# What is the initial torque

1. Oct 18, 2009

### shiby4444

1. The problem statement, all variables and given/known data

A rod is initially along the y axis and has
length ℓ. A force F =ˆı F1 +ˆj F2 + ˆk F3
acts at the upper end of the rod.
What is the initial torque ~τ about the lower
end of the rod?
1. Zero
2. ˆıF3 ℓ
3. −(ˆı F3 +ˆj F2 + ˆk F1) ℓ
4. (ˆıF3 − ˆk F1) ℓ
5. (ˆıF1 +ˆj F2 +ˆk F3) ℓ

2. Relevant equations

3. The attempt at a solution

2. Oct 18, 2009

### Staff: Mentor

Re: Torque

Hint: What's the definition of torque in terms of a cross product?

3. Oct 18, 2009

### shiby4444

Re: Torque

4. Oct 18, 2009

### Staff: Mentor

Re: Torque

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

5. Oct 18, 2009

### shiby4444

Re: Torque

What answer would that be with respect to my choices?

6. Oct 18, 2009

### Staff: Mentor

Re: Torque

That's for you to figure out! All I did was give a definition of torque. Hint: Find an expression for $\vec{r}$ (its magnitude is ℓ) and perform the cross product.