Announcements

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Announcements

• Homework for tomorrow
• (Ch. 5, Problems 5 8)
• Office hours
• M 1-2 pm
• T 1130-1230 pm
• WF 3-4 pm
• R 10-11 am
• Tutorial Learning Center - Houston Hall 110
• MWF 8am-5pm
• T 8-10am, 11am-2pm, 3-5pm
• R 8-9am, 11am-5pm

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Exam answers
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Exam answers
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Exam answers
5
Chapter 5

• Energy
• (Work)

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Energy overview

• It is important to realize that in physics
  today, we have no knowledge of what energy
  is. Richard Feynmann
  Nobel Laureate
• The Newtonian formalism of Mechanics was
  unrivaled for over a hundred years.
• Beginning of 1800s saw the advent of the concept
  of energy, which had enormous predictable power.
• Law of Conservation of Energy
• Scalars versus Vectors

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Energy overview

• Energy
• Property of all matter, not an entity in and of
  itself
• Force is the agent of change,
  Energy is a measure of the change

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Different forms of Energy

• Kinetic
• Potential
• Thermal
• Light (radiant)
• Sound

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Section 5.1Work

• 1D case
• Work done on the body is positive since the
  direction of the force is the same as the
  displacement.
• What are the units of W?

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Work (done by a constant force)

• 2D case
• ? is the angle between vectors tail-to-tail
• What angle yields the maximum work (done by F)?
• What angle yields the minimum work (done by F)?
• What angle yields zero work (done by F)?

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Question 1

• A crate moves to the right on a horizontal
  surface as a woman pulls on it with a 10 N force.
  Rank the situations shown below according to the
  work done by the 10 N force, least to greatest.
  The displacement is the same for all cases.
• 1, 2, 3
• 2, 1, 3
• 2, 3, 1
• 1, 3, 2
• 3, 2, 1

10N
10N
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Frictional Work

• The block undergoes a displacement by sliding
  down the incline.
• What is the work done by friction?
• What is the work done
  by the normal force?

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Dissipative Forces

• Energy is lost to friction by an object which
  goes into
• heating (both the object and its environment)
• sound
• Friction is a dissipative or nonconservative force

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Serway Prob. 5.7

• A sledge loaded with bricks has a total mass of
  18.0 kg and is pulled at constant speed by a rope
  inclined at 20.0º above the horizontal. The
  sledge moves a distance of 20.0 m on a
  horizontal surface. The coefficient of kinetic
  friction between the sledge and surface is 0.500.
• a. What is the tension in the rope?
• b. How much work is done by the rope on the
  sledge?
• c. What is the mechanical energy lost due to
  friction?