Chapter 5 - Physics

1. Chapter 5 - Physics Work and Energy

2. 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.

3. 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.

4. Work • W = Fd(cosθ) • 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

5. 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.

6. Kinetic Energy • Kinetic Energy-The energy of an object due to its motion. • Depends on both mass and velocity. • KE = ½mv2 • Do practice problems 5B, page 173

7. 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. • PEg=mgh

8. Potential Energy • Elastic Potential Energy: The energy in a stretched or compressed spring • Peelastic=½kx2 • 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

9. Mechanical Energy • The sum of the kinetic energy and all forms of potential energy Energy Mechanical Nonmechanical Kinetic Potential Gravitational Elastic Nonmechanical Energy- other forms besides kinetic andpotential

10. 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.

11. 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 • MEi=Mef • ½mv2i + mghi = ½mv2f + mghf • Do practice problems 5D; pg. 182 • When friction is present, mech. E is not conserved – it changes to other forms of nonmech. energy.

12. 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.

13. 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. • Wnet=ΔKE • Work is a method of energy transfer • Do practice problems 5E, pg. 186

14. Work, Power, and Energy • Power- the rate at which energy is transferred. • P=W/ΔT (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

15. Chapter 5 problem set • Pg. 193-199 • #2, 3, 5, 6, 7, 10, 12, 13, 14, 16, 19, 23, 27, 31, 32, 35, 39, 40, 41, 48, 52.