Take an empty plastic Easter Egg (a hard-boiled egg works, but a plastic one is easier and has less potential for egg destruction). Place it on its side on a flat surface and spin it quickly. After a few moments of spinning on its side, it will pop up and spin on one end. Why?
Source: Original.
From Denis Borris:
It has to do with the change of center of gravity and centripital force,
pushing the center of gravity above the balance point. It is like a
gyroscope.
From Rudametkin Sergio Isaa Aguilar:
[KD - I like this answer the most, though I'd like to see just a
little math to support it.]
The answer has to do with minimizing energy and the egg's moment of
inertia. A basic principle of nature is that nature is lazy and will
get away with doing the least amount of work as possible (kind of like
college students). When the egg is spinning on its side it has a larger
moment of inertia and it takes more energy for it to spin at a
particular speed. When it is spinning on its end the mass isn't as
spread out and therefore has a smaller moment of inertia. Hence it is
at a lower energy.
From Ravi Subramanian:
I have a rough explanation for this.
For any non-accelerating object the vertical line through the Center of
Gravity (CG) passes through the point of support.
Let us call this line the axis of support.
When an object is spinning about an axis the "centrifugal forces" cause
an imbalance if the axis is not a principal axis.
The only case when axis of support coincides with the principal axis for
the egg, is when the egg is on either end.
When you spin the egg with sufficient speed, the imbalance causes the
egg to adjust itself till the alignment takes place.
At lower speeds the imbalance is not enough to overcome gravity.
This is a lovely problem and the not 'completely satisfying'
answers point out a fundamental misunderstanding of the underlying
physical principals that is extremely common (among many physics
professors too, I would guess). This system doesn't seek to
minimize its energy. It seeks to minimize its Lagrangian.
Fancy word meaning Potential - Kinetic energy.
I = moment of inertia
omega = angular velocity
h = height of center of mass
Kinetic Energy = T = 0.5 I omega ^ 2
Potential Energy = V = m g h
Lagrangian = mgh - 0.5 I omega^2
Now consider our friend the egg. Let's assign I_2 to be the
angular moment associated with spinning on end, and I_1 to be the
larger moment associated with "wobbling" on its side. Let's also
call the semi-minor axis of our ovoid B, and the semi major axis A.
Now if the initial state is on its side w/ angular velocity,
omega_1, then
Lagrangian = mgB - 0.5 I_1 omega_1 ^ 2
If we allow the egg to stand on end (conserving angular momentum,
but not energy) then
omega_2 = I_1 / I_2 omega_1
Lagrangian(standing) = mgA - 0.5 I_1^2/ I_2 omega_1 ^ 2
difference = mg(B-A) + 0.5 omega_1 ^2 I_1/I_2 (I_1 - I_2)
When you spin the egg faster than omega = sqrt( I_2 2 m g (A-B) )
----------------
I_1 (I_1 - I_2)
then it may flip up.
Notice a couple of things here: If I_1 <= I_2 this wont ever
happen. The more oblong the egg
(A-B) the harder you must spin it.
Also notice that I quietly made a huge assumption about the surface
interaction: We can lose energy but not angular momentum there.
Interestingly it is critical that the total energy of the egg
decrease as it stands up! This is the reason that hard surfaces
make this effect easier to see. Try your egg first on counter top
then on soft carpet.
Colin Bown