# Constant angular acceleration

## Homework Equations

? constant acceleration equations I guess

## The Attempt at a Solution

They didn't get to this part in class today, yet it's expected on our online homework due tonight. No idea how to even get started.

## Answers and Replies

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rock.freak667
Homework Helper
For these questions you can consider conservation of energy.

PE + KE = constant.

For these questions you can consider conservation of energy.

PE + KE = constant.
jtw2e: Your post title makes my think you might not yet have been introduced to rotational kinetic energy. For an alternate approach: consider the relationship between how fast the block lands and how fast the disk rotates. Do you see that you can work out the torque acting on the disk and find the rotational speed?

jtw2e: Your post title makes my think you might not yet have been introduced to rotational kinetic energy. For an alternate approach: consider the relationship between how fast the block lands and how fast the disk rotates. Do you see that you can work out the torque acting on the disk and find the rotational speed?
Thank you. As of today we began rotational kinetic energy but did not get very much covered in the topic. We are supposed to begin torque on Friday.

For these questions you can consider conservation of energy.

PE + KE = constant.
While I would like to use CoE, we're supposed to find our answers with rotational constant acceleration approach. I've actually been doing my other homework with CoE just to get the answers turned in. I don't know how to find them with this rotational stuff.

jhae2.718
Gold Member
If you don't go with the energy approach, you'll want to find the angular acceleration of the pulley system.

You can use these equations:
\begin{align} \vec{\tau} &= \vec{r} \times \vec{F} \\ \vec{\tau}_{net} &= I\vec{\alpha}_{net} \end{align}
In these equations, $\vec{\tau}$ is torque, I is moment of inertia, $\vec{\alpha}$ is angular acceleration, and r is the distance vector from the axis of rotation to the force F.

These are the rotational analogues to force and Newton's second law. You can use Eq. (2) on the pulley system to find the net angular acceleration, which will give the system's acceleration. To find the linear acceleration of the block, $\vec{a} = r\vec{\alpha}$.