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PHY 101 – Introduction to Physics_2_

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					     PHY 101 – Intro to Physics
n   TR 10:30-1:20 pm
n   Combined Lecture/Lab
n   Instructor: Heidi Van Tassell
n   Office Hours:
       Monday: 10:00-10:50 am in PS108
       Tuesday: 9:30-10:20 am in PS211
       Wednesday: 10:00-10:50 am in PS108
       Thursday: 9:30-10:20 am in PS211
       Friday: 10:00-10:50 am in PS108

                MCC PHY101 - Van Tassell - 1/28/2010   1
          Now its your turn!
n   Lab 9 – Conservation of Momentum
    (Cont’d)
             Ch. 6 Momentum
n   To change the momentum, we need to change
    the velocity.
n   To change velocity you need an acceleration.
n   To accelerate, you must have an applied net
    force.
n   The greater the force applied, the greater
    the change in momentum!


                  MCC PHY101 - Van Tassell - 2/14/2011
                Momentum
n   To change the momentum, you must have an
    applied net force.
n   The time the force acts determines by how
    much the momentum is changed.

    Change in momentum= Force x Time




                 MCC PHY101 - Van Tassell - 2/14/2011
               Momentum
  Change in momentum= Force x Time

This comes out of Newton’s 2nd Law:




               MCC PHY101 - Van Tassell - 2/14/2011
Impulse & Momentum
Impulse is the quantity force x time.

   Impulse = change in momentum




         MCC PHY101 - Van Tassell - 2/14/2011
        Impulse & Momentum
n   Example: If you want to slow down
    gradually, you want a long time and a
    small force.

n   Example: If you want to stop suddenly,
    you want a large force, and a small time.


                MCC PHY101 - Van Tassell - 2/14/2011
                Quick Quiz
 Whenever an interaction occurs in an
 isolated system, forces occur in equal and
 opposite pairs. Which of the following do
 not always occur in equal and opposite pairs?
a)   Impulses.
b)   Accelerations.
c)   Momentum changes.
d)   But all of these occur in equal and opposite pairs.
e)   None of these do.

                 MCC PHY101 - Van Tassell - 2/14/2011
                Quick Quiz
An ice sailcraft is stalled on a frozen lake on
a windless day. A large fan (attached to the
sailcraft) blows air into the sail. If all the
wind produced by the fan strikes and
bounces backward from the sail, the craft
will move
a)   To the left (backward)
b)   To the right (forward)
c)   Not at all


                MCC PHY101 - Van Tassell - 2/14/2011
         Impulse & Momentum
n   Bouncing imparts greater impulse than just
    stopping because you actually have to change
    direction.

n   If we want to change velocity, we need a
    force (impulse) to be applied. If there is no
    external impulse then momentum of the
    system is conserved.



                  MCC PHY101 - Van Tassell - 2/14/2011
                    Collisions
n   Elastic Collision – When the colliding objects
    rebound without sound, or lasting
    deformation.
n   Inelastic Collision – When the colliding
    objects become entangled, produce
    deformations or sound.
n   Perfectly Inelastic Collision – When the
    colliding objects stick together after the
    collision.


                  MCC PHY101 - Van Tassell - 2/14/2011
                 Quick Quiz
n    If you throw a raw egg against a wall
     you’ll break it, but if you throw it with
     the same speed into a sagging sheet it
     won’t break. Is this because of
    a)   Decreased impulse
    b)   Increased contact time
    c)   Decreased change in momentum
    d)   All of the above

                 MCC PHY101 - Van Tassell - 2/14/2011
                    Quick Quiz
n    When you are traveling in your car at
     highway speed, the momentum of a bug is
     suddenly changed as it platters onto your
     windshield. Compared to the change in
     momentum of the bug, by how much does the
     momentum of your car change?
    a)   More
    b)   Less
    c)   The same


                    MCC PHY101 - Van Tassell - 2/14/2011
               Quick Quiz

 Would a head-on collision between two cars
 be more damaging to the occupants if the
 cars stuck together or if they rebounded
 upon impact?

a)   Stuck together.
b)   Rebounded.



                MCC PHY101 - Van Tassell - 2/14/2011
          Now its your turn!

n   Practice Page 21




                MCC PHY101 - Van Tassell - 2/14/2011
                 Ch. 7 Work
n   Work is the product of the applied force and
    the parallel distance through which the
    applied force acts.




n   Units:


                 MCC PHY101 - Van Tassell - 2/14/2011
                        Work
n   Work is a scalar quantity. It does not
    include a direction.

n   Work may be positive, negative or zero.

n   Work is measured in units of Joules


                MCC PHY101 - Van Tassell - 2/14/2011
                          Work
n   Positive work means that the force is helping
    the object to move in the direction indicated.
    For example, the work done by gravity in a
    book being dropped a certain distance.
n   Negative work means that the force is trying
    to stop the indicated movement of the object.
    For example, the work done by friction as a
    book slides across a table top.


