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Physical Equilibrium | AP Physics B Types of Equilibrium • A body moving with no acceleration is in equilibrium o Thus, velocity is constant • Static Equilibrium – that constant velocity equals 0 Box in Static Equilibrium The box sits motionless. Since its velocity is 0, the box is in static equilibrium. • Dynamic Equilibrium – the constant velocity is nonzero Box in Dynamic Equilibrium The box slides across the floor at constant, nonzero velocity. It is in dynamic equilibrium. • Suppose a weight on a parachute falls towards Earth o Initially, it accelerates downwards Thus, it is not in equilibrium o But quickly, the upwards force of air resistance matches the downward force of gravity o At this point, velocity becomes constant and the weight is in dynamic equilibrium Weight on a Parachute When air resistance on the parachute matches the downwards force of gravity, there is no more net vertical force. At this point, the weight travels at constant velocity downwards. It’s in dynamic equilibrium. • • • To determine if an object is in equilibrium, check if: o Fupwards = Fdownwards o Fleftwards = Frightwards If so, the body is in equilibrium For the weight: o Fupwards, air resistance, equals Fdownwards, gravity o There is no horizontal force Objects Not in Equilibrium • Say a rocket is accelerating upwards towards space Rocket Accelerating towards Space The rocket is accelerating towards space; that is, it is speeding up in its upwards journey. For this to occur, the upwards force must exceed the downwards force of gravity. So, this rocket is not in equilibrium. 1 © 2017 J Co Review, Inc., Accessed by Guest on 06-15-2017 Physical Equilibrium | AP Physics B Finding the Acceleration of a Body Not in Equilibrium • Whereas our objects in equilibrium had no acceleration, the rocket has an acceleration • To find it, we will use F = ma • F = net force m = mass of the rocket a = acceleration • We first need to find the net force – the combined upwards and downwards forces • The downwards force, gravity, is mg. If this rocket is 100 kg, mg equals 1000 N • And, say we are told the upwards force from the rocket boosters is 1500 N • The net force would be the difference: 500 N upwards • Using F = ma: o 500 N = (100 kg)(a) o a = 5 m/s2, and we know the acceleration is upwards 2 © 2017 J Co Review, Inc., Accessed by Guest on 06-15-2017