Transcript How Things Work - University of Illinois at Urbana–Champaign

2.1 Wind Turbines
We are going to see how turning is similar to going
straight!
But first we need to revisit WORK and ENERGY.
Work is pretty much ( Force|| )( Distance )
Work changes speed; it could go up or down.
WorkTotal 
m v2final
2
2
m vinitial

2
Define these new terms
as kinetic energy – the
energy of motion.
WorkTotal  KEFinal  KEInitial
Because there are two types of force
there are two types of work.
For the first kind of force the exact path matters
to the amount of work done. An example is friction.
For the second kind of force the exact path does not
matter. An example is pretty much anything
but friction. Think of weight.
For the second kind of force, we are going to call
each work term a change in Potential Energy
New BIG IDEA! Conservation of Work-Energy
WorkType One = Change in KE + Change in PE
Until we modify this to allow mass to transform
into energy, this holds for everything!
PhET Simulation - Energy Skate Park
Clicker Question
Which requires more work?
1) Lifting a 10kg mass up by 1m.
2) Lifting a 1kg mass up by 10m.
A) Lifting the 10kg mass
B) Lifting the 1kg mass
C) The work is the same for each
Work = ( Force|| )( Distance )
Clicker Question
Which requires more work?
1) Lifting a 10kg mass up by 1m.
2) Carrying a 1kg mass sideways by 10m.
A) Lifting the 10kg mass
B) Lifting the 1kg mass
C) The work is the same for each
Work = ( Force|| )( Distance )
New ideas for today
What is like Force?
What is like mass?
What is like our motion ideas?
Things rotating and moving at the
same time look complicated!
But we can separate the two kinds of motion
What corresponds to a push?
A Twist!
If a push is a force then
we call a twist a torque
What is the difference?
How do we say how big the torque is?
•Wrench and Bolt
Another example of mechanical advantage
Define Torque
Torque = ( Force )( Perpendicular Lever Arm )
Lever Arm
Force
Pushes can result in accelerations.
How does it work with torques?
Inertia Tubes
It matters where the mass is!
Inertia Table
Close to axel – EASY – like small mass.
Far from axel – HARD – like big mass.
Balance can be complicated!
Center of gravity and torque balance
Center of mass rod with spheres
“Nicely” distributed mass and rotation
Prok chop
Center of mass ignores rotation!
Lack of dynamic balance is bad news.
Observations About Wind Turbines
In absence of forces (wind)
there is no torque and the blades
remain at rest.
 External torque sets blades in
rotational motion.
 Angular velocity increases
as long a torque is present.
 Analogy to linear Motion:
in absence of forces/torque state of
linear/rotational motion remains
unchanged.

Palm Helocopter
Clicker Question
What makes for a good wind turbine?
A)
B)
C)
D)
Long Blades and Low Wind Speed.
Long Blades and High Wind Speed.
Short Blades and Low Wind Speed.
Short Blades and High Wind Speed.
Remember Torque!
Helicopter
Torque from wind drives electric generator
at center of turbine.
New Ideas Today
Rotational position, velocity, acceleration
Rotational “force” is torque.
Torque = ( Force )( Perpendicular Lever Arm )
Rotational mass is both how much mass and
how far it is from the axel.