- Get lab manual and look at first experiment before your first lab. Labs start week of Jan. 25; room Olin 105

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Announcements

• - Get lab manual and look at first experiment
  before your first lab. Labs start week of Jan.
  25 room Olin 105
• Homework 1.1 due this Thursday, in class
• Bring i-clicker to class
• You are allowed 30 missed points in the i-clicker
  total score ( 160 points)

• Homework TAs
• Jillian Bjerke (room 311)
• Colby Meador
• Calli Nguyen
• Lab TAs
• Samrat Dutta, Ashley Carlton, Maggie Baldwin,
  Calli Nguyen, Jack Own, Zach Vance
• (contact TAs if you have questions about lab or
  homework grading first)

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PHY110 TUTOR SESSIONS
Tutor Jillian Bjerke Maggie Baldwin Session
1 Mo, 4-6 pm (Jill) Session 2   We, 4-6
pm (Jill) Session 3 Th, 5-7 pm (Maggie) All
tutorial session will be in Olin 101 (class
room). The tutor sessions in semesters past
were very successful and received high marks from
many students. All students are encouraged to
take advantage of this opportunity. 
There are also private tutors available, contact
Judy Swicegood in the Physics office (Olin 100)
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Chapter 1 The laws of motion, Part I First two
chapters Introduce the language of
physics Subsequent chapters Explore objects and
underlying physical concepts

• Reading assignment for today Chapter 1.2
• Homework 1.2 (Meador Colby)
• due Thursday, Jan. 28 in class
• Exercises 14, 15, 18, 19
• Problem 3, 4, 5, 6, 7

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Chapter 1.2 Falling balls
Concepts
Demos and Objects
Gravity Weight Everything falls at the same
rate Symmetry of up and down motion Vector
components
Falling balls Two falling balls Throwing
balls Shooting a falling apple (target)
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i-clicker question-1

• Suppose that I throw a ball upward into the air.
  After the ball leaves my hand,
• there is a force pushing the ball upward.
• there is a force pushing the ball downward.
• there is no force acting on the ball.
• it moves at constant velocity.

We are ignoring friction, drag, buoyant force
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Observations About Falling Balls

• A dropped ball
• Begins a rest, but soon acquires downward speed
• Covers more and more distance each second
• A tossed ball
• Rises to a certain height
• Comes briefly to a stop
• Begins to descend, much like a dropped ball

? There is a _________ force acting on the ball!
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i-clicker question-2

• You are throwing a ball straight up in the air.
• At the highest point, the balls
• velocity and acceleration are zero.
• velocity is nonzero but its acceleration is zero.
• acceleration is nonzero, but its velocity is
  zero.
• velocity and acceleration are both nonzero.

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Brief Aside Gravity and Newtons Law of
Universal Gravitation
Every particle in the Universe attracts every
other particle with a force of
G Gravitational constant G 6.67310-11
Nm2/kg2 m1, m2 masses of particles 1 and 2 r
distance separating these particles

• On earths surface
• Gravity creates a downward force on any object.
  
• Object attracted directly to towards the center
  of the earth.
• Gravitational force is equal to the objects
  weight.

Weight F mg
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Mass, weight and gravity from a physicists point
of view

• The mass of an object, m, does not change. It
  is measured in kilogram.
• The weight of an object is given by W mg,
• g acceleration due to gravity
• m mass of the object
• Weight is a force!
• Gravity attractive force between two objects.
• (mainly between the earth and objects on earth)

On earth g 9.8 m/s2 On the moon g 1.6 m/s2
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A Falling Ball

• Falling ball accelerates steadily downward
• Its acceleration is constant and downward
• Its velocity increases in the downward direction
• Falling from rest (stationary)
• Velocity starts at zero and increases downward
• Altitude decreases at an ever faster rate

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How can we quantitatively describe motion?
Motion with constant acceleration
1. Velocity
Velocity starting velocity acceleration x
time v v0 a t
2. Position
Position x x0 v0 t 1/2 a t2
x0 starting position v0 starting velocity
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A dropping ball
Quantitatively
1. Acceleration is constant a g - 9.8 m/s2
2. Velocity increases v v0 gt But v0 0
because ball released from rest so v - 9.8
m/s2 t
3. Position changes rapidly x x0 v0t 1/2
gt2 with x0 0 and v0 0 we have x 0.5(-9.8
m/s2 ) t2
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Black board examples

• If it takes you 1.4 s to reach the water from a
  10 m platform, how fast will you go just before
  you enter the water?
• How far will you fall during a 5-second free fall?

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A Falling Ball, Part 2

• A falling ball can start by heading upward!
• Velocity starts in the upward direction
• Velocity becomes less and less upward
• Altitude increases at an ever slower rate
• At some point, velocity is momentarily zero
• Velocity becomes more and more downward
• Altitude decreases at ever faster rate

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i-clicker question-3
You have the ability to throw a rock with a speed
of 30 m/s (67 mi/h). In order to break the ice
of a pond, do you throw the rock up or down? a.
Straight up b. Straight down c. Doesnt matter
We are ignoring friction, drag, buoyant force,
same release height
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Throwing a ball upward
Note that the up and down motions are
symmetric!! It takes the same time to go up as it
does to come back down.
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Throwing a ball straight up and down
Going down
Going up
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Throws and Arcs

• Gravity only affects vertical motion
• A ball can coast horizontally while falling
  vertically

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Bouncing Balls Same Time up as Down
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Vertical and horizontal components of a vector
(velocity, acceleration, force, )
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The vertical velocity gets smaller as the
horizontal component gets larger
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i-clicker question-4
i-clicker question-4

• An apple drops at the same time a shot is fired.
  Do we aim
• At the target?
• Above the target?
• Below the target?

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i-clicker question-5
Dropping balls

• Which ball will hit the ground first?
• Straight drop
• Straight out
• Both at the same time