# Homework Help: Vector Cross Products

1. Nov 23, 2005

### Impathy

I have a two-part problem. The first part gve me a position vector r and a Force vector F acting upon r and I needed to find the magnitude of the torque about the origin. I took rxF and got the correct answer. Part two asks me to find the torque about the a new point (a,b). So instead of r rotating about the origin, it is now rotating about (a,b)? I was okay with the first part, but I just don't understand what's going on with the second part and how to attempt it. I was thinking of a few things regarding the distance between the origin and (a,b) and crossing that with F ... I just don't know what to do ... help! :D Thanks in advance, you guys are always terrific!

2. Nov 23, 2005

### HallsofIvy

If the position vector (the position relative to the origin) is <x,y>, then the position relative to point (a,b) is the vector <x-a, y-b>. Use that instead of vector r.

3. Nov 23, 2005

### Impathy

Alright ... do I just do that to the position vector, or do I do it to the force vector as well and then cross product the new position and new force? Or do I just use the new position and the old force?

4. Nov 23, 2005

### Impathy

Actually, it would make sense to use the "new" position and the "old" force, I suppose, right?

5. Nov 23, 2005

### vaishakh

While finding force you have a force vector, then the vector r, is the position vector drawn to the point at which the force vector is acting on the body with respect to a given point about which the object is rotating. since the point of rotation has changed, the position vector has changed. you must know to give the position vector in notations once the tail and the head points are given.
If you know this fact your doubt is meaningless and if you don't know, you are not good enough to do a torque problem. please read theory parts given in your book before starting to solve a problem. that will be more a healthier habit.