Phys. 121: Tuesday, 23 Sep.
Reading: Finish ch. 10 (skip 10.7 now) for Thurs.
(we will return to ch. 9 later).
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Written HW 5: due Thursday.
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Mastering Phys.: Fourth assign. due tonight.
Fifth assignment due in 9 days (02 Oct.).
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Exam 1: will be Tuesday the 30th (in one week), over
chs. 1-6. Study guide, sample formula sheet, and sample
exam problems are all available on our class web page.
Clickers:
A car turns a corner on a banked
road. Which of the diagrams could
be the car’s free-body diagram?
Clickers: which ball will win the race?





a) The ball on the straight track
b) The ball on the track that curves downward
c) They will arrive at the exact same time
d) Neither ball will reach the end
e) It will be random; can't predict in any
given race which will win
Clickers:
Two coins are on a turntable that
steadily speeds up, starting from rest,
with a ccw rotation. Which coin flies off
the turntable first?
A. Coin 1 flies off first.
B. Coin 2 flies off first.
C. Both coins fly off at the same time.
D. We can’t say without knowing their
masses.
[Clickers: The two masses shown
are spinning together, so that their
angles are always equal (as shown).
R
The acceleration of the outer mass
R
is...]
 a) Equal to zero, like that of the
inner mass
 b) Tangent to the circle there
 c) Equal to that of the inner mass
(but neither are zero)
 d) Half as much as the inner mass acceleration
 e) Twice as much as the inner mass acceleration
Clickers: the masses are also equal, and
the strings massless, for the same
problem. How much is
the tension ratio, T₁ / T₂ ?
 a) 1/2
 b) 1
 c) 2
 d) 4
 e) 3/2
T₂
R
T₁
R
Example: orbit speed
Example: Find minimum speed for water
not to fall out of bucket at top.
R
Example: problem 8.37
Clickers: a heavy chain with total mass M
and total length L is being spun
in a horizontal circle with
constant angular speed
ω. How does the tension
in the chain behave with
distance r from the center?
a)
 b)
 c)
 d)
 e)

stays constant (but not zero)
increases as r increases
decreases as r increases
It will be zero everywhere
It misbehaves (and should be punished)
r
Clickers: a heavy chain with total mass M
and total length L is being spun
in a horizontal circle with
constant angular speed
ω. What is T(r=L/2) compared
with T(r=0)?
a)
 b)
 c)
 d)
 e)

twice the tension of the center
The same tension as the center
3/4 the tension at the center
Half the tension at the center
1/4 the tension at the center
L/2
L
General motion: ac and at
Important Concepts