Swain Hall West- 1st Floor

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Secretarys office
007 Stairs
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Swain Hall West- 2nd Floor
Q. Bailey office on third floor 331
Physics Forum
Library
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CALM system
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CALM system
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P221 Lecture 1

• Welcome to P221 go over the syllabus
• Question What is Physics (science in general)?
  Why do the objects we observe in everyday life
  come to rest when left alone?
• Any quantity in science has three elements
• A number (3, or 2.56, or 6340.094 say).
• A unit (3 meters, 2.56 liters, 6240.094 seconds)?
• An uncertainty (either explicit or implicit).

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Adding numbers requires the right UNITS!!
This sign makes no sense, you cant add years and
people!
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Units and Uncertainty

• Always check your units!! You cant have the
  correct answer if your units are wrong or
  inconsistent!
• An informative website for the definition and
  meaning of units can be found at
• http//physics.nist.gov/cuu/Units/units.html
• Sometimes the uncertainty in a number is
  expressed explicitly, sometimes implicitly
• Explicit 3.2265 /- 0.0003
• Implicit 3.23 or 0.674 (3 sig. figs.
  Uncertainty is understood to be /- 5 in the next
  digit, in these examples it is /- 0.005 and /-
  0.0005 respectively). Note therefore, the
  relative uncertainty in 1.00 is quite different
  from that of 9.99. UNCERTAINTY is the key, not
  number of significant figures.

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Hints for solving problems

• The text gives you a number of strategies for
  solving problems, pay attention to them. Since it
  is often useful to have two views of such
  important matters, here is my version of the key
  strategic points
• RTFQ!!
• Draw a diagram (e.g. identify all forces here).
• Organize the data provided, ask yourself what the
  question is after (explicit and implicit). Why
  was the question asked?
• Identify the crucial concept(s) and equation(s).
  What do you know about the quantity that is being
  sought and what can you relate it to? Can these
  be linked to the data provided?
• SIMPLIFY can the problem be broken into smaller
  parts.
• SIMPLIFY by doing some algebra before inserting
  numbers.
• Put numbers into the equation(s) (CHECK YOUR
  UNITS!!!)?
• Round off to proper uncertainty (number of sig.
  figs.) ONLY at the end of the calculation.
• Check to make sure that the answer makes sense
  (check units, is it manifestly too large or too
  small, etc.) This may be easier if you can make a
  rough guess for the size of the answer (see
  below).

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Estimations

• If someone were to suggest to you that the Earth
  weighed 1 billion billion tons (i.e.
  109x109x103 1021 kg), would you say that was
  about right, too small, or too big?

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Newtons Laws of Motion

• I If no net force acts on an object, then that
  objects velocity (speed and direction of motion)
  will not change.
• II The net force on an object (F) is directly
  proportional to its acceleration (a) ( i.e. the
  rate of change of the objects velocity) the
  proportionality constant is the objects mass, m
• F m a
• III When two objects interact, the force of one
  on the other is precisely equal in size, but
  opposite in direction as the force of the second
  on the first.

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P221 Lecture 2

• The horse pulls forward slightly harder on the
  buggy than the buggy pulls backward on the horse,
  so they move forward. (T 26 F 7 Other 0
  None 12)?
• True. The force the horse is exerting on the
  buggy must be greater than the force the buggy is
  exerting on the horse in order for the buggy to
  move forward in the direction of the force
  provided by the horse.
• This is true because the horse has to pull harder
  and provide more force on the buggy for it to
  move forward. If the buggy pulled backward with
  more force than the horse, then the horse and
  buggy would more backwards.
• False The horse moves forward because the ground
  exerts a force on the horse in reaction to the
  horse's pushing against the road, which results
  in a net force forward This is correct!!!

• BE CAREFUL OF WHAT FORCES YOU ARE TALKING
  ABOUT.RTFQ!!
• The question is, HARDER THAN WHAT?
• DVB THE EXTERNAL FORCES PROVIDE THE KEY TO
  MOTION!!!

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P221 Lecture 2

• The force on the buggy is as strong as the force
  on the horse, but the horse is joined to the
  earth by its flat hoofs, while the buggy is free
  to roll on its round wheels.
  (T 16 F 15 Other 2 None 12)?
• False because if the forces were equal, there
  would be no acceleration of the horse and buggy.
  Even though the buggy is on round wheels, the
  force vectors cancel each other out and provide
  the same necessary force to not move.
• This is not true. Each body has a different mass.
  A different mass results in a difference Force
  since force is dependent of mass.
• It is true that the force on the buggy is the
  same as the force on the horse. But the
  difference between round wheels and flat hooves
  is not the reason the buggy moves forward. The
  horse provides enough force to overcome the
  friction between the street and the buggy so that
  the buggy can move forward. (This is the key, and
  the hooves vs. wheels are in fact, the reason for
  this difference in external forces!)?

• Several answers were like this, which
  anticipates the next question
• Be careful of your concepts. Except for gravity,
  force is independent
• Of mass.

