Ideas of Modern Physics

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Ideas of Modern Physics
Science is an adventure of the whole human race
to learn to live in and perhaps to love the
universe in which they are. To be a part of it is
to understand, to understand oneself, to begin to
feel that there is a capacity within man, far
beyond what he felt he had, of an infinite
extension of human possibilities. I.I. Rabi
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Review

• Weak, color, electromagnetic, and gravitational
  force fields like matter.
• Light is propagating electromagnetic force field.

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Today

• Numbers and the size of some things
• Motion and relativity

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Big and small numbers

• Human scale units (m,s,kg) arent always
  appropriate
• Notation two and one half million meters
  2,500,000 m 2.5x106 m 2.5e6 m
• If we were that big, our natural unit would have
  been about a megameter

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Some scales

• Proton radius 1e-15 m 1 femtometer
• Atomic radius 0.1e-9 m 0.1 nanometer 1e5 rp
• Single cell diameter 1e-6 m 1 micrometer
• Human hair diameter 50 micrometer (visible)
• Earth radius 6e6 m 6,000 kilometer (km)
• Distance to sun 1.5e11 m 1 AU 25,000 rE
• Distance to nearest star 1e17 m 1e6 AU
• Diameter of Milky Way 1e20 m (factor 1e3)
• Distance to nearest galaxy 1e23 m

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Light speed

• c 299,792,458 m/s 3e8 m/s 1 foot/ns where 1
  nanosecond 1e-9 s
• Earth-sun distance 500 light seconds 8.5
  minutes at light speed Astronomical Unit (AU)
• Nearest star 4 light years distant

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Motion

• Motion is the process of changing position.
• It is fascinating to kids and adults!
• It bears careful thought.
• Our eyes and brains are wired to detect motion.
  Evolution is responsible for that!

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The science of motion

• Galileo Galilei (1564 - 1642)

Used telescope to see moons of Jupiter and phases
of Venus and argued the earth orbited the sun.
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Frames of reference

• Position is relative. A thing has position
  relative to something else!

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Position

• The position of object along a line is the
  distance from a reference object or point.

0
1
2
3 m
-1
-2
-3 m
Position can be positive or negative - the
magnitude and sign give distance and direction.
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Dimension of Space

• For no established reason, we live in 3 space
  dimensions.
• The position of an object is in general
  determined by 3 numbers, the projected distances
  along three perpendicular directions

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Velocity

• Motion means position is changing.
• Velocity is the change in position dx in a time
  interval dt divided by the time interval
• v dx/dt

Example A car moves in reverse a distance of dx
-6 m during a time interval dt 2 s. Its
velocity is v -6 m / 2 s -3 m/s
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Acceleration

• The velocity of an object can change too!
• If the change in velocity is dv in time interval
  dt, the acceleration is
• a dv/dt

Example A car accelerates from rest to 10 m/s
in 10 s. The change in velocity is dv10 m/s in
time dt10 s. The acceleration is a dv/dt
(10 m/s)/(10 s) 1 m/s2 .
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Velocity is relative

• Suppose you and I are on train that moves with
  velocity 10 m/s relative to the ground.
• If you are sitting, your velocity relative to me
  is zero but your velocity relative to the ground
  is 10m/s

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Velocity addition

• Suppose you walk forward on a moving train at 1
  m/s so 5 m relative to the train in 5 s. Suppose
  the train moves forward at 2 m/s so 10 m in the
  same 5 s time interval.
• Relative to the earth, your displacement was 5m
  10 m 15 m in 5 s
• v(relative to ground)
• v(relative to train) v(train relative to
  ground)

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Principle of relativity

• Although the appearance of any instance of motion
  depends on reference frame, the same kinds of
  motions appear in all frames of reference.
• Fundamental principle Physics is the same in all
  inertial frames of reference.