# Ch4.5 - conceptual gravity1

8. Mar 2016
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### Ch4.5 - conceptual gravity1

• 1. Gravity Newton rationalized that if the earth puts a force on an apple, then who’s to say that it doesn’t exert a force on the moon? Contact vs. Non-contact Forces ↓  Newer concept: force Must be touching acting at a distance (gravity, magnets, electricity…)
• 2. Gravity must be proportional to both masses because of Newton’s 3rd Law (Equal and opposite forces) F α M m Newton’s Math Proof Force of gravity must decrease by the inverse square of the distance. F α 1/r2 This is known as the inverse square law! It is repeated in electricity and magnetism! Combine These! 2 Mm F r α 2 GMm F r =
• 3. Keeping Relationships Simple • Proportions: Compare two quantities while their outcome remains constant. Symbol: α • Inverse relationship means you are comparing two quantities and as one quantity gets larger the other gets smaller. • F = ma • When the F is kept constant the m is inversely proportional to the a.
• 4. Still Keeping it Simple Inverse Square relationship means that as one quantity gets larger the other gets smaller by the square of the smaller quantity. F α 1/r2 When r goes up, F goes down.
• 5. Keep is Simple Direct • Direct Relationship means that as one quantity gets larger the other gets larger as well. • F α a • As F increases so does the a. • F = ma, mass will be kept constant.
• 6. Law of Universal Gravitation Every particle in the universe attracts every other particle with a force that is proportional to the product of their masses and inversely proportional to the square of the distances between them. This force acts along a line joining the 2 particles. 2 GMm F r = G = gravitational constant (small or large?) G = 6.67 x 10-11 Nm2 /kg2 It took 100 years later for this law to be proven. Henry Cavendish performed the experiment.
• 7. Sample Problem A 50 kg girl & a 70 kg boy are 50 cm apart on a park bench. Find the force of attraction… Field  Area around an object that affects other objects. Gravitational Field  Area around a mass that affects other masses What represents the gravitational field around the earth? At Crown Point, the radius of the Earth is 6.37101 x 106 m. If the Earth has a mass of 5.98 x 1024 kg. Then calculate the Earth’s gravitational field in Crown Point. 11 24 2 6 2 2 6.67 10 (5.98 10 ) (6.37101 10 ) 9.83 / GM x x g r x g m s − = = =

### Hinweis der Redaktion

1. Show picture of Henry Cavendish’s experiment on the board. 1700’s
2. Draw picture of girl and boy on park bench (bubble “physics”) F = 1x 10-6 N