Finding revolution time at surface

[ross moldvoer]

Homework Statement

For a 0.90km radius cylinder, find the time for one revolution if "gravity" at the surface is to be 9.8 m/s2.

Homework Equations

rω^2=a

The Attempt at a Solution

i tried solving for omega but i couldn't find a solution that i only had 1 variable in it.

 

[haruspex]

Homework Statement

For a 0.90km radius cylinder, find the time for one revolution if "gravity" at the surface is to be 9.8 m/s2.

Homework Equations

rω^2=a

The Attempt at a Solution

i tried solving for omega but i couldn't find a solution that i only had 1 variable in it.

Which other variable do you think you have no value for?

 

[ross moldvoer]

Which other variable do you think you have no value for?

i don't know the velocity for solving for omega, is this what you meant?

 

[gneill]

ω is a velocity: an angular velocity. How is ω related to the frequency and the period of one "cycle" (one revolution in this case)?

 

[haruspex]

i don't know the velocity for solving for omega, is this what you meant?

I don't know where you are stuck. The first step is to calculate ω - can you do that from the formula you quoted? Next, as gneill says, you can find the time of one revolution from ω. Is that part your problem?

 

[ross moldvoer]

i know that ω = v/r but i do not know v. I guess i am confused about what to do in order to solve for ω.

 

[haruspex]

i know that ω = v/r but i do not know v. I guess i am confused about what to do in order to solve for ω.

In the OP you quoted the right equation to be using. That has three variables and you are given the values of two of them. Forget v for now.

 

[ross moldvoer]

wow can't believe i missed that. so then 450ω^2=9.8, ω=.133
i converted .9 km to 900 m

 

[haruspex]

wow can't believe i missed that. so then 450ω^2=9.8, ω=.133
i converted .9 km to 900 m

Ok, but why did you halve it to 450? Either way, I don't get .133.

 

[ross moldvoer]

Ok, but why did you halve it to 450? Either way, I don't get .133.

the cylinder's diameter is 900m and i need the radius. 900/2=450. then i did 9.8/450=ω^2=.022, ω=.147
(i had a math error in the previous post, this is what i actually got)

 

[haruspex]

the cylinder's diameter is 900m

It says radius.

 

[ross moldvoer]

It says radius.

my bad

then i get .104

 

[haruspex]

my bad

then i get .104

Sounds right.

 

[ross moldvoer]

so after this do i just plug it into the equation? the answer i get seems high

 

[haruspex]

so after this do i just plug it into the equation? the answer i get seems high

You know what I'm going to say, surely? Please post your working and answer.