# A block spins around a circle with thrust

oneamp

## Homework Statement

A 400g steel block rotates on a steel table while attached to a 1.20m -long hollow tube. Compressed air fed through the tube and ejected from a nozzle on the back of the block exerts a thrust force of 4.71N perpendicular to the tube. The maximum tension the tube can withstand without breaking is 60.0N . Assume the coefficient of kinetic friction between steel block and steel table is 0.60. (Figure 1)

If the block starts from rest, how many revolutions does it make before the tube breaks?

## The Attempt at a Solution

Fbreak = 60 N = mv^2/r → v = sqrt(60r/m) m/s = 180 m/s
a = F/m m/s^2 = 11.775 m/s^2
t = v/a = 15.2866242
s = at^2/2 = 1372.875167
theta = s/r = 1372.875167
revs = theta/(2pi) = 218.499...

Where am I going wrong? I only get a couple of more attempts then the question is locked out..... please help.

Thanks

## Answers and Replies

Homework Helper
Gold Member
Hello, oneamp.

In the first line of your solution, did you forget to take the square root?

Homework Helper
Gold Member
Also, did you take into account the friction?

oneamp
Yes those are both mistakes I made, thank you. Now I am trying to figure out where to put friction...

Homework Helper
Gold Member
Now I am trying to figure out where to put friction...

Think about the direction of the friction force.

oneamp
Ok I got it! Thank you very much for pointing those things out to me!