velocity stay the same with a net force acting on an object? I'm trying to figure this out and it's making me crazy! Any answers, opinions are welcome thank you.
    Last edited: Nov 10, 2016
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  3. Nov 10, 2016 #2

    Simon Bridge

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    Welcome to PF;
    By definition, the force on an object is the rate if change of it's momentum.
    If the mass does not change, then this means that force is proportional to acceleration by definition.

    For your example - if you let go of an object, it falls. It started out stationary, then, right after you let go, it is moving.
    Even if it moves at a constant speed as it falls, it still had to change it's speed right at the start.
    Changing the speed means acceleration happened, so "objects falling at a constant speed" does not mean that there is no acceleration to account for.
    On top of that, we'd have to explain why the acceleration stopped at just that speed. This makes the resulting theory of gravity more complicated.
    So in the end, we define force the way we do because it makes the maths simpler while still describing reality.

    You keep asking "why?" That may not be a useful question here: we know that acceleration happens, and we would like to be able to make predictions about that.
    All Newton's laws are doing is giving us a maths language that is useful for this.

    For more discussion on this: see Leonard Susskind's lectures on classical mechanics on youtube.
    I think the first one discusses why Newton's laws have the form they do in some detail using maths.
  4. Nov 10, 2016 #3
    Thanks for your input but this wasn't my question at all. I'm talking about when the object is already moving and not when it starts moving . You are taking on an whole different question. (I'm going off topic to tell and answer you that an object cannot directly start from rest position to a constant moving velocity which you pretty much based you unrelated opinion on). And thanks for the welcome, it's a really nice site that I just discovered recently :)
    Last edited: Nov 11, 2016
  5. Nov 10, 2016 #4

    Simon Bridge

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    I was attempting a reducio ad absurdam by addressing a consequence of what you were proposing as an alternative - ie. that it begs the question.

    I did also attempt to answer the question as you asked it - ie. force goes to acceleration because of how we define force.
    Perhaps it will be clearer if I point out that forces in physics do not cause acceleration in the way we usually think of causation. You do not have a force and then an acceleration happens. Is this more what you are asking about?

    If this second one does not relate to your question then please restate the question so it is clearer or, at least, explain how it fails to match up.

    Note: v=0m/s is a state of motion ... if you throw a ball directly upwards it will come to rest then start movig down. This example satisfies your requirement that the ball starts out in motion so, in the description that a force causes a constant velocity, you still have a deceleration followed by an acceleration.
  6. Nov 10, 2016 #5

    Stephen Tashi

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    (As Aristotle pointed out:)
    If we say "a net force produces an acceleration because of reason X1" then you can ask "Why should reason X1 be true". Then someone can reply "Reason X1 is true because of reason X2" Then you can ask "Why should reason X2 be true". What criteria are you going to use to end the chain of reasons X1,X2,X3,.... ?

    In Newtonian physics F = MA is an assumption based on empirical experiments and observations.

    Here is an interesting thought experiment: Suppose object A is pushing object B along a horizontal frictionless surface. If object B is moving along with constant velocity V, how can object A exert any force upon object B unless object A "tries to go faster" than velocity V ?
  7. Nov 10, 2016 #6
    Wow ok, where do I begin...: Let's go with the obvious ones first:
    1. 0m/s is not motion... it is called "rest".. here, I googled motion for you: https://en.wikipedia.org/wiki/Motion_(physics). The explanation after this is just either complete non sense or just ignorance, so much that I dont even feel noting all of the things wrong with it...
    2. Forces in physics can cause acceleration, depending on the force you are talking about. So "forces in physics do not cause acceleration in the way we usually think of causation." and"You do not have a force and then an acceleration happens", that depends of the force you are talking about.
    3. Your answer to my question ".force goes to acceleration because of how we define force" is useless because I say that I know this in the question..
    4. Your reducio ad absurdam attempt is complete failure because again, You cannot go from a rest state directly to a constant moving velocity.
    So again thanks for trying to contribute.

    I totally agree with this, but I always like to think things through but I guess this is the maybe the end of the chain. I threw the question out to see people's point of view. And as for the thought experiment, it's just an impossible scenario since you say that A is pushing B but B moves at constant velocity. :)
  8. Nov 10, 2016 #7
    Position is where something is - measured in a distance unit... you're 5 meters away, you're 15 feet away etc
    Velocity is derivative of position - measured in distance per time: you're moving at 60 miles per hour
    Acceleration is derivative of velocity - measured in distance per time per time: you're accelerating at 9.8 meters per second per second
    Jerk is the derivative of acceleration - measured in distance per time per time per time.
    There's also snap, crackle and pop, which are names given to the 4th, 5th, and 6th derivatives of position.

