Let $\mathbf F_i$ be forces each of which is applied on $\mathbf r_i$ of a rigid body. Then is there a position vector $\mathbf r$ that satisfies

$$\displaystyle\sum_i\mathbf r_i\times\mathbf F_i =\mathbf r\times\displaystyle\sum_i\mathbf F_i~ ? \tag{1}$$

Well, what I get is by letting $\mathbf r_i=(r_{i,x},r_{i,y},r_{i,z})$, $\mathbf F_i=(F_{i,x},F_{i,y},F_{i,z})$, $\mathbf r=(r_x,r_y,r_z)$ is

$$\begin{bmatrix} 0 & \sum F_{i,z} & -\sum F_{i,y}\\ -\sum F_{i,z} & 0 & \sum F_{i,x}\\\sum F_{i,y} & -\sum F_{i,x} & 0 \end{bmatrix} \begin{bmatrix} r_x\\ r_y\\r_z \end{bmatrix}= \begin{bmatrix} \sum (r_{i,y}F_{i,z}-r_{i,z}F_{i,y})\\\sum (r_{i,z}F_{i,x}-r_{i,x}F_{i,z})\\\sum (r_{i,x}F_{i,y}-r_{i,y}F_{i,x}) \end{bmatrix}, \tag{2}$$

and $$\begin{vmatrix} 0 & \sum F_{i,z} & -\sum F_{i,y}\\ -\sum F_{i,z} & 0 & \sum F_{i,x}\\\sum F_{i,y} & -\sum F_{i,x} & 0 \end{vmatrix}=0. \tag{3}$$

Since the matrix is singular, the system might not have a unique solution.

So is it the case that generally such $\mathbf r$ may not be unique (or even nonexistant)? If so what is the criterion for uniqueness of $\mathbf r$?


2 Answers 2


Yes. The solution is:

$$ \bf{r} = \dfrac{\left( \sum {\bf F}_i\right) \times \left( \sum ({\bf r}_i \times {\bf F}_i) \right)} {\| \sum {\bf F}_i \|^2} =\dfrac{{\bf F} \times {\bf \tau}}{{\bf F}\cdot{\bf F}}$$

Then you can show that

$$ {\bf r}\times \left(\sum {\bf F}_i \right)= \sum ({\bf r}_i \times {\bf F}_i) = {\bf }\tau$$

Use ${\bf F} =\sum {\bf F}_i$ and ${\bf \tau} = \sum {\bf r}_i \times {\bf F}_i $ and the vector triple product $\vec{a}\times(\vec{b}\times\vec{c}) = \vec{b}(\vec{a}\cdot\vec{c})-\vec{c}(\vec{a}\cdot\vec{b})$

$$\begin{aligned} {\bf r}\times {\bf F} &= \left(\dfrac{{\bf F} \times {\bf \tau}}{{\bf F}\cdot{\bf F}}\right)\times {\bf F} \\ & = \dfrac{\left({\bf F} \times {\bf \tau}\right)\times {\bf F}}{{\bf F}\cdot{\bf F}} \\ & =- \dfrac{{\bf F}\times \left({\bf F} \times {\bf \tau}\right)}{{\bf F}\cdot{\bf F}} \\ & = - \dfrac{{\bf F} ({\bf F}\cdot{\bf \tau})-{\bf \tau} ( {\bf F}\cdot{\bf F})}{{\bf F}\cdot{\bf F}} \\ & = {\bf \tau} \end{aligned} $$ Since ${\bf F}\cdot{\bf \tau}=0$

See more details in this answer https://physics.stackexchange.com/a/70445/392. Each loading defines a screw axis, and screws can be combined linearly (addition of two screws is a screw, and a scalar times a screw is a screw). For the resultant screw you can extract its direction $\vec{e} = \frac{\bf F}{\| {\bf F}\|}$, its position $\vec{r} = \frac{{\bf F}\times{\bf \tau}}{{\bf F}\cdot{\bf F}}$ and its pitch $h=\frac{{\bf F}\cdot{\bf \tau}}{{\bf F}\cdot{\bf F}}$.

