Download Chapter2_2

Chapter 2.2 Announcements:
Homework 2.2:
due Thursday, Feb. 11, in class (Colby Meador)
Exercises: 21, 22, 24, 26, 27, 29, 31
Problems: 7, 11, 16
Homework rules:
- Show your work, use proper units
- You can collaborate, use tutors, but do your own work in the end.
- In-class:
100%
- Same day, late:
80%
- Two days later:
50%
Chapter 2.2
Wheels
Demos and Objects
- wheels
- pulling boxes
- burning wood by
rubbing
- lighting a match
- rubbing hands
- ball bearings
- moving and spinning
things
Concepts
- static friction
- sliding friction
- sliding friction  thermal
energy
- work, power, energy
- kinetic energy
i-clicker-1
You are in a tremendous hurry and you want your car to accelerate as quickly as
possible. Will you accelerate faster if you
A. “burn rubber” (skid your wheels),
B. or if you just barely avoid skidding your wheels?
C. Not enough information
Related: What are the three advantages of anti-lock brakes? (later)
Observations About
Wheels
•
•
•
•
•
Without wheels, objects slide to a stop
Friction is responsible for the stopping effect
Friction seems to make energy disappear
Wheels reduce friction enourmously
Wheels can also propel vehicles, but how?
Friction
• Opposes the relative motion of two surfaces
• Consists of a matched pair of forces:
– Object one pushes on object two
– Object two pushes on object one
– Forces have equal magnitudes, opposite directions
• Comes in two types: static and sliding
Types of Friction
• Static Friction
– Acts to prevent objects from starting to slide
– Forces can vary from zero to an upper limit
• Sliding Friction
– Acts to stop objects that are already sliding
– Forces have fixed magnitudes (for a given system)
Frictional Forces
• Increase when you:
– push the surfaces more tightly together
– roughen the surfaces
• Peak static force is greater than sliding force
– Surface features can interpenetrate better
– Friction force drops when sliding begins
Friction and Wear
• Static friction
– No work is done (no distance)
– No wear occurs
– Rolling is mostly static friction (microscopically
like gears (sprockets))
• Sliding friction
– Work is done (distance in the direction of force)
– Wear occurs
– Work is turned into thermal energy
The traffic light turns red, and now you need to stop as quickly
as possible. What are the three advantages of anti-lock brakes?
A. _______________________________________
B. _______________________________________
C. _______________________________________
Rollers
• Eliminate sliding
friction at roadway
• Are inconvenient
because they keep
popping out from under
the object
Wheels
• Eliminate sliding
friction at roadway
• Wheel hubs still have
sliding friction
Bearings
• Eliminate sliding
friction in wheel
hub
• Behave like
automatically
recycling rollers
Kinetic energy of objects
The kinetic energy of an object with
mass, m, moving with velocity, v is:
1
2
K  mv
2
The kinetic energy of an object with
rotational mass, I, spinning with angular
velocity, w is:
1
2
K  I w
2
Friction converts kinetic energy to thermal energy (objects heat
up). Thermal energy is ‘disordered’ energy.
A cannon ball of mass 1 kg moves at 100 m/s.
A truck of mass 10,000 kg moves at 1 m/s.
A tire with rotational mass 2 kg‧m2 spins at 100 rad/s.
Which has more kinetic energy?
A. The tire
B. The truck
C. The cannon ball
D. Same
E. Need more information
i-clicker-2
Is it true that you use up the same amount of energy when going
up a hill faster, vs. slower?
If so, why does it feel so different?
 More power output when you go faster.
Power
Work
Power 
time
Energy
Power 
time
W
P
t
The units of power are joule/sec (J/s) = Watt (W)
James Watt (1736-1819); Scottish inventor and engineer whose
improvements to the steam engine were fundamental to the changes
wrought by the Industrial Revolution.
(from Wikipedia)
(1 horsepower = 746 W)
An elite athlete (mass 70 kg) has a power
output of 450 W (at the aerobic
threshold).
How long will it take him to climb a 500
m (1640 ft) high mountain?
(Power output for a good athlete is 200 – 300 W)