                  MCC PHY101 - Van Tassell - 2/14/2011
                         Work
n   No work means that the force is neither
    helping nor hindering the indicated movement
    of the object, or no movement takes place.
    For example, the work done by gravity in
    holding a barbell over your head, or the work
    done by gravity in moving a book horizontally
    across a table.




                 MCC PHY101 - Van Tassell - 2/14/2011
             Quick Quiz
 How much work is done against gravity
 in a bowling ball rolling down a bowling
 alley?
a)   Positive amount of work
b)   Negative amount of work
c)   No work



              MCC PHY101 - Van Tassell - 2/14/2011
              Quick Quiz
 According to the scientific definition
 of work, pushing on a rock
 accomplishes no work unless there is
a)   movement
b)   a net force
c)   an opposing force
d)   movement parallel to the direction of the
     force


               MCC PHY101 - Van Tassell - 2/14/2011
                       Power
Power is a measure of how quickly work is done.
  The rate at which work is done.




  Units:

  Power is a scalar quantity. It does not include
  a direction.
                MCC PHY101 - Van Tassell - 2/14/2011
                       Energy
n   Energy is the ability to do work. When work is
    done on or by an object it results in a change
    in energy to the object.
n   Energy is a scalar quantity. It does not have
    direction.
n   Energy comes in many flavors. In this chapter
    we will study two particular forms of energy,
    but others exist and some will be discussed
    later.


                  MCC PHY101 - Van Tassell - 2/14/2011
             Types of Energy
n   Kinetic Energy is the energy of motion.

n   Potential Energy is the energy of position.
n   Mechanical Energy is the combination of
    kinetic and potential energies.
n   Chemical Energy is the type of energy
    involved in chemical reactions.


                  MCC PHY101 - Van Tassell - 2/14/2011
             Types of Energy
n   Thermal Energy is the energy of heat.
n   Radiant Energy is light energy that can travel
    through space.
n   Electrical Energy is energy due to
    electromagnetic interactions.
n   Nuclear Energy is energy derived from the
    atom.


                  MCC PHY101 - Van Tassell - 2/14/2011
             Potential Energy
n   Potential Energy is energy of position. When
    work is done to move an object against
    gravity, the object will have the potential to
    move or do work on its own. The energy it has
    is equal to the work done in moving the object
    against gravity.




                  MCC PHY101 - Van Tassell - 2/14/2011
         Potential Energy




For the purpose of potential energy, you can
choose where to measure the height from.
The height itself is unimportant, but changes
in height are very important. Thus you can
choose the zero point.

             MCC PHY101 - Van Tassell - 2/14/2011
           Potential Energy
Example: A 1kg book is found on the floor, and
  is lifted to a countertop which is 1 m above
  the ground. Find the initial potential energy
  and the final potential energy of the book. Do
  this first using the floor as a zero point, and
  repeat the calculations using the ceiling (3
  meters above the floor) as the zero point.



                MCC PHY101 - Van Tassell - 2/14/2011
Potential Energy




  MCC PHY101 - Van Tassell - 2/14/2011
Potential Energy




  MCC PHY101 - Van Tassell - 2/14/2011
              Kinetic Energy
n   Kinetic Energy is the energy of motion. When
    an object is moving it can do work on other
    objects.




                 MCC PHY101 - Van Tassell - 2/14/2011
              Work & Energy
    The result of work done against a resistance
    is a change in energy. There are many ways to
    change the energy of an object.
n   Increased kinetic energy – Work against
    inertia often results in an increase in the
    energy of motion.
n   Increased potential energy – Work against
    gravity and work against shape result in an
    increase in the energy of position.
                  MCC PHY101 - Van Tassell - 2/14/2011
              Work & Energy
n   Increased temperature – Work against
    friction results in an increase in temperature.
    This is actually an increase in the kinetic
    energy of the particles making up the object.
    We will learn more about this in chapter 15.

n   Increased combinations of kinetic energy,
    potential energy, and/or temperature. Again
    this is the most common and thus the most
    realistic, but these complexities are often
    ignored.
                  MCC PHY101 - Van Tassell - 2/14/2011
              Work & Energy
n   So when work is done on an object, its energy
    increases allowing it to do work on another
    object. In this way we say that energy is
    flowing into and out of objects all the time.
n   We can use the energy of an object to
    determine its motion under different
    circumstances. If we know what work is being
    done on or by the object we can predict the
    changes in mechanical energy and thus the
    motion of the object.

                 MCC PHY101 - Van Tassell - 2/14/2011
              Work & Energy
n   At this time we have not learned how to
    include the energy gain due to increased
    temperature, so we will not be able to employ
    the concept of conservation of to problems
    involving friction.
n   We also will be limited to problems that do
    not involve the other types of energy we have
    yet to discuss.
n   Within these limitations, the energy of a
    system should be conserved.
                  MCC PHY101 - Van Tassell - 2/14/2011
            Work & Energy
n   Work-Energy Theorem:




               MCC PHY101 - Van Tassell - 2/14/2011
       Conservation of Energy
n   Conservation of Energy Theorem:




               MCC PHY101 - Van Tassell - 2/14/2011

				
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