December 18th, 2016, 08:51 AM  #1 
Newbie Joined: Dec 2015 From: us Posts: 8 Thanks: 0  Centripetal force
i got a problem in physics class i dont understand. it says that an object is moving on a circle with const. speed. what is his normal force compared to his weight. i thought that it is bigger b/c there is force of gravity pointing down and centripetal force also pointing down towards center of circle (the object is exactly at top of circle) so because it's not falling down or flying, normal force equals centripetal force plus mg. Am i wrong? 
December 18th, 2016, 11:20 AM  #2  
Math Team Joined: Jul 2011 From: Texas Posts: 2,663 Thanks: 1328  Quote:
At the top of a vertically oriented circular path, $F_c = F_N + mg \implies F_N = F_cmg$ ... the relationship cannot be determined unless one knows the speed and radius of the circular motion. Suppose the "object" has a fixed weight, $mg = 200 \, N$, and the circular path has radius, $r = 10 \, m$, with a speed of $v=20 \, m/s$ ... $F_N = \dfrac{(20 \, kg)(20 \, m/s)^2}{10 \, m}  200 \, N= 600 \, N$ clearly, $F_N > mg$ Change the radius of rotation to $r = 25 \, m$ ... $F_N = \dfrac{(20 \, kg)(20 \, m/s)^2}{25 \, m}  200 \, N = 120 \, N$ $F_N < mg$  
December 18th, 2016, 02:01 PM  #3 
Newbie Joined: Dec 2015 From: us Posts: 8 Thanks: 0 
Why is centripetal force equal normal plus gravity force? I thought it should be added to gravity force since they point in same direction.
Last edited by skipjack; December 18th, 2016 at 02:46 PM. 
December 18th, 2016, 02:17 PM  #4  
Math Team Joined: Jul 2011 From: Texas Posts: 2,663 Thanks: 1328  Quote:
If not either of the above, what is the physical set up? Last edited by skipjack; December 18th, 2016 at 02:47 PM.  
December 18th, 2016, 03:35 PM  #5 
Newbie Joined: Dec 2015 From: us Posts: 8 Thanks: 0 
its a skateboarder on a circular hill

December 18th, 2016, 03:56 PM  #6 
Math Team Joined: Jul 2011 From: Texas Posts: 2,663 Thanks: 1328 
riding inside or outside the loop at the top? Last edited by skeeter; December 18th, 2016 at 04:32 PM. 

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