1. Newton’s Laws: The New Physics
Planetary Motions
Newton’s Laws:
The New Physics

2. Newton’s Laws
LAW 1: A body is in natural motion when it is at rest or when moving along at a constant speed along a straight line
Definition of natural motion

3. Newton’s Laws
LAW 2: A net force applied to a mass results in an acceleration OR
Acceleration is directly proportional to the net force and inversely proportional to the mass

4. Newton’s Laws Speed is the rate of motion (km/s)
Velocity is speed with direction
Acceleration is a change in velocity (speed, direction, or both!)
Accelerations result from net forces (add them all!)

5. Newton’s Laws
Circular motion is accelerated motion (even if speed constant)
Why? Direction of motion is constantly changing!
Planetary motions are accelerated; implies a force

6. Newton’s Laws
LAW 3: Every force involves another equal but opposite force OR
Forces act in simultaneous pairs

7. Falling Bodies
Observation: All masses near surface of earth fall with same acceleration
Inference: Gravity acts so that the greater mass is pulled by a greater force

8. Gravitation Interaction at a distance between masses (any size, shape)
More mass => more force (directly proportional to mass)
Greater distance => less force (inversely proportional to square of distance: inverse square law)

9. Newton’s Law of Gravitation
Force is proportional to (M1 x M2)/Distance2
Outside of a body, acts as if all mass is concentrated at center
Weight is a force!
Predicts escape speed of one object leaving another (space travel!)

10. Newton’s Law of Gravitation
Results in Kepler’s laws (new version of 3rd; masses!)
Predicts/explains motions of planets with great precision
Physical basis of heliocentric model (earth’s rotation, revolution)
Powerful theory of gravitation

11. Newton’s Law of Gravitation
Predicts interactions of masses even outside solar system: a universal physical law!
Erased separation of earth and sky
Newton thought cosmos infinite; otherwise all masses would have already collapsed together