1. Chapter 2.2 Announcements:
Homework 2.2: (due Tuesday Feb. 12)
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%

2. Chapter 2.2 Wheels Concepts Demos and Objects wheels pulling boxes
burning wood by rubbing
lighting a match
rubbing hands
ball bearings
moving and spinning things
static friction
sliding friction
sliding friction  thermal energy
work, power, energy
kinetic energy

3. 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
“burn rubber” (skid your wheels),
or if you just barely avoid skidding your wheels?
Not enough information
Related: What are the three advantages of anti-lock brakes? (later)

4. 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?

5. 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

6. Types of Friction Static Friction Sliding 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)

7. 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

8. Friction and Wear Static friction Sliding 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

9. The traffic light turns red, and now you need to stop as quickly as possible. What are the three advantages of anti-lock brakes?
_______________________________________

10. Rollers Eliminate sliding friction at roadway
Are inconvenient because they keep popping out from under the object

11. Wheels Eliminate sliding friction at roadway
Wheel hubs still have sliding friction

12. Bearings Eliminate sliding friction in wheel hub
Behave like automatically recycling rollers

13. Kinetic energy of objects
The kinetic energy of an object with mass, m, moving with velocity, v is:
The kinetic energy of an object with rotational mass, I, spinning with angular velocity, w is:
Friction converts kinetic energy to thermal energy (objects heat up). Thermal energy is ‘disordered’ energy.

14. i-clicker-2 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

15. 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

16. How long will it take him to climb a 500 m (1640 ft) high mountain?
The units of power are joule/sec (J/s) = Watt (W)
James Watt ( ); 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)