# Rotating disk find the radial and transverse velocity

1. Oct 13, 2016

### nysnacc

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

2. Relevant equations

rXF ??
3. The attempt at a solution

T= 6 N (r =1)
T = 12 N (r=2)

2. Oct 13, 2016

### Simon Bridge

So far so good - do you have a question?
Please describe how you are thinking about the problem ... given the initial condition, what do you think will happen?
Why is it important that the string is elastic (how would you model the string?)

3. Oct 13, 2016

### andrevdh

There is a lot going on in this problem and we have to make some (hopefully correct assumptions).
First the disk is spinning about the centre at 1 meter with a tangential speed of 4 m/s.
Then it motions out to 2 meter and they want both the radial and tangential speed when it is spinning about the centre at this distance.
What comes to mind is maybe trying energy conservation?
Energy is stored in the elastic as it is stretched, thankfully it obeys Hooke's law!
Also the tensions at the various radii supplies the centripetal forces for the spinning motion.

Last edited: Oct 13, 2016
4. Oct 13, 2016

### nysnacc

I mean what does those contribute to the finding of v?

Can I just use F= m V2 / r which i put F = T at 0.25m, and r = 0.25m ... so that V is the Vtheta??

5. Oct 13, 2016

### kuruman

Conserving mechanical energy is a good starting point and gives you one equation. You need to find two quantities so you need a second equation. What else is conserved? Hint: The force on the mass is along the radius.

6. Oct 13, 2016

### Simon Bridge

mv^2/r is the centripetal force needed to keep the disk in circular motion... what could be providing the centripetal force, and does it provide that much?
Note: the other two are jumping ahead. You need to understand the motion before deciding on what laws to use, and what assumptions are valid.
Nysnac's assumptions may not be valid, and how kuruman's modification works will depend on that.