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Name ____________________________________ NOTES – CHAPTER 12 WORK AND ENERGY HONORS PHYSICAL SCIENCE I. Work, Power and Machines a. What is Work? i. Work is the transfer of energy to a body by the application of a force that causes the body to move in the direction of the force ii. Work is done only when a force causes an object to move in the direction of the force. This is different from the everyday meaning of work. iii. Work Equation 1. Work = force x distance iv. Work is measured in joules 1. Because work is calculated as force times, it is measured in unites of newtons times meter, N x m 2. These units are also called joule (j) in terms of SI base units, a joule is equivalent to 1 kg m2/s2 b. Power i. Power is a quantity that measures the rate at which work is done or energy is transformed ii. Power Equation iii. Power is measured in watts 1. A watt (W) is equal to a joule per second (I J/s) c. Machines and Mechanical Advantage i. Machines multiply and redirect forces 1. Machines help people by redistributing the work put into them 2. They can change either the size or the direction of the input force ii. Different forces can do the same amount of work 1. A machine allows the same amount of work to be done by either decreasing the distance while increasing the force or by decreasing the force while increasing the distance iii. Mechanical advantage tells how much a machine multiplies force or increase distance iv. Mechanical Advantage Equation II. III. Simple Machines a. The Lever Family i. The most basic machines are called simple machines ii. The six types of simple machines are divided into two families 1. The lever family a. Simple lever b. Pulley c. Wheel and axis 2. The inclined plane family a. Simple incline plane b. Wedge c. Screw iii. Levers have a rigid arm and a fulcrum iv. Levers are divided into three classes 1. All first-class levers have a fulcrum located between the points of application of the input and output forces 2. In a second-class lever, the fulcrum is at one end of the arm and the input force is applied to the other end 3. Third-class levers multiply distance rather than force. As a result, they have a mechanical advantage of less than 1. 4. Pulleys are modified levers a. The point in the middle of a pulley is like the fulcrum of a lever b. A single, fixed pulley has a mechanical advantage of 1. c. Multiple pulleys are sometimes put together in a single unit called a block and tackle 5. A wheel and axle and axle is a lever or pulley connected to a shaft a. The steering wheel of a car, screwdrivers, and cranks are common wheel-and-axel machines b. The Incline Plane Family i. Incline planes multiply and redirect force 1. An incline plane turns a small input force into a large output force by spreading the work out over a large distance ii. A wedge is a modified inclined plane iii. A screw is an inclined plane wrapped around a cylinder c. Compound Machines i. A machine made of more than one simple machine is called a compound machine ii. Examples of compound machines are: 1. Scissors, which use two first class levers joined at a common fulcrum 2. A car jack, which uses a lever in combination with a large screw Energy and Work a. Energy is the ability to do work i. When you do work on an object, you transfer energy to that object ii. Whenever work is done, energy is transformed or transferred to another system b. Energy is measured in joules i. Because energy is a measure of the ability to do work, energy and work are expressed in the same units c. Potential Energy i. The energy than an object has because of the position, shape, or conditionof the object is called potential energy ii. Potential energy is stored energy 1. Elastic potential energy is the energy stored in any type of stretched or compressed elastic material, such as a spring or a rubber band 2. Gravitational potential energy is the energy stored in the graviatational field which exists between any two or more objects iii. Gravitational potential energy depends on both mass and height iv. Gravitational Potential Energy Equation v. The height can be relative 1. The height used in the above equation is usually measured from the ground 2. However, it can be a relative height between two points, such as between two branches in a tree d. Kinetic Energy i. The energy of a moving object due to the object’s motion is called kinetic energy ii. Kinetic energy depends on mass and speed iii. Kinetic Energy Equation iv. Kinetic energy depends on speed more than mass e. Other Forms of Energy i. The amount of work an object can do because of the object’s kinetic and potential energies is called mechanical energy ii. Mechanical energy is the sum of the potential energy and the kinetic energy in a system iii. In addition to mechanical energy, most systems contain non-mechanical energy iv. Non-mechanical energy does not usually affect systems on a large scale v. Atoms and molecules have kinetic energy 1. The kinetic energy of particles is related to heat and temperature IV. vi. Chemical reactions involve potential energy 1. The amount of chemical energy associated with a substance depends in part on the relative positions of the atoms it contains vii. Living things get energy from the sun 1. Plants use photosynthesis to turn the energy in sunlight into chemical energy viii. The sun gets energy from nuclear reactions 1. The sun is fueled by nuclear fusion reactions in its core ix. Electricity is a form of energy 1. Electrical energy is derived from the flow of charged particles, as in a bolt of lightning or in a wire x. Light can carry energy across empty space 1. Light energy travels from the sun to Erath across empty space in the form of electromagnetic waves Conservation of Energy a. Energy Transformations i. Energy readily changes from one form to another ii. Potential energy can become kinetic energy 1. As a car goes down a hill on a roller coaster potential energy changes to kinetic energy iii. Kinetic energy can become potential energy 1. The kinetic energy a car has at the bottom of a hill can do work to carry the car up another hill iv. Energy transformations explain the flight of a ball v. Mechanical energy can change to other forms of energy 1. Mechanical energy can change to non-mechanical energy as a result of friction, air resistance, or other means b. Law of Conservation of Energy i. The law of conservation of energy states that energy cannot be created or destroyed ii. Energy doesn’t appear out of nowhere 1. Whenever the total energy in a system increases, it must be due to energy that enters the system from an external source iii. Energy doesn’t disappear, but it can be changed to another form iv. Scientists study energy systems 1. Boundaries define a system v. Systems may be open or closed 1. When the flow of energy into and out of a system is small enough that it can be ignored, the system is called a closed system 2. Most systems are open systems, which exchange energy with the space that surrounds them c. Efficiency of Machines i. Not all of the work done by a machine is useful work 1. A machine cannot do more work than the work required to operate the machine 2. Because of friction, the work output of a machine is always somewhat less than the work input ii. Efficiency is the ratio of useful work out to work in. 1. Efficiency is usually expressed as a percentage 2. The efficiency of a machine is a measure of how much useful work it can do 3. Efficiency Equation 4. Perpetual motion machines are impossible a. Energy is always lost to friction or air resistance 5. Machines need energy input a. Because energy always leaks out a system, every machine needs at least a small amount of energy input to keep going