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P221 Lecture 2

• Newtons third law states clearly that the force
  of the horse on the buggy is equal and opposite
  to the force of the buggy on the horse. Hence the
  two forces cancel each other out and the buggy
  cannot move forward. (T 9 F 21 Other 3
  None 12)?
• True, but is should be stated that the forces are
  equal because of the absence of acceleration. If
  the horse and buggy are on a moving floor,
  accelerating the horse and buggy in one
  direction, the magnitude of the forces change.
• It's true that the two forces are equal and
  opposite, but it's false that the two cancel out
  since the forces do not apply to the same object.
  The buggy moves forward from the force of the
  horse if the horse overcomes the force of the
  wagon by some other means such as using the
  ground to propel itself forward. THIS IS THE
  RIGHT ANSWER!!!

• NO! These two forces are equal irrespective
  of whether the horse and buggy
• are accelerating or not!. DVB
• Only 3 or 4 of you really nailed the reason for
  the statement being false.

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Sample Warm-up (CALM) Q

• In the diagram below, what is the element that
  provides the force holding the weight up against
  the force of gravity? Why/How?

Ceiling
String
Wall 1
Wall 2
Weight
ground

• Answer

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P221 Lecture 2
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P221 Lecture 2
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Newtons Laws of Motion

• I If no net force acts on an object, then that
  objects velocity (speed and direction of motion)
  will not change.
• II The net force on an object (F) is directly
  proportional to its acceleration (a) ( i.e. the
  rate of change of the objects velocity) the
  proportionality constant is the objects mass, m
• F m a
• III When two objects interact, the force of one
  on the other is precisely equal in size, but
  opposite in direction as the force of the second
  on the first.

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Explaining motiontake-home lessons

• FORCES are the key we need to develop tools for
  identifying and quantifying forces.
• CONTACT, GRAVITY (for now) only!!
• Forces cause CHANGES in motion, they DO NOT CAUSE
  motion itself!!
• Be careful with our (rather precise) use of
  terminology in this course, it could differ in
  subtle but important ways from your previous
  experience.
• Watch your units!!

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Terminology

• The previous slide introduced a number of terms
  (in italics) that have particular meaning to
  physicists. Please be careful in using the ideas
  associated with these words.
• Velocity (v) Gives the direction of an objects
  motion as well as its speed in that direction
  (our first example of a VECTOR).
• Forces (F) clearly we need to get practice
  identifying forces, since they play a key role in
  determining motion (though not necessarily the
  role your intuition might lead you to believe).
  We consider
• Contact (arise from physical contact between two
  objects, e.g. pushes, pulls, spring forces,
  friction, etc.)?
• Fundamental (gravity, nuclear (2 types), and
  electro-magnetic).
• For now we will only concern ourselves with
  contact forces and gravity. The other fundamental
  forces we ignore for p221.
• Acceleration (a) Time rate of change of an
  objects velocity (more on this later) NOTE If
  a0, then the velocity DOES NOT CHANGE.
• Mass (m) measure of an objects resistance to
  changes in its velocity (i.e. a measurement of
  the objects inertia).

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• The figure shows a displacement vs. time graph
  for a ball rolling along the floor. During the
  third second, what was the approximate the speed
  of the ball? Please describe how you obtained
  your answer.
• The class did quite well on this one, no need to
  review (25 correct 6 incorrect sloppy with
  units, or insufficient info to tell the mistake
  17 no ans.)?
• It could be approximated by the average speed.
  The displacement/total time. 8m/4s 2m/s
  (810-2, 4 5-1 to get the most accurate
  number).

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What is the average velocity over the time from 1
sec to 9 sec?

• The figure shows a displacement vs. time graph
  for a ball rolling along the floor. During the
  third second, what was the approximate the speed
  of the ball? Please describe how you obtained
  your answer.
• The class did quite well on this one, no need to
  review (25 correct 6 incorrect sloppy with
  units, or insufficient info to tell the mistake
  17 no ans.)?
• It could be approximated by the average speed.
  The displacement/total time. 8m/4s 2m/s
  (810-2, 4 5-1 to get the most accurate
  number).

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What is the average velocity over the time from 1
sec to 9 sec? What is the average speed?

• The figure shows a displacement vs. time graph
  for a ball rolling along the floor. During the
  third second, what was the approximate the speed
  of the ball? Please describe how you obtained
  your answer.
• The class did quite well on this one, no need to
  review (25 correct 6 incorrect sloppy with
  units, or insufficient info to tell the mistake
  17 no ans.)?
• It could be approximated by the average speed.
  The displacement/total time. 8m/4s 2m/s
  (810-2, 4 5-1 to get the most accurate
  number).

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• During aerobic exercising, people often suffer
  injuries to knees and other joints due to HIGH
  ACCELERATIONS. When do these high accelerations
  occur? (15 vertical 12 lateral 4 unclear 17 no
  ans.)?
• These high acceleration occur when the athlete
  speeds up too quickly. The speed needs to be
  increase slowly and gradually. This can also
  happen when the motion suddenly comes a complete
  stop.
• These accelerations occur when the legs move up
  and down at high speeds. (don't confuse speed an
  accel.)?
• High accelerations occur when one speeds up or
  jumps. Also, high accelerations occur when one
  changes direction. (what do you mean by changing
  direction here?)?
• Virtually everyone figured that it is
  acceleration that is the key (good), but a large
  number left it at that, or talked about changes
  of velocity (many with an explicit statement that
  the motion along the ground was the key motion).
  IN FACT it is the abrupt (ltlt1sec) change in
  vertical motion when you impact with the
  ground/floor.
• Don't get trapped into thinking that velocity is
  ONLY along the ground (more on this next week)!