The Work Energy Principle Part 5 By Heather Britton

Conservative Forces Conservative forces are defined as those that only the initial and final positions are taken into account. The path taken does not matter Examples are gravity and the force of a spring

Conservative Forces When only conservative forces are taken into account the Law of Conservation of Energy is easily validated When nonconservative forces are taken into account it is not so easy to see

Nonconservative Work Conservation of energy states that energy cannot be created or destroyed Why when you drop a ball does it not return to its original height? Why does an object rolling along the ground eventually stop?

Nonconservative Work The answer to these questions is a nonconservative force is acting upon the object Nonconservative forces make energy appear to disappear When they act over a distance, they do nonconservative work

Nonconservative Forces Nonconservative forces are dissipative forces that make energy appear to disappear The best example of a nonconservative force is friction

Nonconservative Forces Other examples of nonconservative forces include air resistance (friction) tension propulsion a force exerted by a person

Nonconservative Work If you rub your hands together they get warm The potential energy stored inside your body is transformed into kinetic energy

Nonconservative Work The energy you expend is also turned into heat (which is a form of energy) When a dropped ball hits the floor, both the ball and the floor get a bit warmer Thus the ball cannot return to its original height

Nonconservative Work Also, notice when a ball hits the ground you can hear a sound Sound is another form of energy The ball will never return to its original height, but nevertheless, energy has been conserved

Nonconservative Work In real world situations the nonconservative work must be accounted for to achieve accurate results The formula for this is W nc = ΔPE + ΔKE

Nonconservative Work To fully use this equation we must be able to substitute equal terms If you want to know how much energy dissipated in the form of heat (W nc = Q) Q = heat energy (more on this in another chapter)

Nonconservative Work If the amount of energy lost is due to friction (W nc = F f Δx) Remember F f = μF n Fn = mg(cos θ)

Example 10 A skier starts from rest and slides down a 20 degree incline 100 m long. a) If the coefficient of friction is 0.090, what is the skiers speed at the base of the incline? b) If the snow is level at the base of the incline and has the same coefficient of friction, how far will the skier travel from the base?