Homework

1
Homework

• Work Mastering Physics Assignments
• Study for Exam
• rotational motion
• torque and angular momentum
• static equilibrium
• gravity
• Review Session - 7 PM - HL 188
• Work Reading Assignment for Friday
• Work on Lab Presentation

2
Example

• A 150 kg satellite is in circular orbit of radius
  7.3 Mm about the earth.
• Determine the potential energy.
• U -8.2 x109 J
• Determine the orbital speed.
• v 7400 m/s
• Determine the kinetic energy.
• K 4.1x109 J
• What is the escape speed from this altitude?

3
Exercise

• Why are rocket launch sites located nearer to the
  equator (rather than the poles)?

4
Angular Momentum in the Bohr Model of the Atom
In the Bohr model The electron orbits like a
planet. The electron can only have certain values
of angular momentum. L n (h/2?)
where n is an integer
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Angular Momentum in Quantum Mechanics

• L ? (h/2?)
• gives length of angular momentum vector
• Lz ml (h/2?)
• gives orientation of the angular momentum vector
• The Bohr quantum number

Lz
L
?
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Quantum Numbers determine the shape of the state
1s
2s
2p
l 0 for s-state l 1 for
p-state l 2 for d-state L r x mv
Only s-states (L 0) can have electrons at the
origin.
7
More Static Equlibrium

• How does a truck scale work?
• If the front wheels give a weight of 10000 N, the
  middle wheels (which are 3 m back) give a weight
  of 30000 N and the rear wheels (which are 10 m
  further back) give a weight of 25000 N, where is
  the center of mass of the truck?

8
Opening the paint can

• A man tries to pry the lid off a can of paint
  with a very light screwdriver.
• A force F is applied a distance L from the edge
  of the can. The screwdriver pushes up on the lid
  a distance d from edge of the can.

• What is the upward force on the lid?
• What is the normal force supplied by edge of the
  can on the screwdriver?

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Example

• If the wall is frictionless, the floor is not,
  what are the forces acting on the ladder?

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Followup

• If the wall is frictionless, the coefficient of
  static friction between the ladder and the floor
  is 0.8, how large is the force of static friction?

11
Example

• At what angle will the contents spill?

12
Example

• A 9 m flagpole hangs in a horizontal position
  from the side of a building. It is supported by a
  hinge and a 15 m cable. Find the tension in the
  cable and the magnitude and direction of the
  force supplied by the hinge if the flagpole
  weighs 1500 N?