SMDEP Physics

1
SMDEP Physics

• Work, Power, Potential Energy

2
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

3
Fridays Mechanics Quiz

• No calculators needed/allowed
• No clickers needed
• Dont 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?

4
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 krines_at_astro.yale.edu
• AC andrew.cahoon_at_yale.edu
• Office hours KR today 5-6 pm Marigolds

5
Ch 8, 1 distance spring stretched

• 0.09 m
• 0.9 m
• 9 m
• 90 m
• Other
• Didnt finish

6
Ch 8, 8 is air resistance conservative?

• Yes
• No
• Sometimes
• Didnt finish

7

• 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, Earths gravity

8
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?

• T3 gt T2 gt T1 T4
• T3 T2 T1 T4
• T1 gt T2 T4 gt T3
• None of the above

9
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

10
Consider a person standing in an elevator that is
accelerating upward. The upward normal force
exerted by the elevator floor on the person is

• Larger than
• Equal to
• Smaller than

..the downward weight of the person
11
You stand on a scale in your bathroom. It reads
150 lb. In which of the following situations will
it read less?

• 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

12
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 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

• Twice
• The same as
• Half

..what it was before
13
Consider a weightlifter holding a 500 pound
barbell stationary above his head. The
weightlifter is doing

• Positive work on the barbell
• Negative work on the barbell
• No work on the barbell