Follow up on Barlow's wheel

1. Jun 21, 2015

NooDota

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

In the Barlow's wheel experiment, let's say I want to find the torque of the forces that affect it. (Friction is ignored)

I have W (weight) has no torque
R (reaction) also has no torque
F (the force that moves the wheel) does have torque

I can calculate F's torque individually.

Now my question is, if I use ΣΓ = I*α, I'll get 0, because the angular acceleration is 0 since the angular velocity is constant.

Why do I get 0 when I do have torque?

Thanks

2. Relevant equations

ΣΓ = I*α

3. The attempt at a solution

Last edited: Jun 21, 2015
2. Jun 21, 2015

Staff: Mentor

Without friction, the wheel is constantly accelerating (unless your current breaks down at some unrealistic speed).

3. Jun 21, 2015

NooDota

But alpha in this case is 0 because the velocity is constant, no?

4. Jun 21, 2015

Staff: Mentor

Why do you expect the velocity to be constant in the absence of friction?

5. Jun 21, 2015

NooDota

Because the force that's causing the velocity is constant? Neither the current, the radius or the magnetic field are changing.

6. Jun 21, 2015

Staff: Mentor

Which force?
Without friction, you have an accelerating force (more precise: torque), so the wheel keeps accelerating.

7. Jun 21, 2015

NooDota

I don't know what you call it in English. The F = I*L∧B force.

It's what's moving the wheel, and it's constant, and so the angular velocity is constant and the acceleration is 0, since the velocity isn't changing.

8. Jun 21, 2015

Staff: Mentor

No, a constant force leads to a constant acceleration.

What you actually need is the integral $\int_0^L I B r dr$ for the torque, but that follows the same rules.