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Energy & Work Energy • What is energy? • The capacity to do work • What is Work? • The transfer of energy from one physical system to another Bottom line: it is the outward manifestation of energy which causes a change in something (e.g. changes in motion or position). • Energy is measured in Joules • Joules = newton*meter = kg*m2/s2 • In English units we call them calories or kilocalories Types of energy • Kinetic energy – energy of motion • Potential energy – stored energy Kinetic Energy • Mechanical energy – energy inherent in objects in motion • Light energy – highly organized packets of energy called photons • Electrical energy – energy generated by charged particles • Heat energy – atomic and molecular motion create heat Potential Energy • Positional energy – energy stored due to position of object relative to some force • Electrical energy – stored charged particles • Bond energy – energy stored in the chemical bonds holding 2 or more atoms together Laws of thermodynamics • laws of the movement of heat?...why heat? • Push your book of the desk! – 1) Positional energy to simple kinetic energy – 2) positional energy also converted to heat due to friction – 3) book hits ground, the kinetic energy is converted to heat energy in the book and the ground – 4) pick it back up and bond energy is converted into mechanical energy in my muscles – 5) both bond and mechanical muscle energy are converted to heat – 6) the heat is eventually radiated into the atmosphere Laws of thermodynamics • 1st Law - Conservation of Energy • Energy is neither created nor destroyed • Energy within a system is only converted from one form of energy to another • Energy can be moved into or out of a system • Total energy in the universe is a constant Laws of thermodynamics • 2nd Law – Entropy • Energy naturally flows from more ordered forms of energy to less ordered forms of energy • Corollaries: – The total disorder in any closed system always increases. – Every conversion of energy from one form to another converts some energy into disordered energy in the form of heat. Back to Work • If work is the transfer of energy and energy can be neither created or destroyed…….the work done within a closed system…….. »Decreases over time? »Remains constant ? How is Work measured? • The force applied to an object times the distance the object moves due to that force. • W = F*d (Recall, F=ma) • Peculiarities: – 1) Movement perpendicular to the direction of the force does not result in work – 2) Supporting an item does not result in work. If the distance moved is zero, then there is no work. Animal Movement • Moving a body is doing work • There are a variety of ways of dealing with forces and distance that relate to the ease of accomplishment of work • Humans, being animals, have made a habit of using these ways to accomplish work more easily in the form of Simple Machines A machine is a device that helps make work easier to perform • Transferring a force from one place to another • Changing the direction of the force • Increasing the magnitude of a force • Increasing the distance or speed a force is applied 4 Basic Simple Machines • Levers • Inclined planes • Pulleys • Wheel and axle Incline Plane Horizontal distance Vertical distance How much force is required to lift a 50kg weight 3 meter off of the ground? F=ma F = 50 kg (9.8 m/s2) = 490N 3m 50kg How much work vs. gravity needs to be done to move a 50 kg weight up 3 meters? Work = F*d = 490N*3m = 1470 Joules 50kg 3m 5m How much force do you need to apply to move the same weight up the ramp? Work = F*d F = W/d = 1470J/5m = 294 N 3m 5m 50kg How much easier is it to do the work (1470J) with the inclined plane than without? Mechanical advantage = force without incline / force with incline Force w/o Force w 490 N = 294 N = 1.66 For inclined planes you get the same number by: Length of plane Height of plane 3m 5m 5m = 3m = 1.66 Mechanical Advantage • force produced by the machine divided by force applied to the machine –F out / F in Levers Resistance force Effort Force de Resistance force dr Effort Force de What is the mechanical advantage of the lever system? 5 kg 15 kg R E de 15 kg * 9.8 m / s2 F out = F in 5 kg * 9.8 m / s2 If the mechanical advantage is 3 :1 can you calculate dr and de? 15 kg R 5 kg E de If you need to move the Fr 9 meters what is the distance the Fe will have to move? This is a different type of lever; what is different from the previous lever? Fr Fe Where are dr and de? Fe dr de This is the last type of lever; what is different? Fe Fr Where are dr and de? de dr