# Homework Help: Calculating angular acceleration and angular velocity

1. Dec 8, 2011

### Dave_1984

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

Calculate the angular acceleration and angular velocity of a 2kg object rotating in a
circle of 1.5m radius in a time of 3s.

2. Relevant equations

w=v/r
2*pie*r

3. The attempt at a solution

Hi all, every attempt so far has been a failure, I just can't work out how to calculate this, hours spent, Please help.

Thanks
Dave.

2. Dec 8, 2011

### Staff: Mentor

Is that a complete statement of the problem? Is it moving at constant speed? Or does it start from rest, by any chance?

What have you done so far?

3. Dec 8, 2011

### MrB8rPhysics

Correct me if I am wrong, but cant this be calculated pretty much directly assuming the body is at a constant speed? Really dont wanna overhelp here.

4. Dec 8, 2011

### Dave_1984

Hi, Yes that is the complete question so I guess it will start from rest if they are asking for the acceleration?

My course information provides very short explanitions and no examples of angular acceleration and angular velocity. (ICS)

As for what I have done so far - mainly just looking at it. I know for angular accelaration I need to use w=v/r If i'm honest Im not sure how to get v?

I'm doing quite well apart from this question so i'm sure i'm missing something simple?

5. Dec 8, 2011

### Staff: Mentor

If that's the word for word exact problem statement, it's very poorly worded. You shouldn't have to guess what they mean. (Please double check!)

That formula relates angular speed (ω) to tangential speed (v).

Here's how I interpret the problem, although this is just a guess: It starts from rest and travels, with constant acceleration, one complete circle in the given time.

What kinematic formulas for accelerated motion might you apply?

6. Dec 8, 2011

### MrB8rPhysics

Nah, it wont be at rest to start, its going to be a constant velocity. The term angular acceleration refers to the fact that its constantly changing direction, and hence velocity (which is a vector). As for finding this velocity...

Average speed = distance over time. You now need to consider how to find the distance the object travels in one rotation... :)

7. Dec 8, 2011

### Staff: Mentor

Not usually. Angular acceleration refers to the rate at which the angular velocity is changing. (And if the angular velocity is constant...)

8. Dec 8, 2011

### MrB8rPhysics

Yea agreed, I didnt word my explanation very well at all... :)

9. Dec 8, 2011

### Dave_1984

Hi yes that is the question word for word, it has definitely confused me.

I could use s = ut + 1/2at if it starts at rest that will give me the acceleration in ms-2

2 pi r= 9.42m

9.42 / 3 = 3.14ms-1
Thats about as far as I can go.

10. Dec 8, 2011

### Staff: Mentor

I think you mean s = ut + 1/2at2 (you left out the square). You can let s, u, and a represent angular quantities.

Angular speed has units of rad/s; angular acceleration has units of rad/s2.

That would be a calculation of average speed. (What would be the final speed?)

Use the formula above (modified for rotational quantities) to find the acceleration.

There are several ways to solve this. Just dive in.

11. Dec 9, 2011

### Dave_1984

Hi, this is the last question of my assessment and I have spent over 10 hours looking at it and I really dont understand it. (I have had no problem with the other 25)

I'm sure there is an easy and simple explanation as to where i'm going wrong? I can remember doing this in school and I'm sure it was not this h

Thanks
Dave.