Chapter 8 Work and Energy

8.1 Work, Power, and Machines Work What is Work Work is done only when force cause a change in the motion of an object. Work is calculated by multiplying an applied force by the distance over which the force is applied. Work = force x distance W = F x d

8.1 Work, Power, and Machines Work In the case of trying to lift a heavy object, you might apply a large force, but if the distance that the object moves is zero, there is no work done. An object has to move at least a small amount for work to be done, because work is the product of force times distance. Work = force x distance W = F x d

8.1 Work, Power, and Machines Work Work is measured in Joules, J. Because work is the product of force times distance, it is measured in units of Newtons times meters or N . m 1 N . m = 1 J = 1 kg.m2/s2

8.1 Work, Power, and Machines Power What is Power Power is defined as the rate at which work is done. Power = work / time P = W / t

8.1 Work, Power, and Machines Power What is Power Power is defined as the rate at which work is done. Because power is the rate at which work is done, it is measured in units of Joules per second or Watts. 1 N . m /s = 1 J / s = 1 watt

8.1 Work, Power, and Machines Machines & Mechanical Advantage Machines multiply and redirect forces Work input equals work output Mechanical advantage tells how much a machine multiplies force or increases distance Defined as the ratio of the output force over the input force or the input distance to the output distance. Mechanical advantage = output force / input force Or = input distance / output distance

8.2 Simple Machines The six (6) Simple Machines The lever family simple lever pulley wheel & axle The inclined plane family simple inclined plane wedge screw

8.2 Simple Machines The Three Classes of Levers First class lever All first class levers have a fulcrum in the middle of an arm; the input force acts on one end, and the other end supplies an output force. [Claw hammer] MA > 1 Input force Output force

8.2 Simple Machines The Three Classes of Levers Second class lever All second class levers have a fulcrum at one end of an arm; the input force is applied to the other end. [wheel barrow] MA > 1 Output force Input force

8.2 Simple Machines The Three Classes of Levers Third class lever All third class levers also have a fulcrum at one end of an arm; however, these levers multiply distance rather than force. [forearm] MA < 1 Output force Input force

8.2 Simple Machines Pulleys are modified levers The mechanical advantage of pulleys Output force = 150 N Input force = 150 N MA = 1 Fixed pulley

8.2 Simple Machines Pulleys are modified levers The mechanical advantage of pulleys Input force = 75 N Output force = 150 N MA = 2 Moving pulley

8.2 Simple Machines Pulleys are modified levers The mechanical advantage of pulleys Input force = 75 N Output force = 150 N MA = 3 Multiple pulleys

8.2 Simple Machines A wheel and axle is a lever or pulley connected to a shaft Output force fulcrum

8.2 Simple Machines The Inclined Plane Family Inclined Planes multiply and redirect force Output force Input force An inclined plane changes both the magnitude and direction of force

8.2 Simple Machines The Inclined Plane Family A wedge is a modified inclined plane

8.2 Simple Machines The Inclined Plane Family A screw is an inclined plane wrapped around a cylinder.

8.2 Compound Machines Many devices that you use every day are made of more than one simple machine. A machine that combines two or more simple machines is called a compound machine.

8.3 Energy and Work Energy is measured in joules While work is done only when an object experiences a change in its motion, energy can be present in an object or a system when nothing is happening at all. There are several types and forms of energy: potential, kinetic, chemical, nuclear, electromagnetic, electrical, etc.

8.3 Energy and Work Potential Energy Potential energy is stored energy. Gravitational potential energy depends on both mass and height: Gravitational PE = mass x free-fall acceleration x height. PE = m g h Do practice problems on page 265

KE = ½ x mass x velocity squared 8.3 Energy and Work Kinetic Energy The energy that a n object has because it is in motion is called kinetic energy. Kinetic energy depends on mass and velocity. KE = ½ x mass x velocity squared KE = ½ m v2

8.4 Conservation of Energy Potential energy can become kinetic energy Kinetic energy can become potential energy Energy can neither be created or destroyed!