3D Equilibrium of a rigid body

**Oblivionwarrior** · November 1st 2008, 03:44 PM

I am stuck on this question

I generally know how to solve the problem but the force is throwing me off. Since it is parallel to the XY plane there will no be k component of the force. But how should I about breaking the 970N force into i,j components? Thanks.

**TriKri** · November 3rd 2008, 09:57 AM

The distance from $O$ to the point where $\vec{F}$ takes effect is $\vec{r} = (250,\ 200,\ -150)\ mm$ . $\vec{F}$ is in the x-y plane, so $\vec{F}=(F_x,\ F_y,\ 0)$ . The momentum will then be

$\vec{M}=\vec{r}\times\vec{F}$

Now, we probably want to charge the support at $O$ as much as possible, so we set $|\vec{F}| = 970\ N$ . Besides, we want $\vec{r}$ and $\vec{F}$ to be orthogonal to each other, so

$\vec{r}\cdot\vec{F} = 0 \Leftrightarrow 250 F_x + 200 F_y = 0 \Leftrightarrow F_y = -\frac{5}{4}F_x$

$F_z = 0$ , so $\vec{F}$ is already in the x-y plane.

So we could parametrisize $\vec{F}$ and write $\vec{F} = t\cdot(4,\ -5,\ 0)$ , implying that $|\vec{F}| = \sqrt{41}\ |t|$ , which means that $\vec{F}$ could have basically any non-negative length.

Now we know that there exist $\vec{F}$ with the in the x-y plane with the length 970 N, orthogonal to $\vec{r}$ . Let's use that in our momentum formula:

$\vec{M} = \vec{r}\times\vec{F} \Rightarrow$

$|\vec{M}| = |\vec{r}\times\vec{F}| = |\vec{r}|\cdot|\vec{F}|\cdot\sin(\alpha) = \sqrt{250^2+200^2+(-150)^2}\ mm\ \cdot\ 970\ N\cdot\ \sin(\alpha) =$ $\frac{485}{\sqrt{2}}\ Nm\cdot\sin(\alpha)$ , where $\alpha$ is the angle between $\vec{r}$ and $\vec{F}$

Now, since $\vec{r}$ and $\vec{F}$ are orthogonal, $\sin(\alpha) = 1$ , so $|\vec{M}| = \frac{485}{\sqrt{2}}\ Nm$ , which should be the maximum magnitude of the momentum.

Alternativelly, you could turn $\vec{F}$ into poolar coordinates:

$F_x=970\ N\cdot\cos(\varphi)$
$F_y=970\ N\cdot\sin(\varphi)$
$F_z=0$

perform the cross product between $\vec{r}$ and $\vec{F} = 970\ N\cdot(\cos(\varphi),\ \sin(\varphi),\ 0)$ to get the momentum, extract the magnitude of the momentum (the vector lenth), and analyse the expression to se which $\varphi$ that gives the greatest magnitude of the momentum or to directly extract the greatest value of the expression.

**Oblivionwarrior** · November 3rd 2008, 03:55 PM

I got the answer, thanks man

**TriKri** · November 3rd 2008, 09:31 PM

I'm studuing mechanics myself right now.

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