# Cross product question

1. Jan 13, 2019

### sp3sp2sp

1. The problem statement, all variables and given/known data
Force F⃗ =−11j^N is exerted on a particle at r⃗ = (8i^+5j^)m.
What is the torque on the particle about the origin? Express your answer using two significant figures. Enter coordinates numerically separated by commas.

2. Relevant equations

3. The attempt at a solution

F: 0i, -11j, 0k
r: 8i, 5j, 0k

F X r = (0-0)i - (0-0)k + (0 - -88)j
= 0, 0, 88

this is wrong because it should be -88, but I dont see why? It was my understanding that I should plug values into formula i - j + k. Thanks for any help.

2. Jan 13, 2019

### PeroK

What's the definition of torque?

3. Jan 13, 2019

### Ray Vickson

In future, please avoid using symbols like j^N; in typewriter math, this means $j^N$, which is not what you intend. Just use j N, or perhaps enclose the "N" in parentheses, so write either F = - 11j N or F = -11j (N). Better still, use LaTeX to typeset your formulas and equations, so you could write $F = - 11\, j$ N or $F = -11\, j$ (N). You could even make them look like vectors by using a bold font, or an arrow on top, like this: $\mathbf{F} = -11 \, \mathbf{j}$ N or $\vec{F} = -11 \vec{j}$ N.

Anyway, back to your question. Is torque equal to $\mathbf{r \times F}$ or is it $\mathbf{F \times r}$?

4. Jan 13, 2019

### sp3sp2sp

Thanks for the replies . Torque is not communicative so torque = r X F
Its the cross product of the Force and the distance from pivot.

5. Jan 13, 2019

### sp3sp2sp

OK I think i get it now. it should be in form rXF which when you apply the i-j+k gives 0i, 0j, -88k
thanks for help

6. Jan 13, 2019

### haruspex

I am not familiar with such a formula. I assume it is some way to remember how to get the signs right, but if so I dislike it. It loses the symmetry.
It is really quite simple. The result is the next one around in cyclic order:
ixj->k
jxk->i
kxi->j
If you have to switch the inputs to get them in cyclic order, switch the sign too:
jxi->-k
kxj->-i
ixk->-j

7. Jan 13, 2019

### sp3sp2sp

Yup thats exactly what it was .. its not a formula, just way I was remembering it because it was confusing me. thanks for the help