  • $\begingroup$ The notation $\Sigma\vec{F}^2_i$ in the first line is highly misleading, a reader might read it as the sum of the squares of the forces, when it is supposed to be the square of the sum of the forces. Furthermore, you assume $\vec{F}\cdot\vec{\tau}=0$, when the whole question was about whether there is a solution. $\endgroup$
    – Timaeus
    Mar 13, 2015 at 17:23
  • $\begingroup$ Do I need to add why ${\bf F}\cdot{\bf \tau}=0$ ? $\endgroup$ Mar 14, 2015 at 0:23
  • $\begingroup$ Since it is not always true, you should definitely "add it" $\endgroup$
    – Timaeus
    Mar 14, 2015 at 0:49
  • $\begingroup$ Well, that vector triple product thing seems very useful. Otherwise I would have needed to do it the brute-force way(i.e., expanding it to scalars) again to see if the answers correct. :) $\endgroup$
    – Allen
    Mar 14, 2015 at 15:42

Let me introduce the notation

$$\sum F_{i,x} = F_x, \ \ \ \sum F_{i,y} = F_y, \ \ \ \sum F_{i,z} = F_z, \tag{i}$$

Since the determinant is zero, there may be indeed, no solution of the system. But if the system of equations has a solution, recall that the body doesn't rotate around a point, but around an axis. So, your $\vec r$ is bound to be on an direction perpendicular to the resultant force $\vec F$ and on the rotation axis. A direction in the space may be described by a simple system of equations, e.g.

$$\begin{cases} {y = a x + b} \\ {z = a' x + b'} \end{cases}. \tag{ii}$$

Now, for simplicity let's introduce one more notation

$$\begin{cases} {\sum (r_{i,y}F_{i,z}-r_{i,z}F_{i,y}) = \tau_x} \\ {\sum (r_{i,z}F_{i,x}-r_{i,x}F_{i,z}) = \tau_y} \\ {\sum (r_{i,x}F_{i,y}-r_{i,y}F_{i,x}) = \tau_z} \end{cases}, \tag{iii}$$

From your system $(2)$ and using the notations $\text {(i)}$ and $\text {(iii)}$ one obtains three equations, two of which of the form $\text {(ii)}$.

Well, for this system of equations to have a solution, between the constants $\tau_x, \tau_y, \tau_z$ has to be a linear and homogeneous relation. From now on the job is yours. Find the relation.

  • $\begingroup$ Doing the maths gives me $F_x\tau_x +F_y\tau_y +F_z\tau_z = 0 $ assuming $F_x\neq0$ or $F_y\neq0$ or $F_z\neq0$ ,which is in vectors, $\mathbf F\cdot\boldsymbol{\tau}=\mathbf 0$ $(\mathbf F\neq\mathbf 0) $. So is it correct to say that if this is the case then there are infinitely many solutions, if else then none? $\endgroup$
    – Allen
    Mar 12, 2015 at 15:04
  • $\begingroup$ @Allen What you obtained is that the torque $\tau$ is perpendicular to $\vec F$, as expected from your relation (1). Therefore, if the total force $\vec F$ is not zero, and also the torque $\vec \tau$ is not zero, the solution for $\vec r$ is possible. That means that there exists a unique line $\text {(ii)}$. However, the vector $\vec r$ is not unique, it may connect the origin with whatever point on this line. A special vector $\vec r$ among all these vectors, would be the one perp. on both $\vec F$ and $\vec \tau$. I would recommend you to calculate explicitly this line i.e. $a, b, a', b'$. $\endgroup$
    – Sofia
    Mar 12, 2015 at 15:32
  • $\begingroup$ Calculation gives me $\mathbf r= t\mathbf F+\frac{\mathbf F\times\boldsymbol{\tau}}{F^2} (t\in\mathbb R)$ for $\mathbf F\cdot\boldsymbol{\tau}=0$. Seems like a brute-force way to get some simple solution. $\endgroup$
    – Allen
    Mar 13, 2015 at 14:24
  • $\begingroup$ @Allen Who is $t$? You maybe mean $\tau$? The result I got was something like $\mathbf r = \frac {1}{F^2} \left[ \mathbf F (\mathbf F \cdot \mathbf r) + \mathbf F \times \mathbf {\tau} \right]$. $\endgroup$
    – Sofia
    Mar 13, 2015 at 14:38
  • $\begingroup$ It's an arbitrary real number as stated. Since $\mathbf r= \frac{F\times\boldsymbol{\tau}}{F^2}$ is a solution (it's the 'special' perpendicular one), so should $\mathbf r= t\mathbf F+\frac{\mathbf F\times\boldsymbol{\tau}}{F^2}$ be one for $\forall t\in\mathbb R$, for $t\mathbf F\times\mathbf F=0$. It is indeed a unique line whose directional vector is $\mathbf F$ and passes through $\frac{\mathbf F\times\boldsymbol{\tau}}{F^2}$. $\endgroup$
    – Allen
    Mar 14, 2015 at 15:13

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