Physics 121

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Physics 121 General Physics IFall 2009

• Prof. Vinod Menon
• Office SB 204
• Phone [phone]
• Email vmenon_at_qc.cuny.edu
• http//www.physics.qc.edu/pages/vmenon/PHYS121/ind
  ex.html

Office Hour Tu, Th 300 pm 400 pm
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Physics

• Explains Nature
• Fundamental science
• Approach of reductionism
• First explained Chemistry - Now Biology
• Description of evolution, stock market etc.
• Most technology of today (cell phones, DVD
  player, etc) are result of discoveries that
  happened in physics last century.
• Develop problem solving and logical reasoning
  skills very important in any field of work!!!

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Units

• Physics is an experimental science
• Units define a quantity
• VERY IMPORTANT to know the units in which the
  measurements were made.
• - SI - m, kg, sec
• - CGS - cm, g, sec
• - BE - ft, slug, sec
• Consequences Mars Orbiter mission failure

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Types of Units

• Length Meter Distance that light travels in
  vacuum in a time of 1/299792458 sec
• Why speed of light Universal Constant 3 x
  108 m/s
• Mass Standard bar of Pt- Ir alloy
• Time Atomic clocks
• electromagnetic radiation emitted by Cesium
  133
• 1 second time needed for 9192631770 wave
  cycles to occur
• Accuracy 1 part in billion estimated to lose
  one sec in 1.7 million years (why do we need so
  much accuracy?)

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Atomic Clock
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Quantity in terms of base or derived units

• 1000 m 103 m 1km
• 0.001m 10-3m 1mm
• 1000 g 1Kg
• 0.001g 1mg
• Tera 1012, Giga 109, Mega 106
• Femto 10-15, Pico 10-12, Nano 10-9

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Role of UNITS in problem solving

• Need to know conversion
• Do problems with all units in the same system.
• Units should combine algebraically to give
  desired units
• Only quantities with same units can be added or
  subtracted.

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Dimensional Analysis

• Dimension Physical Nature of the quantity
• Fundamental Dimensions L, M, T
• Most physical quantities can be expressed in
  terms of these fundamental units. (exceptions -
  temperature)
• Dimensional analysis is used to check the math
  relation for consistency

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Trigonometry

• Sine, Cosine, Tangent

Sin ? ho/h Cos ? ha/h Tan ? ho/ha
h
ho

• Sin-1(ho/h)
• ? Cos-1(ha/h)
• ? Tan-1(ho/ha)

?
ha
NOTE Tan-1 ? 1/Tan
h2 ho2ha2
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Scalars and Vectors

• Scalars One that can be described by a single
  number (along with the unit)
• Examples volume, time, temperature, mass etc.
• Vectors Magnitude is only part of the story
• A quantity that deals with magnitude and
  direction is called a vector quantity.
• Textbooks use either A or A

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Vector Addition and Subtraction

• Should take into account both magnitude and
  direction.
• How to add perpendicular vectors
• How to add vectors at arbitrary angles
• Graphical method
• Subtraction

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Components of a vector

• r x y
• Components of a vactor can be used instead of the
  vector itself in any calculation
• X and Y are Perpendicular

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Scalar Components

• Ax and Ay have magnitude and direction
• Scalar Components magnitude of Ax or Ay
• example 8 meters

A
Ay
Ax
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Vector Components

• If the magnitude and direction of a vector are
  known one can break it into its components.
• For a vector to be ZERO every vector component
  must be zero
• Two vectors are EQUAL if and only if they have
  the same magnitude and same direction

?
r
?
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Addition of Vectors means of components
By
C
B
C
Cy Ay By
Bx
A
Ay
Cx Ax Bx
Ax
C2 Cx2Cy2 ? Tan-1(Cy/Cx)
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Strategy for problem solving

• For each vector, determine the X and Y components
  in a conveniently chosen coordinate system
• Find the algebraic sum of the
• x components x component of resulting vector
• y components y component of resulting vector
• Use x and Y components along with Pythagoras's
  theorem to get the magnitude of the resultant
  vector
• Use trigonometric functions to derive the
  direction of the resultant vector.