# Homework Help: Measuring torque

1. Nov 26, 2014

### AFIST2

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

hi
i need to perform the experiment described in this link

so i need to measure the torque
in the experiment they use Torsion dynamometer to do the job
but i need to measure the torque without this device
how can i do this manually?

2. Relevant equations

τ = m × B = mBsinθ

3. The attempt at a solution

i really don't have a good idea how to perform this but i may suggest the following ways:

we may use the equation:
τ=Iα
α is the angular acceleration and I is the inertia moment
but i think there is a problem with this because the torque is not constant but it change with time
because the angle between the field and the moment changes in time
so i don't really understand how this is works (what τ exactly they measure?)

another way is to attach a spring to the loop so there is anothe torque (in the opposite direction) that acts on the loop
therefore at equilibrium the magnetic torque will be the same as the spring torque
m × B = kx × r
therfore the torque i need is
τ=kx × r
and these quantities i can measure

please give me your opinions or better ideas to measure the torque

Last edited by a moderator: May 7, 2017
2. Nov 26, 2014

### haruspex

The spring sounds good to me, but there are some practical difficulties.
The torque will be small, so you want a very light, stretchy spring. To prevent the loop twisting out of the vertical, I suggest two identical springs, one attached to each side of the loop. If the springs are ferrous, you'll need to make sure they are well outside the magnetic field, attached by long thin strings to the loop.
I assume you want multiple readings, at different rotation angles. You can move the springs closer/further away to adjust the balance point.

3. Nov 26, 2014

### AFIST2

thank you very much.. i will take your notes into consideration

4. Nov 26, 2014

### haruspex

Forgot to mention... you'll need to take into account the angle that the strings/springs make to the plane of the loop.

5. Nov 26, 2014

### AFIST2

you mean in the expression kx × r = kx⋅r⋅sinβ right?

6. Nov 26, 2014

Yes.