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This is a school exercise.

We tend to think that the action of a constant force produces a constant movement speed as well. How can you explain this situation in accordance with Newton's second law?

Because as the force will end up in a certain time that means there will not be a movement of constant speed; I'm correct?

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Hi Igor! Your question seems to be based on a false premise. In reality, the action of a constant force does not produce a constant speed. So I don't understand why you're asking for an explanation of this incorrect statement. Could you clarify what it is that you're asking? – David Z May 3 '12 at 18:14
It's a school question. – Zignd May 3 '12 at 18:16
What do you mean by "a school question"? Is this part of an assignment you were given? – David Z May 3 '12 at 18:16
Not exactly like that, this is an exercise from a school book of physics. – Zignd May 3 '12 at 18:25
OK, well, it still falls under our homework policy. – David Z May 3 '12 at 18:34
up vote 1 down vote accepted

A couple of point that might help clarify the situation.

  1. When you say "the action of constant force..." you really mean the action of a force that is being intentionally harnessed for it's motive power. That is you've given only a partial description of the situation.

  2. Essential all travel on Earth occurs in fluid (liquid or gas) environments and much of it occurs in contact with solid surfaces. These facts have physical consequences in the form of dissipative processes (friction in many guises). Friction in a force, and it always acts to oppose motion.

So what does this imply about the full description of the situation?

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1. Something, someone pushing a box, with a constant force – Zignd May 3 '12 at 18:35
Yes. And the box is (1) moving in air and (2) sliding on some surface. Those facts have consequences and you have not yet taken them into account. – dmckee May 3 '12 at 18:37
I think this is the best way to say it is with an example. eg: A man pushing a box constantly and with constant speed on Earth. – Zignd May 3 '12 at 18:46

We don't think that. If you have a constant ongoing force, then you have constant ongoing acceleration, which by definition is an ongoing change of the velocity, which implies a constant change of speed.

We get a "constant movement speed" by a short push after which that force ends or if you change the frame, so everything seems moving. If there is something like constant friction, then you might construct a situation where you have to apply constant force to get a steady movement - maybe your impression is coming from this situation.

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Now I understand, because as the force will end up in a certain time that means there will not be a movement of constant speed; I'm correct? – Zignd May 3 '12 at 18:23
@Igor: I'm not sure if I understand you. If you are in a enviroment without any noticable forces around you (say you're floating through empts space, and there is no gravity, which would make things more compliated to compute) and you push one object (say your sunglasses) then you act on that object by a force for only one moment. This is where you accelerate the object and it will then have some velocity relative to you. Because of this velocity, i.e. because of this small push for one moment, it will float away from you forever into deep space. – NikolajK May 3 '12 at 18:35
This object is for example a local gravitational field value 10 N / kg, and there is friction, everything the same way as on Earth, eg: A man pushing a box constantly and with constant speed on Earth – Zignd May 3 '12 at 18:44

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