Physics 101: Lecture 12 Work and Energy

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Physics 101 Lecture 12Work and Energy

• Chapter 6, sections 6.1, 6.2, 6.3 (Work and
  Energy)
• Reminders
• Exam I, Tuesday September 30th at 5 PM
• See PHY101 Web page for room assignments
• Do not forgot to bring your UB ID card !

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Work Energy

• An important concept in physics
• Alternative approach to mechanics
• Many applications beyond mechanics
• Thermodynamics (movement of heat)
• Quantum mechanics...
• Very useful tools
• You will learn new (sometimes much easier) ways
  to solve problems

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Work done by a Constant Force

• The work done on an object by a constant (i.e.
  displacement independent) force F is given by
• W Fs s
• Unit W N m J (Joule)
• s magnitude of displacement
• Fs magnitude of the force in the direction of
  the displacement
• W can be positive or negative
• W Fs s if Fs points in the same
  direction as s
• W - Fs s if Fs is opposite of s

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Lecture 9, Preflight 1 2

• You are towing a car up a hill with constant
  velocity. The work done on the car by the normal
  force is
• 1. positive2. negative3. zero

The normal force is perpendicular to the
displacement, hence, does no work.
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Concept Question

• You are towing a car up a hill with constant
  velocity. The work done on the car by the
  gravitational force is
• 1. positive2. negative3. zero

With the surface defined as the x-axis, the x
component of gravity is in the opposite direction
of the displacement, therefore work is negative.
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Concept Question

• You are towing a car up a hill with constant
  velocity. The work done on the car by the
  tension force is
• 1. positive2. negative3. zero

Tension is in the same direction as the
displacement.
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Work/Kinetic Energy Theorem
Wnet Fnet s m a s m (v2 v02)/2 Ekin
m v2/2 is called the kinetic energy of an object.

• Net Work done on an object
• change in kinetic energy of object

• Also works for a variable force !

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Work done by Gravity

• Object falling vertically upward or downward
• Wgravity Fgravity s m g (h0-hf)
• Epot m g h is called gravitational potential
  energy
• Object falling downward Wgravity gt 0
• Object moved upward Wgravity lt 0
• Gravity is an example for a conservative force
• work is independent of path or force does no net
  work on
• object moving around a closed path.

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Concervation of Mechanical Energy

• Total mechanical energy of an object remains
  constant
• provided the net work done by
  non-conservative forces is zero
• Etot Ekin Epot constant
• or
• Ekin,fEpot,f Ekin,0Epot,0
• Otherwise, in the presence of net work done by
• non-conservative forces (e.g. friction)
• Wnc Ekin,f Ekin,0 Epot,f-Epot,i