    To answer the question: "Why wouldn't the velocity stay the same with a net force acting on an object?"
    The same reason an object doesn't stay in the same place if it has a non-zero velocity.

    Velocity acts on position the same exact way acceleration acts on velocity.


    As for what causes acceleration in the first place: why doesn't stuff just experience an instant change in velocity? The same reason why stuff doesn't just experience an instant shift in position. You don't just teleport from position A to B, you experience a jerk, acceleration, velocity and position change.
    Last edited: Nov 10, 2016
  9. Nov 11, 2016 #8

    Simon Bridge

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    I don't seem to be communicating very well...
    I'm sorry you feel the need to respond like that.

    An equation of motion may be something like ##v(t)=v_0-gt## ...
    How would you answer the question: what is the state of motion at time ##t=v_0/g##?

    "zero" is a valid quantity.
    Let me repeat: "at rest" is a state of motion. "Not moving" is a kind of motion in the same way that inaction is a form of action.
    Physics can be zen like that.
    I am introducing you to this concept because many students who have the kinds of confusion you have asked about find it useful to help them reason through things.
    I won't insist that you to accept the concept, but please do not assume ignorance as a result.

    Anyway: you seem to have got an answer that helps you from someone who understands you better.
    Is this correct?
  10. Nov 11, 2016 #9


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    The short answer is because that's the way the universe works.

    If, in another universe, a net force did produce a constant velocity you would have to wonder what happens when that force is removed? Presumably the object slows down because (in this other universe) a force is needed for a constant velocity. In which case what would cause the velocity to reduce? It can't be an opposing force because (in this other universe) forces result in constant velocity.
  11. Nov 11, 2016 #10


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    An example is the "massless blocks on a [hidden] floor covered in syrup" universe. Velocity is proportional to force. In the absence of force objects stop. For the inhabitants in such a universe, the idea of "what would cause the velocity to reduce?" does not arise. The blocks stop ...

  12. Nov 11, 2016 #11
    The question may be asking for for a derivation. Often in physics we can answer "why" by starting with the theories and postulates we believe to be true and demonstrate by logic and mathematics that the phenomenon is a necessary consequence. However all scientific theories and in fact all logical reasoning must have a basis of postulates: statements we assume to be true and then reason from them. The postulates are not derived from anything. We accept and believe these postulates not because they can be derived from other ideas, but rather because the logical consequences derived from them match our observations. Newton's laws are the postulates of classical dynamics. We believe that is how the universe works because when we take that as a given we can accurately explain the dynamics of physical systems. When we find behavior that the postulates do not correctly explain, then we must make new postulates that fit all of the observations.
  13. Nov 11, 2016 #12
    Replacing old postulates in the face of observations that can't be explained by the existing theory has been the history and success of science. The black body spectrum the photoelectric effect, and a few other things were a crisis in physics showing the failure of the existing paradigm and leading directly to the development of quantum mechanics. The Michelson Morley experiment and the invariance in the speed of light regardless of the observers motion violated the existing understanding and led directly to the development of relativity.

    Well, as it happens the switch to the idea that force causes acceleration was a similar observation driven paradigm shift. Aristotelian physics did NOT believe that forces caused acceleration. Before Galileo reasoning was fooled by friction. You had to keep pushing on things or they would stop. Force clearly causes constant motion. It imparts impetus and after a while objects lose their impetus. However, that reasoning got many things wrong, most importantly the trajectory of cannon balls. That was the crisis in natural philosophy that spurred Galileo to study trajectory and arrive at the modern model.
  14. Nov 15, 2016 #13
    I think he was asking for a simple answer... that answer in my mind is that when things stop accelerating, there is no "net" force being applied.. meaning the force once used to accelerate, get matched by an equal and opposite force (aero drag for example) and the object continues at a final terminal velocity. (newtons first law)
  15. Nov 15, 2016 #14

    Simon Bridge

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    ... so when post #1 says:
    ie. "why do things accelerate when there is a net force on them?" - that is poorly worded??
    Well I'll agree that OP's question is unclear whatever.
    Without feedback I don't think we can tell what was meant.
    This does not look likely as OP has not returned since post #6.
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