# Torque question (qualitative)

1. Jun 3, 2016

### pennywise1234

1. The problem statement
what role does torque play in body stability and ergonomics ?

2. Jun 3, 2016

### bananabandana

I'm not sure what level of explanation you want... Most simply, torque is a turning moment. The idea of torque is that if you apply a force at a greater distance, you get a bigger rotation - it would be easier to swing a cat by its tail than its middle (if you wished to swing cats), and it requires less effort to open a door from the edge, rather than at the pivot.

If you want a mathematical definition:

$$\tau_{Total} = \sum_{i} \tau_{i} = \sum_{i} \vec{r_{i}} \times \vec{F_{i}}$$

Where $\tau_{Total}$ is the total torque on the body, and is the sum of a number of torques, $\tau_{i} = \vec{r_{i}} \times \vec{F_{i}}$ where, $\vec{r_{i}}$ is the position vector, and $\vec{F_{i}}$ is the force.

The angular momentum of a body is related to the torque applied:

$$\vec{L} = \vec{r} \times \vec{p}$$

$$\frac{d\vec{L}}{dt} = \frac{d\vec{r}}{dt} \times \vec{p} + \vec{r} \times \frac{d\vec{p}}{dt}$$

So

$$\frac{d\vec{L}}{dt} = \vec{r} \times \vec{F}$$

i.e -if we don't have any torque, then the angular momentum must be constant in time. The body cannot be spinning up or spinning down - in cannot be changing its motion in any rotational sense. So in this way, we see that a necessary (but not sufficient! - e.g we also have to balance forces) condition for equilibrium is that the total torques on a body must be zero...

Does that help at all?

3. Jun 3, 2016

### CWatters

Penny... have a think about the joints in the human body. I think all (?) Involve rotation.

4. Jun 3, 2016

### pennywise1234

The question is more geared towards when trying to improve stability or how do stability and torque correlate. I was thinking it had to do with Centre of mass or base of support. I have no idea what could be said for ergonomics