1
Key Concepts:
• Unstable Equilibrium
• Static and Dynamic Stability
• Precession
Observations about Bicycles
• They are hard to keep upright while
stationary
• They stay upright easily while moving
forward
• They require leaning during turns
• They can be ridden without hands
Question 1
Why is a stationary tricycle so stable?
Vehicle’s Static Stability (1)
Static stability is determined by
1.
Base of support (polygon
formed by ground contact
points)
2. Center of gravity (effective
point at which gravity acts)
Static stability occurs
when the center of gravity
is above the base of
support
c
‐
o
‐
m
Vehicle’s Static Stability 2
If center of gravity lies above base of support, when
tipped:
– Gravitational potential energy rises
– Acceleration is back toward equilibrium and vehicle or
object returns to equilibrium.
Vehicle or object is
then:
“statically stable”, or “in stable equilibrium”.
Gravitational
potential energy
increases
mgh
mg
h
Question 2
Why is stationary bicycle so unstable?
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Vehicle’s Static Stability (3)
If center of gravity does not
lie above base of support,
when tipped:
– Gravitational potential energy decreases
– Acceleration is away
from equilibrium and vehicle or
object does not
return to equilibrium, it tips over
.
Therefore vehicle or object is :
“is statically unstable”, or
is “in an unstable equilibrium”.
Tips over to a new
stable equilibrium
Energy
becomes
smaller in this
direction
mgh
Tricycle:
statically
stable
Bicycle:
statically
unstable
You can also understand this using torque
Stationary Vehicles:
• Tricycles
– have 3 contact points
– are statically stable and
hard to tip over
•B
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 Fall '07
 Russell
 Physics, Energy, Force, Potential Energy, Bicycle, Unstable equilibrium, Static Stability

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