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Consider I have a simple formula for the work along some path (in 1 dimension):

$$W~=~\int_{x_0}^{x_1}\vec{F}\cdot d\vec{x}.$$

If I now move from left to right ($x_1 > x_0$) along the axis (assuming the X-axis points right) it is obvious that (using unit vectors along the axis):

$\vec{F}=F\vec{i}$

$\vec{x}=x\vec{i} \ \ => \ \ d\vec{x}=dx\vec{i}$

$\vec{F}d\vec{x} = (F\vec{i})(dx\vec{i})=Fdx$

Integration gives (assuming constant force along path for simplicity):

$\int_{x_0}^{x_1}\vec{F}d\vec{x} = \int_{x_0}^{x_1}Fdx = F \int_{x_0}^{x_1}dx = F(x_1-x_0) > 0$

Consider now I move from right to left ($x_1 < x_0$) but the force is also reversed, then:

$\vec{F}=-F\vec{i}$

$\vec{x}=-x\vec{i} \ \ => \ \ d\vec{x}=-dx\vec{i}$

$\vec{F}d\vec{x} = (-F\vec{i})(-dx\vec{i})=Fdx$

$\int_{x_0}^{x_1}\vec{F}d\vec{x} = \int_{x_0}^{x_1}Fdx = F \int_{x_0}^{x_1}dx = F(x_1-x_0) < 0$

Obviously, there is some terrible mistake here in my reasoning... If we move from one point to another along the force direction the work must be positive.

Qmechanic
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rk85
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  • I believe in the second integral the limits should switch places, shoudl they not? EDIT: Wait, hold on, i am reading it once more. – DakkVader Nov 11 '18 at 09:43
  • But this integral does include orientation, this it should always be from $x_0$ to $x_1$. I believe. – rk85 Nov 11 '18 at 09:45
  • I caught that as well, i am rereading it. As you say, something could be wrong here, we just need to find it – DakkVader Nov 11 '18 at 09:46
  • The limits on the integral tell you the direction of motion. Don't put a sign on the $dx$. – Bill N Nov 19 '18 at 15:33

2 Answers2

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Your mistake is in thinking that $dx$ doesn't account for the direction of motion. More explicitly, $$d\boldsymbol{x} \neq -dx \hat{\boldsymbol{i}}$$ in the second scenario. Instead, $$d\boldsymbol{x} = dx \hat{\boldsymbol{i}}$$ as in the first case.

  • Thanks. But in the first case I move from left to right, in the second one, from right to left, thus the sign should change. Is that correct? In fact in seems it doesn't but why? Thanks in advance. – rk85 Nov 11 '18 at 10:06
  • @rk85 It follows from the definition of a Riemann integral. Revisit it. –  Nov 11 '18 at 10:17
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    @rk85 PiKindOfGuy is correct. The quantity dx always points in the positive x direction. Just look at that quantity itself. When x increases it always increases in the positive x direction, regardless of the limits of integration. By flipping the sign you are doing a change of variable, which would require you to change your limits of integration in the normal fashion. – Dale Nov 11 '18 at 10:21
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This answer amplifies @PiKindOfGuy's answer and @Dale's comment.

I think there is a confusion in the use and/or interpretation of $d\vec x$

  • as an infinitesimal element of a directed path and
  • as an increment of the x-coordinate.

Let's use $d\vec s$ for the infinitesimal element of a path.
Let P and Q refer to start and end of the path.

So, $$W=\int_P^Q \vec F\cdot d\vec s$$

Before I begin, I want to emphasize that a conservative Force $\vec F$ is generally independent of the path (and its direction) of the displacement. For instance, the gravitational force always points down... but the path could be arbitrary.

In your example, when you turn, you are changing the direction of the force and the displacement... let's say forward from P to Q then backward to R, where $x_P ,x_R < x_Q$.

Along a horizontal displacement and constant magnitude force ...with no mention of any directions... \begin{align} W_{AB} &=\int_A^B \vec F\cdot d\vec s\\ &=\int_A^B (F_x \hat i + F_y \hat j) \cdot (dx\ \hat i)\\ &=\int_{x_A}^{x_B} F_x dx\\ &=F_x (x_B-x_A)\\ \end{align}

So, for the forward force $F_x=F$ along the forward path (displacement $(x_Q-x_P)>0$), we have $$W_{PQ}= (F )(x_Q-x_P)>0.$$

For the backward force $F_x=-F$ along the backward path (displacement $(x_R-x_Q)<0$), we have $$W_{QR}= (-F)(x_R-x_Q)>0.$$

Note that the lower limit and upper limit refer to the start and end of the oriented path... the limits control the orientation of the path.

Note the lower limit and the upper limits are not necessarily the smaller and larger of x-coordinates.

robphy
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