Every object on the surface of the Earth feels the force and this force is directed toward the center of the Earth.

What about accepting this idea: force, “acting at a distance” ?

What is the Centripetal Acceleration of the Moon?

a M ~ g/3600

The Universal Law of Gravity Force ~ mass 1 x mass 2 /distance 2

Newton’s Law of Universal Gravitation Every particle in 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 distance between them. This force acts along the line joining the two particles.

F = G m 1 m 2 /r 2

The Universal Gravitational Constant

G F = Gm 1 m 2 /d 2 G = 6.67 10 -11 Nm 2 /kg

Gravity and Distance The Inverse Square Law The inverse square law holds for gravity and for all phenomena wherein the effect from a localized source spreads uniformly throughout the surrounding space: the electric field of an electron, light from a match….

Questions 1. If there is an attractive force between all objects, why do we not feel gravitating toward massive buildings in out vicinity?

2. Consider an apple at the top of the tree that is pulled by Earth’s gravity with a force of 1 N. If the tree were twice as tall, would the force of gravity be only ¼ as strong? Defend your answer.

1. If Earth were hollow but still had the same mass and radius, would your weight be different? Why?

2. Skylab caused quite a commotion when it returned to Earth in July 1979. Why would it suddenly return to Earth after it had been in orbit for many years?

3. Why we use different formulas for gravitational force for object on the surface of Earth and for object far from Earth?

Weight and Weightlessness

To be more general we now refine the definition of weight: weight of something is the force it exerts against a support.

Question In what sense is drifting in space far away from all celestial bodies like stepping off the table?

Question We known that both the Moon and the Sun produce our ocean tides, and we know that the Moon plays the greater role because it is closer. Does it closeness mean it pulls with more gravitational force than the Sun on the Earth oceans?

Ocean Tides

Tides in the Earth and Atmosphere

Tides on the Moon

Gravitational Fields The field concept plays in-between role in our thinking about the forces between different forces.

Gravitational Field Inside of Planet

Your acceleration will be progressively less as you continue toward the center of the Earth. Why???

Black Holes

How may a black hole detected if there is literally no way to “see” it?

Universal Gravitation

Why is our Earth round?