Chapter 5 - Physics Work and Energy

Section 1 objectives  Recognize the difference between the scientific and ordinary definition of work.  Define work, relating it to force and displacement.  Identify where work in being performed in a variety of ways.  Calculate work done when many forces are applied to an object.

Work  Work – The product of the magnitudes of the component of a force along the direction of displacement and the displacement.  Work is not done unless the object is moved.  Work is only done when components of a force are parallel to a displacement  Components of the force perpendicular to a displacement do no work.

Work  W = Fd(cos θ )  Do sample problems 5A on page 169.  Sign of work  Page 170; figure 5-3  Work is + when the force is in the same direction of the displacement  Work is – when the force is in the opposite direction of the displacement

objectives  Identify several forms of mechanical energy.  Calculate kinetic energy for objects.  Distinguish between kinetic and potential energy.  Classify different types of potential energy.  Calculate an object’s potential energy.  Relate kinetic and all forms of potential energy to the idea of mechanical energy.

Kinetic Energy  Kinetic Energy-The energy of an object due to its motion.  Depends on both mass and velocity.  KE = ½mv 2  Do practice problems 5B, page 173

Potential Energy  The energy associated with an object due to its position.  Different types of potential energy:  Gravitational Potential Energy: The energy assoc. w/ an object due to its position relative to the Earth or some other gravitational source.  PE g =mgh

Potential Energy  Elastic Potential Energy: The energy in a stretched or compressed spring  Pe elastic =½kx 2  k= spring constant  x=distance compressed or stretched  Spring constant= A parameter that expresses how resistant a spring is to being compressed or stretched.  Do practice problems 5C; page 177

Mechanical Energy  The sum of the kinetic energy and all forms of potential energy Energy Mechanical Nonmechanical Kinetic Potential GravitationalElastic Nonmechanical Energy- other forms besides kinetic and potential

objectives  Identify situations in which conservation of mechanical energy is valid.  Recognize the forms that conserved energy can take.  Solve problems using conservation of mechanical energy.

Conservation of Energy  Energy is conserved  See example pg 180; figure 5-1  In the absence of friction, mechanical energy is conserved, but can change forms  ME i =Me f  ½mv 2 i + mgh i = ½mv 2 f + mgh f  Do practice problems 5D; pg. 182  When friction is present, mech. E is not conserved – it changes to other forms of nonmech. energy.

objectives  Objectives  Apply the work-kinetic energy theorem to solve problems.  Relate the concepts of energy, power, and time  Calculate power in two different ways  Explain the effect of machines on work and power.

Work, Power, and Energy  Work-Kinetic Energy Theorem  The net work done on an object is equal to the change in the kinetic energy of the object.  W net = Δ KE  Work is a method of energy transfer  Do practice problems 5E, pg. 186

Work, Power, and Energy  Power- the rate at which energy is transferred.  P=W/ Δ T (Power = work/time)  Remember W=Fd, so P =Fd/t, but d/t = v, so this can be simplified to say that P=Fv.  You can use any of these equations depending on the given information.  SI unit of power = Watt (W)  1 W = 1 J/s  1 hp = 746W (hp-horsepower is the English unit)  Do practice problems 5F, PG. 188

Chapter 5 problem set  Pg  #2, 3, 5, 6, 7, 10, 12, 13, 14, 16, 19, 23, 27, 31, 32, 35, 39, 40, 41, 48, 52.