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?

Question

welshfella · Answer

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 · Answer

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

welshfella · Answer

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

welshfella · Answer

w = 8.89 pi rads/sec

anonymous · Answer

5x10^(-3) ?

welshfella · Answer

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

welshfella · Answer

what is 5x10^-3 ?

anonymous · Answer

the answer for the centripetal force?

welshfella · Answer

hmm..

welshfella · Answer

not quite - you need to multiply that by pi

anonymous · Answer

sorry i forgot to put pi, is it 0.0157?

welshfella · Answer

yes 
thats in newtons

welshfella · Answer

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

anonymous · Answer

the answer is not in the choices :(

anonymous · Answer

i know other formulas maybe we can try that :)

welshfella · Answer

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

anonymous · Answer

$$T = 2\pi(r) \over v$$

anonymous · Answer

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

jhannybean · Answer

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

welshfella · Answer

yes  where v = linear velocity

anonymous · Answer

yes @Jhannybean  :)

jhannybean · Answer

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

jhannybean · Answer

cool, I still remember $\checkmark$ haha

welshfella · Answer

yea  my memory has failed me here  I should have checked

welshfella · Answer

i was too confident   lol!

anonymous · Answer

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

anonymous · Answer

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