Transcript Document 7509957

Planetary Motions
Newton’s Laws:
The New Physics
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
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
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!)
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
Newton’s Laws
• LAW 3: Every force involves
another equal but opposite
force
OR
• Forces act in simultaneous
pairs
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
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)
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!)
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
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