Lecture 7
1
PHY121 – Physics for the Life Sciences I
Lecture 7
1.
Rotational
Dynamics
: Torque
2.
Newton’s Law for Rotations
3.
Rotational Inertia: Moment of Inertia
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2/14/2012

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Lecture 7
2
Recap: Newton’s Gravity
Gravitational attraction is the force acting between objects
that have a “
gravitational charge
”, i.e. gravitational mass
(one of the mysterious facts of nature is that inertial mass and gravitation charge are
proportional (or identical – in SI units) to another! This, among other facts, led Einstein to
the Theory of Relativity)
–
Force of Gravity between point-masses
m
1
and
m
2
at distance
r
12
from
one another:
•
Magnitude
:
Gm
1
m
2
/
r
12
2
, INIFINTE RANGE
with
G
= 6.673×10
–11
Nm
2
/kg
2
•
Direction: attractive; notation:
r
1
2
is pointing
outwards
• For
SPHERICALLY SYMMETRIC
objects:
as if all mass was at center!
–
e.g. on mass
m
by
M
E
of Earth at
sealevel
:
1
,1 on 2
1
1
2
2
2
2
G
m m
r
G
F
r
2
E
G
E
R
mM
G
F
j
extremely weak for
“normal” masses
2
E
E
GM
g
R
11
24
2
6
6.67
10
5.97
10
6.38
10
2
9.80m/s
m
g
j
2/14/2012