# Homework Help: Impossible Physx Homework

1. Oct 1, 2006

### thebigeis

Impossible Physx Homework!!

I have a few problems left that I'm completely stumped on how to do.

#1) A typical lab centrifuge rotates at 4000rpm. Test tubes have to be placed into a centrifugre very carefully because of the very large aceelerations. What is the acceleration at the end of a test tube that is 10cm from the axis of rotation in?

#2) A 5m-diameter merry-go-round is initially turning with a 4s period. It slow down and stops in 20s. Before slowing what is the speed of a child on the rim? How many revs does the merry-go-round make as it stops?

Out of my 6, these trouble me the most. For #1, I made the radius 10cm, the velocity constant, but am unsure what equations I need to plug those in. As for #2, it's thet same deal. In one, I have the period, and in the other I'm left with the velocity, and with both I have the radius, but have no idea where to put them. Please, help with a point in the right direction.

2. Oct 1, 2006

### grant9076

The accelleration is equal to the tangential velocity squared divided by the radius (v^2/r). If you multiply the radius (.1m) by 2*Pi, you will get the distance traveled in one revolution. You then multiply this distance by 4000 rpm to get the velocity in meters/minute and divide by 60 to get the velocity in meters per second. Since you now have the radius, and the velocity, you can now square the velocity and divide by the radius to get the accelleration. I can tell you right away that the accelleration is going to be very high.