# Homework Help: Centripetal Acceleration check my reasoning

1. Sep 13, 2009

### pointintime

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

ok well then...

we did a lab were you have a hanging mass attached to a stirng that went through a straw and was attached to a rubber stopper. The lab was to find the mass of the rubber stopper once you know the velocity. So I was woundering if this looks correct

2. Relevant equations

f = ma
m1 = mass of stopper
m2 = mass of hanging mass

3. The attempt at a solution

net force radial = m (acceleration radial) = force of gravity on mass m2
net force radial = (m1 + m2) (acceleration radial) = m2 g

divided both sides by acceleration radial

m1 = (acceleration radial)^-1 ( m2 g - m2 (acceleration radial) )

at which point I plugged in the value I found for the radius
the value I found for the period

2. Sep 13, 2009

### pointintime

ok hmmm

m2 g = m1 (acceleration radial)

so then if this is the correct equation my question... is how do you know when to consider the masses as a system and when not to?

net force = mass times acceleration

by deffintion
it dosen\'t mean this necessarily

net force = net mass times acceleration

when do I know when I should use the net mass and when not to as I have gotten points taken off before because i didn\'t use the net mass and am not sure when to consider the masses as one system and when not to...

like how come this is wrong

net force radial = (m1 + m2) (acceleration radial) = force of gravity on m2

and this is right

net force radial = m1 (acceleration radial) = force of gravity on m2

I need help on when to make this distinction between the masses and when not to as I don\'t know when i should...

THANKS!!!
This has been eating me away and I need to know this or the whole problem is wrong so it\'s important that I know when to make this distinction and when not to. So can you tell me when I should and when I shouldn\'t???

3. Sep 13, 2009

### pgardn

M1g = Msv^2/r

M1 was your known mass hanging. g is the acceleration due to gravity. Ms is the mass of your rubber stopper. v is the speed of your rubber stopper and r was the length of the string from the straw to the rubber stopper (the radius of the circle the rubber stopper was making around the straw)

I guess you counted how many times it took to make a certain number of revolutions in a certain amount of time. 1 rev. =2 x pi x r. Divide the number of revolutions by the time in seconds it took to make those revolutions and you got v. At this point you know everything but Ms so put all the numbers in and solve for Ms

4. Sep 14, 2009

### pointintime

can you answer my question in #3

5. Sep 14, 2009