NNNOOOOOO 0 Report post Posted April 6, 2011 I don't think I'm doing it correctly. Here's my data:Mass:bob (what's being spun)- .55kgradius:16.8cmRPM:88.2I think the centripetal force should be about .17kg(?). At least that is an equivalent mass for it. I think I keep trying to do the wrong thing. Share this post Link to post Share on other sites
rvalkass 5 Report post Posted April 6, 2011 The equation for centripetal force in this instance is: F = mr|w|2 Where F is the force, m is the mass, r the radius and w the angular velocity. Your data give us this (converted to SI units): m = 0.55kg r = 16.8cm = 0.168m w = 88.2RPM = (2*PI/60)*88.2 rad s-1 Plug all those in to the equation: F = 0.55kg * 0.168m * ( 2 * PI * 88.2 rad s-1 / 60 )2 F = 7.8 N (to 2sf, the greatest accuracy given) Share this post Link to post Share on other sites
NNNOOOOOO 0 Report post Posted April 6, 2011 I was wrong. I was doing T=mv2/r. Share this post Link to post Share on other sites
rvalkass 5 Report post Posted April 6, 2011 The two equations are equivalent. You just need to remember the key difference between velocity and angular velocity. The magnitude of each of them can be related by the following equation: |v| = r * |w| Or, if you're happy with vectors: v = w x r where x denotes the vector cross product, and r is the vector from the centre of rotation to the body's centre of mass. Share this post Link to post Share on other sites
