Physics 101 lecture 12 work and energy
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Physics 101: Lecture 12 Work and Energy. Chapter 6, sections 6.1, 6.2, 6.3 (Work and Energy) Reminders: Exam I, Tuesday September 30 th at 5 PM See PHY101 Web page for room assignments Do not forgot to bring your UB ID card !. Work & Energy. An important concept in physics

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Physics 101: Lecture 12 Work and Energy

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Physics 101 lecture 12 work and energy

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 !


Work energy

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


Work done by a constant force

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 = - Fss if Fs is opposite of s


Lecture 9 preflight 1 2

FN

V

T

correct

W

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.


Concept question

FN

V

T

correct

W

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.


Concept question1

FN

V

T

correct

W

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.


Work kinetic energy theorem

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.

{NetWork done on an object}

=

{change in kinetic energy of object}

  • Also works for a variable force !


Work done by gravity

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 > 0

    Object moved upward: Wgravity < 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.


Concervation of mechanical energy

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,f+Epot,f = Ekin,0+Epot,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


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