Gravity Equation
F = force of gravity
G = gravitational constant (6*10-11) 0.00000000006
M1 = mass of body 1
M2 = mass of body 2
S2 = distance between
M1 & M2 squared
F= G
M1* M2
S2
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Gravity
F= G
M1* M2
S2
1. The force of gravity is not effected by other
bodies present.
2. The force of gravity is equal but oppositely
directed.
F=ma Newton’s second law
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Gravity
F= G
M1*M2
S2
F
M1
M1A1
If the force is equal and opposite, then:
M2
=
M2A2
F = MA for both
M1A1 = M2A2
M2 is larger than M1
so, A1 must be larger than A2 if they are equal.
[Across a crowded room, you and an airplane]
Gravitational Constant
G ~ 6*10-11
F= G
- a very small number
0.00000000006
M1* M2
S2
What is the attraction between you and the
person next to you?
What is the attraction between you and the
earth?
Gravity
F= G
M1*M2
S2
If masses stay the same,
but the distance increases,
how does the force of
gravity change?
M same
= F<
S increases
Gravity
F= G
M1*M2
S2
If distance stays the same,
but the masses increase,
how does the force of
gravity change?
increase
same
= F>
Gravity
Will a body always have the same mass if the
volume remains constant? NO
Same volume (size)
Density may be different, so masses are different
Venus (0.95 radius of earth, density 0.53)
Acceleration due to Gravity (g)
The acceleration of a body arising from the
earth’s gravitational pull.
Units are ft/sec2 or m/sec2
At the earth’s surface g is: 9.8 m/sec2
or
32 ft/sec2
(g) depends on position
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Acceleration due to Gravity (g)
A body falling will continue to accelerate until
air friction causes the body to reach a
terminal velocity.
Acceleration due to gravity is independent of mass
F=MA
Force
Acceleration 
Mass
The Beam Balance and Weight
Known mass
1 kg
?
unknown mass
Gravity
Weight = pull of gravity (a force)
[ Will it work everywhere? on Earth, another planet? in space? ]
Weight
F= G
M1*M2
S2
Weight is the force of gravity between me and the
earth.
1. How can I reduce my weight?
A) move scales to second floor?
B) move to the mountains?
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Gravity and the Earth
F= G
N
M1*M2
S2
-Rotates west to east
-Velocity greater at equator, zero at poles
-Centrifugal force bulges earth at the equator
S
N
diameter
S
-So, the earth is slightly squashed at equator
-equator diameter is 42 km > than
pole diameter