Physics 211 Lecture 8, Slide 1

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Physics 211 Lecture 8
Today's Concepts Potential Energy Mechanical
Energy
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Stuff you asked about
25x pudding
Non-conservative forces and their relationship
with Potential and Mechanical energy.
Everything. All of it.
not really any, all seem easy
Why is potential energy represented by "U"?
i hate problems that deal with space and things
away from the earth.
I would like to discuss more on Gravitational
Potential energy, from distances far away from
Earth's surface.
I still want a pudding pizza party. I could put
it together if you want, maybe like 5-7 per
person depending on how many people and the kinds
of pudding (cups or homemade). I could get the
jello brand and make large quantites. I think
that would work well. I could delegate drinks to
someone else if I had to.
more spring problems. and if i find out who keeps
typing pudding i will punch them in the face.
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MIDTERM NEXT WEEK
October 1, 7pm (Thursday Night)
We will give a conflict exam at 515pm on the
same night. Anyone can sign up for this in your
grade book
Best way to study Do old exams online
Exams rooms TBA on website
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Will this be your first exam at the U of I ? A)
YES B) NO
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Total Mechanical Energy E Kinetic Potential
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There is nothing new here -
Its just re-writing the work-KE theorem
More intuitive
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Preflight
Three balls of equal mass are fired
simultaneously with equal speeds from the same
height h above the ground. Ball 1 is fired
straight up, ball 2 is fired straight down, and
ball 3 is fired horizontally. Rank in order from
largest to smallest their speeds v1, v2, and v3
just before each ball hits the ground.
A) v1 gt v2 gt v3 B) v3 gt v2 gt v1 C) v2 gt v3 gt
v1 D) v1 v2 v3
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A) v1 gt v2 gt v3 B) v3 gt v2 gt v1 C) v2 gt v3 gt
v1 D) v1 v2 v3
They all have the same potential energy and
launching speed, so when it's all turned
transformed into kinetic energy, the final speeds
are the same.
path does not matter, gravity is conservative
I'm going to answer this in the form of a
limerick. Excuse my abuse of physics. Three balls
of same mass are fired One up, one right, and one
downward. Kinetic energy's the same. Same
velocity is to blame. But more energy is still
acquired. All balls started at a fixed height.
Meaning potential energy is a might. And the
displacement's the same. So the energy to gain Is
the same for all three balls, alright? The energy
for all balls is the same. At the end of this odd
little game. So they have the same speed I'm so
glad we agreed! The answer is C! this is so very
lame
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Act
A block of mass m is launched up a frictionless
ramp with an initial speed v and reaches a
maximum vertical height h. A second block having
twice the mass (2m) is launched up the same ramp
with twice the initial speed (2v). What is the
maximum vertical height reached by the second
block?
A) h B) h C) 2h D) 4h
v
m
h
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Act
A block attached to a spring is oscillating
between point X (fully compressed) and point Y
(fully stretched). At point X, which of the
following quantities would reach its maximum
value?
A) The blocks kinetic energy B) The spring
potential energy C) The magnitude of the blocks
acceleration D) Both A) and C) E) Both B) and C)
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Act
A block attached to a spring is oscillating
between point X (fully compressed) and point Y
(fully stretched). At point O, which of the
following quantities would reach its maximum
value?
A) The blocks kinetic energy B) The spring
potential energy C) The magnitude of the blocks
momentum D) Both A) and C) E) Both B) and C)
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Preflight
A box sliding on a horizontal frictionless
surface runs into a fixed spring, compressing it
a distance x1 from its relaxed position while
momentarily coming to rest. If the initial speed
of the box were doubled, how far x2 would the
spring compress ?
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Preflight
In Case 1 we release an object from a height
above the surface of the earth equal to 1 earth
radius, and we measure its kinetic energy just
before it hits the earth to be K1. In Case 2 we
release an object from a height above the surface
of the earth equal to 2 earth radii, and we
measure its kinetic energy just before it hits
the earth to be K2. Compare K1 and K2.
A) K22K1 B) K24K1 C) K24K1/3
D)K2 3K1/2
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