# The motion of a spring

I have a question about the force set by this spring, I saw many times that $\overrightarrow{F}=-Kx\overrightarrow{i}$. I'm asking why not using $\overrightarrow{F}=Kx\overrightarrow{i}$ without the minus.

And supposing that $\overrightarrow{F}$ is changing during the motion of the solid object as well as the form of the spring.

Here $K$=the Hooke's constant, $K>0$.

P.S:I need a detailed explanation of this phenomenon.

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Did you notice how as positive $x$ is to the right, and the force illustrated points to the left? Hence the minus sign. –  ja72 Sep 10 '13 at 20:33

$x$ measures the difference in length of the spring in relation to its relaxed state. If you increase the length (positive $x$), the spring creates a force in the negative $x$ direction, because it wants to return to its relaxed state. Accordingly, if you compress the spring (negative $x$) the spring wants to expand (force in positive $x$ direction) in order to be relaxed again. That's why the negative sign in $$\vec F = - K x \vec i$$ gives the physically appropriate behaviour.

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If the force exerted by the spring on the attached object / the acceleration of the object is in the same direction as its displacement, you can imagine that the object will continue to go to infinity because there is no opposite force bringing the object back to the equilibrium position. Hence, the minus sign give us the sense that the acceleration of the object always opposes its displacement.

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Tie a spring to a wall and pull it. Do you feel a resistive force? Or do you feel a force that is somewhat helping the force that you are applying? In other words, Does the spring pull on you or push you away when you pull on it?

It pulls you, of course, and this is a direct result from Newton's third law, which in this case gives:

$$F_{\mbox{restorative}}=-F_{\mbox{applied}}$$

Similarly, for your question, the spring is being compressed, so it pushes back on the block.

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