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Physics 101: Lecture 12 Work and Energy

Physics 101: Lecture 12 Work and Energy. Chapter 6: 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 Alternative approach to mechanics

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Physics 101: Lecture 12 Work and Energy

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  1. Physics 101: Lecture 12Work and Energy • Chapter 6: 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 !

  2. 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

  3. 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 (Fs=F cosΘ) 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

  4. 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.

  5. 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.

  6. 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.

  7. Work/Kinetic Energy Theorem: Work done by a constant net force: Wnet = Fnet s = m a s = m (vf2 –v02)/2 KE = m v2/2 is called the kinetic energy of an object. {NetWork done on an object} = {change in kinetic energy of object} = ½ m vf2 – ½ m v02 = KEf - KE0 Wnet=ΔKE • Also works for a variable force !

  8. Work done by Gravity • Object falling vertically upward or downward : Wgravity = Fgravity s = m g (h0-hf) PE = m g h is called gravitational potential energy Object falling downward: Wgravity > 0 Object moving upward: Wgravity < 0 Gravity is an example for a conservative force: work done is independent of path or force does no net work on object moving around a closed path.

  9. Conservation of Mechanical Energy • Total mechanical energy of an object remains constant provided the net work done by non-conservative forces is zero: Etot = KE+ PE = constant or KEf + PEf = KE0 + PE0 => ΔKE + ΔPE = 0 Otherwise, in the presence of a net work done by non-conservative forces (e.g. friction): Wnc = ΔKE + ΔPE = KEf – KE0 + PEf-PEo

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