1. Phy 101: Fundamentals of Physics I Chapter 9 Lecture Notes

2. Henry Cavendish (1731-1810) English chemist and physicist Studied electricity –Investigated the property of capacitance –Measured the strength of electrical current by shocking himself and estimating the magnitude of the pain Devised a method to measure the Universal Gravitational Constant (G) (he used this value to calculate the mass of the Earth)

3. Newton’s Law of Universal Gravitation Gravitational force (weight) is the mutual attraction of two bodies due to their mass Depends on –The product of the masses of each object –The inverse square of the distance between the objects –The universal gravitational constant (G) {more on this constant later} In symbolic form: F gravity ~ m 1 m 2 /r 2

4. The Universal Gravitational Constant (G) A fundamental constant in nature (it is the same everywhere in the universe) The value is G = 6.67x10 -11 N. m 2 /kg 2 Allows gravitational force to be calculated when using mass and distance values F gravity = G. m 1 m 2 /r 2

5. Inverse Square Law Reflects how an object’s influence gets diluted in space at increased distances Whenever any force (or physical quantity) obeys an inverse square law its form is: F ~ 1/r 2 Other notable inverse square force: –Electrical force

6. Gravitational Force Inside a Planet As you go deeper into the planet (or any solid body) less of its mass directly affects its gravitational acceleration/force When inside a solid body (i.e. a planet), gravitational influence (or field) is proportional to the remaining mass of the “sphere” from your location to its center, or: F ~ r (not 1/r 2 ) The closer to the center you get, the smaller the gravitational effect (its zero at the center of the earth!) This is due to the fact that that as you go inside it some of its mass now pulls you outward rather than inward

7. Ocean Tides Affected by both the moon and the sun Important factors to consider: –The moon is closer to the earth than the sun –The gravitational force between the moon and the earth is smaller than that between the sun and earth The moon has a greater influence on the ocean tides since its gravitational influence varies more during its cycle than the sun. Tides are due to a “bulging” of the ocean water due to the moon’s gravitation force on the earth and its movement