What is the Initial Torque of a Rod with an Acting Force?

[shiby4444]

Homework Statement

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

Homework Equations

The Attempt at a Solution

 

[Doc Al]

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

 

[shiby4444]

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

I am not sure. Please help

 

[Doc Al]

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

 

[shiby4444]

What answer would that be with respect to my choices?

 

[Doc Al]

What answer would that be with respect to my choices?

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.