It takes 0.225 seconds/revolution for a body of mass of 25 grams to rotate a circle with a radius of 15 cm. What is the centripetal force acting on that body?

- anonymous

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- welshfella

force = m w r^2
F is in Newtons . mass in kgs and r in meters
w = the angular velocity in radians / sec
you can work this out from the given speed in revs / sec.

- anonymous

how can I convert seconds/revolution to radians/sec?

- welshfella

1 rev = 2 pi radians
it travels 2 pi radians in 0.225 secs
so in 1 second it travels 2 pi / 0.225 radians

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## More answers

- welshfella

w = 8.89 pi rads/sec

- anonymous

5x10^(-3) ?

- welshfella

now you need to convert 25 gms to kgs and 15 cms to meters.

- welshfella

what is 5x10^-3 ?

- anonymous

the answer for the centripetal force?

- welshfella

hmm..

- welshfella

not quite - you need to multiply that by pi

- anonymous

sorry i forgot to put pi, is it 0.0157?

- welshfella

yes
thats in newtons

- welshfella

you can write it as 1.57 * 10^-2 N

- anonymous

the answer is not in the choices :(

- anonymous

i know other formulas maybe we can try that :)

- welshfella

oh - maybe i've gone wrong somewhere - its been a while since i did these
what are the choices?

- anonymous

\[T = 2\pi(r) \over v\]

- anonymous

0.731 N, 749.47 N, 187.368 dynes, 2,924 N

- Jhannybean

Isn't centripetal force \[\sf F_c = \frac{ mv^2}{r}\]

- welshfella

yes where v = linear velocity

- anonymous

yes @Jhannybean :)

- Jhannybean

since centripetal acceleration = \(\sf a_c = \dfrac{v^2}{r}\)

- Jhannybean

cool, I still remember \(\checkmark\) haha

- welshfella

yea my memory has failed me here I should have checked

- welshfella

i was too confident lol!

- anonymous

\[T = \frac{ (2\pi(r)) }{ v }\]

- anonymous

|dw:1438464437672:dw|

- anonymous

is it correct?

- welshfella

my original equation should have been m w^2 r
so F = 0.025 * (8.89pi)^2 * 0.15

- anonymous

oh it's 2.924 thank you :D

- welshfella

= 2.925 N

- anonymous

Can you help me with another problem ? :)

- welshfella

2.924

- welshfella

well i only have 10 minutes but see what i can do

- anonymous

thank you :)

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