Friday, February 1, 2008
Announcements:
- If you find a mistake in your textbook (especially a
physics-related error), let me know, and you will receive extra credit
if you are the first to identify the error. So far, I have only
found one such mistake (see the note below related to problem #9 in
chapter 5).
Assignments:
- Answer the recommended conceptual exercises and problems for
Chapter 5, and check your understanding by
looking at the answers in the back of the textbook.
- Note:
The answer to problem 9 should be 3.6 x 10^22 N (not 3.6 x 10^33 N as
stated in the back of the textbook).
Class Discussion:
Chapter 4 - Why things move
Ponderable: What is the tension in the rope during a
tug-of-war contest? Related
story.
Demo: Fan cart with sail
Demo: Medicine ball with skateboards
Other questions?
Chapter 5 - Gravity
Isaac Newton was a busy thinker at the age of most college
students. In 18 months, he laid the foundation for a theory of
gravity and a theory of light, and he invented calculus along the way
to explain these theories!
Orbital motion is a natural extension of objects falling on Earth and
projectile motion.
Fg = G*m1*m2/d^2 where G = 6.7 x 10^-11 N*m^2/kg^2 is the
Universal Gravitational Constant
Exercise: Estimate the force of attraction between you and the
person sitting next to you.
The circumference of the Earth is 40,000 km, and the moon is about 10
times this distance away (400,000 km)
Where should the moon be in Fig 5.6?
Which body exerts a stronger gravitational pull on the moon: the
Earth or the Sun?
Useful solar system data:
mass of Earth = 5.97 x 10^24 kg
mass of moon = 7.35 x 10^22 kg ~ 1/81 mass of Earth
mass of Sun = 2 x 10^30 kg
radius of Earth = 6370 km = 3950
radius of moon = 1740 km = 1080 mi ~ 1/4 radius of Earth
radius of Sun = 700,000 km = 432,000 mi
Estimate the weight of an object on the moon compared to Earth given
the facts that m(Earth)/m(moon) = ~100, and r(Earth)/r(moon) ~ 4.
What are the consequences of Newtonian thinking?
What limitations are there in our world today due to Newtonian thinking?
Examine Fig. 5.16 - Excellent summary of physics
Why not use the more modern and accurate theories instead of F = ma?