Tangential acceleration and centripetal acceleration

[siapola1]

a disc or radius r = 16cm starts spinning from rest with a uniform angular acceleration of 8.0 rad/s^2. at what time is its tangential acceleration twice the centripetal acceleration.

i figured out the tangential acceleration is:
A_tan = α/R = 8 / .16 = 50 m/s^2

and the centripetal acceleration is :
2(α*R) = 2.56 m/s^2then i got stuck. will the centripetal increase as its tangential acceleration increase?
please help

 

[haruspex]

A_tan = α/R

Not a good start.

2(α*R) = 2.56 m/s^2

That's a better attempt at the tangential acceleration, but not quite right.
Can you find a correct formula for centripetal?

 

[siapola1]

Not a good start.

So can you help me please?

 

[haruspex]

So can you help me please?

Don't you have any notes or textbook that give you the relationship between angular and tangential accelerations?
What about angular and tangential velocities? Angular and tangential displacements?

 

[siapola1]

I wasnt in class that day and homework are usually much harder than lectures.
He gave is us this formula in class

Centripetal acc. = R * √ (ω⁴ + α²)
But I don't know how to use it in this problem

 

[haruspex]

I wasnt in class that day and homework are usually much harder than lectures.
He gave is us this formula in class

Centripetal acc. = R * √ (ω⁴ + α²)
But I don't know how to use it in this problem

Oh. That's not helpful at all. In fact it is wrong.
If the angular acceleration is α, radius r, then the tangential acceleration is αr.
Likewise, if a disc is rotated by angle θ then a point on its rim travels an arc length θr, and if rotating at rate ω then the tangential velocity is ωr.
The centripetal acceleration is that component of the total acceleration which is normal to the velocity. For a rotating disc it is pointing towards the disc's centre. Its magnitude is v²/r = rω².
The centripetal and tangential accelerations are at right angles, so the total acceleration has magnitude √((rω²)²+(αr)²) = r√(ω⁴+α²).