Physics 100 Chapt 10 Newton’s Universal Law of Gravitation
Feb 12, 2016
Physics 100Chapt 10
Newton’s Universal Law of Gravitation
Very fast horizontal toss
t = 0st = 1sx= 8km
5m
t = 2sx=16km
t = 3sx=24km
20m 45m
V=8km/s
Orbital motion is free fall
V = 4 km/sV = 6 km/sV = 8 km/sV = 10 km/s
Circular Orbit!Eliptical Orbit
Centripetal acceleration
a = v2/r for a circular orbit (v = 8km/s = 8x103m/s)
a = (8 x103 m/s)2
6.4 x 106 m = 64 x106 m2/s2
6.4 x 106 m
= 10 m/s2
Toward Earth’s center = g
Artificial satellite
av
a = v2/r = g
Moon-earthv
a=v2/r
Is the Moon in free-fall around the Earth?
r=3.84x1
05 km
v
a = v2/r
what is v?v = dist/time = 2r
28d =2x3.14x 3.84x108 m
28dx(24h/d)x3.6x103s= 24 x 108 m
2.4x106s = 1.0 x 103 m/s
Moon’s centripetal acceleration
amoon = v2/r; v = 1.0 x103 m/s)
amoon=(1.0 x103 m/s)2
3.84 x 108 m = 1.0x106 m2/s2
3.84 x 108 m
= 2.7 x 10-3 m/s2
Toward Earth’s center g1
3600
Newton’s dreams
Hmmmmm
The Moon is in free-fall around the Earth
It’s acceleration is only 1/3600 g (accel
at the Earth’s surface)
Distances
r=3.84x108m = 60 x 6.4x106 m
The moon is 60x further fromthe Earth’s center than objectson (near) the Earth’s surface
= 60 x RE
160
13600( )2=
RE = 6.4x106m
Newton’s big idea
The force of gravity gets weaker as
distance squaredThe moon is 60x further from
the Earth’s center than objects on
(near) the Earth’s surface
The strength of Earth’s gravity
near the Moon is(1/60)2 t=1/3600
times weaker
Gravity gets weaker as 1/dist2
Universal law of gravity
m Mr
F mF M
F 1r2
combine: F mMr2 F = G mM
r2
Proportionality constant:
“Newton’s Constant”
Universal
Universal:applies to all objects!!!
What is G?
W= GmMERE
2W
W= mGMERE
2
W= m g
g=
GMERE
2
Force of gravity between “ordinary-sized” objects
80kg60kg
1mF = G mM
r2
F = 6.7x10-11Nm2/kg260 kg 80kg(1m)2
F = 6.7x60x80x10-11N
Boy’s weight = mg = 80kg x 10m/s2 = 800 N
30x109 times bigger!
F = 32160.x10-11 N = 3.2x10-7
N
Measuring gravity force between “ordinary-sized” objects is very hard
Moon’s gravity causes tides
So does the Sun’s
Full Moon
New Moon
Half Moon
Measuring Weight
mg
N
Weightlessness
N =mg
N >mg N <mg
N =0
Weightlessness means =0N
compensating upward
Weightlessness in action
“Floating” is space isreally free-falling in space
What is g on the moon?
W= GmMMRM
2
W= mGMMRM
2
W= m gM
gM=
GMMRM
2
WRM=1.7 x 106m
mMM
gM on the Moon
g =
GMMRM
2
6.7x10-11Nm2/kg2 x 7.4x1022Kg(1.7x106m)2
=
gM = 1.7 m/s2
1/6 x gEarth
summary
Obey the law!