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Physics 101: Lecture 12 Work and EnergyPowerPoint Presentation

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 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

- 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

- 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

FN

V

T

correct

W

Lecture 9, Preflight 1 & 2You 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.

FN

V

T

correct

W

Concept QuestionYou 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.

FN

V

T

correct

W

Concept QuestionYou 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:

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

- 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

- 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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