Download Types of Equilibrium • A body moving with no acceleration is in

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.
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© 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
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© 2017 J Co Review, Inc., Accessed by Guest on 06-15-2017