Question
Fri May 20, 2011 By: Nilanjana Lodh

my book declares-a body in uniform circular motion is an example of dynamic equilibrium. but i know that in order for a body to be in equilibrium, the net force has to be zero. but how can that be possible in circular motion? the direction of velocity of the body is changing at every instant, so there is an acceleration. plus,there is also a net force acting on the body i.e. the centripetal force... also can you tell me what do you exactly mean by dynamic equilibrium? thank you plz help! i am in great confusion..

Expert Reply
Sat May 21, 2011
In dynamic equilibrium all the forces are balanced, if you include the reaction process.

Newton's 2nd Law

F = MA

The left is the Applied Force and the right is the Reaction Force. Note that they are equal as required by Newton's 3rd Law. 

In Dynamic Equilibrium the Reaction Force is constant so the acceleration is constant. In the case of Centripetal Force the Centripetal Force is at right angle to the direction of motion (tangential to the circle) thus no Work is done====> No energy is lost===> motion can continue for ever ( orbiting planets for example). Notice that though the object is being accelerated everything is steady state and if you include the Centrifugal Force (Reaction Force) the object is in Equilibrium

The reality of Centrifugal Force is clear in a rotating frame of reference. You are sitting on an amusement park ride that is a seat rotating on the end of a set of chains. The Frame of Reference is the ride itself and it rotates with you and your seat. Relative to that Frame of Reference you are in equilibrium and motionless. If you are in equilibrium then the Sum of the Forces acting on you must equal zero. The seat is exerting a force on your butt (Centripetal Force), By Newton’s 3rd Law, if you are motionless then, your butt must exert an equal and opposite force on the seat (Centrifugal Force).
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