Transcript Class 11, 3 July

SMDEP Physics
Work, Power, Potential Energy
New Syllabus
• Available at front
• Shifting topics by one day through
“gravity”; one day less on gravity
– NO HOMEWORK FOR WEDS!
– Also, no TA session Weds
• Andrew teaching WTh, MTu
– KR available by email
• Quiz on Friday
Friday’s Mechanics Quiz
• No calculators needed/allowed
• No clickers needed
• Don’t memorize formulas
– Any formulas needed will be provided
• Mostly qualitative, conceptual questions
• Length: About 30 minutes
• Material: Kinematics, Mechanics, Energy
– Circular motion, friction, springs
– NOT: vectors, dot products, momentum
• Grades not instant
• Also: Midcourse evaluations
– What would you change about the course?
Resources
• Textbook: especially end-of-chapter
“Questions”
• New web site
– http://www.astro.yale.edu/krines/smdep/
– Powerpoints of first two weeks, more to come
• Email
– KR: [email protected]
– AC: [email protected]
• Office hours: KR today 5-6 pm Marigolds
Ch 8, #1: distance spring stretched
0.09 m
0.9 m
9m
90 m
Other
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Ch 8, #8: is air resistance
conservative?
Yes
No
Sometimes
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• Normal forces
– Always act at right angles to surface
– Force of electron clouds repelling each other
– Friction always normal to normal force, so friction
is always parallel to surface
• “Centrifugal forces” – NO!
• “The centripetal force” – No
• “A centripetal force” – Yes
– Centripetal force is always exerted by something
touching object or gravity: e.g., normal force from
rollercoaster track, string/rope tension force,
Earth’s gravity
A stone is attached to a string is whirled in a
vertical plane. Let T1, T2, T3, and T4 be the
tensions required (at the locations shown) for
the stone to have a constant speed v0
throughout. Which is true?
...
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=.
..
T2
=.
..
N
>
T1
T3
=
T2
>.
..
13% 10%
5%
T3
T2
>
T3 > T2 > T1 = T4
T3 = T2 = T1 = T4
T1 > T2 = T4 > T3
None of the above
T3
1.
2.
3.
4.
73%
T2
T4
T1
You are pushing a wooden crate across the floor
at constant speed. You then turn the crate on
end, reducing by half the surface area touching
the floor. In the new orientation, to push the
same crate across the same floor with the same
speed, the force that you apply must be about:
Four times greater
Twice as great
Equally great
Half as great
One-fourth as great
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Consider a person standing in an elevator that is
accelerating upward. The upward normal force
exerted by the elevator floor on the person is:
1.
2.
3.
Larger than
Equal to
Smaller than
..the downward weight
of the person
You stand on a scale in your bathroom. It reads
150 lb. In which of the following situations will
it read less?
1.
2.
3.
4.
5.
In an elevator accelerating
upward
In an elevator accelerating
downward
In an elevator moving up at
constant velocity
In an elevator moving down at
constant velocity
Two of the above
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Twice
The same as
Half
..what it was before
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2.
3.
Tw
70
In the 17th century, Otto von Güricke, a
physicist in Magdeburg, fitted two hollow
bronze hemispheres together and removed the
air from the resulting sphere with a pump. Two
eight-horse teams could not pull the halves
33% 33% 33%
apart even though the hemispheres
fell apart
when air was re-admitted. Suppose von Güricke
had tied both teams of horses to one side and
bolted the other side to a heavy tree trunk. In
this case, the tension on the hemispheres would
be:
10
Consider a weightlifter holding a 500
pound barbell stationary above his head.
The weightlifter is doing:
86%
1. Positive work on
the barbell
2. Negative work on
the barbell
3. No work on the
barbell
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