1. Phys. 121: Thursday, 06 Nov. Written HW 10: Due by 5 pm.
HW 11: ch. 13, probs. 25, 36, and ch. 14,
probs. 15, 18, 36, and 43. Due in one week.
Mast. Phys.: Assign. 9 due on Tuesday.
Reading: Finish ch. 14 by Tuesday (You may
skim sects and 14.8.)
Exam 2: average was 12.8/20; there was one
perfect score (20/20) - congratulations! I have
added a curve of 1.5 points to the score shown on
your exam, when calculating the scores shown on
Moodle. Test corrections through the OSL are
available on problems 3, 4, 10, and 14; return them
to me with your original exam by Tues., 18 Nov.

2. Test 2: let's go over it...

3. Clickers:
A baseball bat is cut in half at its center of mass. Which end is heavier?
The handle end (left end).
The hitting end (right end).
The two ends weigh the same.
Answer: B 3 3

4. Clickers:
Which third force on the wheel, applied at point P, will make the net torque zero?
Answer: A 4 4

5. Clickers: which torque vector goes with the
figure in (a)? (F is into the screen and slightly
to the right.)
a) b) c) d) e) 6

6. Clickers: What does the scale read?
500 N.
1000 N.
2000 N.
4000 N.
Answering this requires reasoning, not calculating.
Answer: C
Slide 12-91 6 6

7. Ch. 13: Newton's Law of Universal Gravitation:
Any two objects with mass produce a
gravitational force between them! The
direction of the force is attractive (straight
toward each other), and the magnitude is
where r is the distance between the (centers of
mass of the) two objects, and G is a constant.
(G = 6.67 x 10^(-11) N m²/kg².)‏

8. Clickers:
Three stars are aligned in a row. The net force on the star of mass 2M is
To the left.
To the right.
Zero.
Not enough information to answer.
Slide 13-33 8 8

9. Notice that the usual relation holds:
F_r = - dU/dr
(force = negative slope)‏

10. Decrease of g with Distance
A satellite orbits the earth at height h above the earth’s surface.
The value of g at height h above sea level is:
where gearth = 9.83 m/s2 and Re = 6.37  106 m.

11. Decrease of g with Distance

13. The parabolic orbit (total E = 0) has
exactly (the local) escape speed at
all times:
(This follows from its total energy being exactly
zero, with the convention that U(r) goes to zero
as r goes to infinity.)

14. Fun Fact: putting v_esc. = c, the
speed of light, gives a size that Michell
and Laplace called a “dark star”...
R_dark = 2 G M/c² --- this turns out
to be the size of a black hole!

15. The Principle of Equivalence
The inertial mass of an object relates the net force acting on it to its acceleration:
The gravitational mass of an object appears in Newton’s law of gravity and determines the strength of the gravitational attraction:
The assertion that mgrav = minert is called the principle of equivalence.

16. Clickers: For the non-circular orbits, which of the following is true?
a) The orbiting object moves slower as it gets
closer to the Sun.
b) The orbiting object moves faster as it gets
c) The orbiting object will have a constant speed.
d) The orbiting object will constantly gain speed.
e) The orbiting object will fly away if you cut the
invisible cord which holds it to the Sun.