 Test Repair, Thursday before school (7:20) or
Lunch
 Inuit Yoyo lab (circular motion) due Wednesday,
9/9
 My Solar System Phet Lab due Friday, 9/11
 Unit 3 “exam” and HW due Friday, 9/11
 Rocket Lab III Monday 9/14- need more bottles!!!

Circular Motion and Gravitation
Part 2: Universal Gravitation and G

 Newton, Newton, Newton, Newton… Arrrgggghhhh!!!

 actually stars the moon.
 Newton realized that the force that attracted the apple to the Earth was the same as the force that attracted the Moon.
 He set up the problem and…….
it didn’t work!!! Ha Ha!!

20 years later he gave it another try:
1680, with Edmund Halley
Gravitational Force, F
G
, depends directly on the product of the masses of the two objects and inversely with the square of the distance between them.

Let’s write that more neatly using math:
F
G a m
1 x m
2
F
G a
1/d 2
So…
F
G a m
1 x m
2 d 2
Still can’t find the a button so let’s change it to =

In order to make the equation work we need a constant, G
 G = the universal gravitational constant.
 G = 6.67 x 10 -11 Nm 2 /kg 2
 This value was determined by a
British physicist, Henry Cavendish, in 1798 in a very slick experiment called “Weighing Earth”

F
G
=
Gm
1 m
2 d 2
CARD!!

This was used to “Weigh Earth”
 The force Earth exerts on a 1 kg object at Earth’s surface is
9.8 N.
 The distance between the object and Earth’s center is
Earth’s radius, 6.4 x 10 6 m.
 So, plug all this in……..
9.8 N = 6.67 x 10 -11 Nm 2 /kg 2
1kg x m
E
(6.4 x 10 6 m) 2

E
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