# Homework Help: Help with a angular velocity question

1. Jan 25, 2008

### someboy

hi i am a beginner at physics. my teacher just gave me this problem for a make up work but he did not give me any equations on solving this problem due to some misunderstanding he got mad at me. so here i am stuck..... please help!!

A discus thrower (arm length 1.2 m) starts from rest and begins to rotate
counterclockwise with a constant angular acceleration of size 2.7 rad/s2.
a. How long does it take for the discus thrower's angular speed to reach 5.7
b. How many revolutions does it take for the discus thrower's angular speed to
c. What is the linear speed of the discus at 5.7 rad/s?
d. What is the linear acceleration of the discus at this point?
e.What is the size of the discus's centripetal acceleration?
f. What is the size of the discus's total acceleration?

i searched all over the internet and i still can't find any equations that applys to my questions here is the only question i found.. i don't even know how to use this equation on solving those problems..

O=O(initial)+ W(intial)T + 1/2 XT(square)

please begging on my knees for some help on solving this problem!!

any helps would be appreciated!!

2. Jan 25, 2008

### physixguru

here the arm length becomes the radius of rotation..

initial angular velocity=0
>>5.7=0+2.7 *t

calculate t...

for next part=1 revolution=2 pi rad..

for next part..

for next part..
linear acceleration=linear velocity/time

for next part..

total accln=[centripetal accleration^2+tangential accln^]^1/2

tangential accln=linear accln=v/t

3. Jan 25, 2008

### physixguru

are ya now able to solve it???

4. Jan 25, 2008

### someboy

well is there also a equation for linear velocity and angular velocity?

because i need to show work i am trying to write out the equation in letters from the helps you provided.

thank you very much!!!

5. Jan 25, 2008

### someboy

i need a little more help how do i get the linear velocity and angular velocity is?

6. Jan 25, 2008

### Go Angels

I'm having trouble on a similar problem, involving tangetial acceleration and radial acceleration; which I guess equates to linear and angular. Sorry I can't help you.

Btw, what is the name of your textbook?

7. Jan 25, 2008

### someboy

it is call the physics of everyday phenomena ... it is basically uselesss it doesn't give the equation at all... only explains stuff

8. Jan 25, 2008

### Go Angels

Hmmm, my textbook doesn't have anything about "angular velocity." But I think linear velocity is just the normal velocity equation of v = x/t. Don't take my word for it though. I'm horrible at physics.

9. Jan 25, 2008

### someboy

yeah i am very horrible at physics but i waited for 1 hour infront of my couselor's officer and she won't even let me drop one class >_<. i still got finals tomorrow ! i guess.. i will go now... hope you can somehow solve your problems too GO angels...

really appreciate your help physixguru! you are my live saver!

10. Jan 25, 2008

### Go Angels

Wow finals. Best of luck to you!

11. Jan 25, 2008