Angular acceleration homework problem

[Dr_bug]

Homework Statement

What is the tangential acceleration of a bug on the rim of a 8.0 in. diameter disk if the disk moves from rest to an angular speed of 75 revolutions per minute in 5.0 s?
(b) When the disk is at its final speed, what is the tangential velocity of the bug?

(c) One second after the bug starts from rest, what is its tangential acceleration?

What is its centripetal acceleration?

What is its total acceleration (magnitude and angle relative to the tangential acceleration) ?

Homework Equations

atan= R*angular acceleration
v= R*w
ac=R*w^2

The Attempt at a Solution

I was able to answer a, b, and the first part of c but I can't get the centripetal acceleration . For (a) I got .1596 m/sec^2 (b) .79796 m/ sec (c) the atan after 1 sec is the same as (a). To find the ac I used the third equation but didn't get the right answer... is that the right equation to use to find centripetal acceleration? I converted 4 in into .1016 m and used that for R and then multiplied it by 7.85 rad/sec^s... i could use some help

 

[Delphi51]

I don't agree with your (a) and (b) answers . . . can you show the calcs?
Once you have the v value from (b) it should be easy to find a = v²/r.

 

[Dr_bug]

sure (a):
(4 in* 2.54 cm*75 rev*2pi rad)/(100 cm*60 sec*5 sec)= .1596 m/sec^2 -- I know its right because I already submitted it online and it accepted the answer.
(b):
(4 in* 2.54 cm*75 rev*2pi rad)/(100 cm*60 sec)= .79796 m/s -- this was also accepted

 

[Delphi51]

Oh, I didn't notice the inches! Agree.

 

[Dr_bug]

so should I then take .79796 and square it and then divide by .1016 (thats the radius converted to meters)

 

[Delphi51]

Right!