Unit 8 (Chapter 10 & 11) Work, Energy and Power

Work “Work” means many things in different situations. When we talk about work in physics we are talking about applying a force over a certain distance. For work to occur a force must produce motion in the direction of the force. Work = Force x Distance

Power How fast work gets done is called power Power is measured in a unit called watts A powerful machine can do the same amount of work in less time than a less powerful machine 1 watt = 1 joule/sec Horsepower is the English unit for power. 1 horsepower = 746 watts Power = work time

Energy Energy is defined as the ability to do work –6 Types of energy Kinetic Energy: associated with motion Heat: associated with the speed of internal particles Chemical: bonds atoms together Electromagnetic: moving electric charges Nuclear: in the atomic nucleus Potential Energy: energy of position (energy at rest)

Energy The units of energy are joules joules are also the same unit as work! (How Confusing!) You can think of energy as stored work – that is ready to be released! Energy cannot be created or destroyed – only changed from one type of energy to another!

Kinetic Energy Energy with motion Energy an object has because it’s in motion All moving objects that have mass have kinetic energy As mass increases, the KE increases As speed increases, the KE increases KE changes more with speed than with mass KE = mass x speed 2 2 Kinetic Energy Kinetic energy (joules) KE = ½ m v 2 m - mass of the object in kg v - speed of the object in m/s KE - the kinetic energy in J

Kinetic Energy depends on two things: mass and speed The amount of kinetic energy the cart has is equal to the amount of work you do to get the cart moving A cart at rest has NO Kinetic energy Applying a force can give the cart speed, and therefore kinetic energy. Applying a greater force increases the speed and therefore the kinetic energy Increasing the mass also increases kinetic energy because it takes even more force to push.

Work-Energy Theorem the net work done on an object is equal to its change in kinetic energy The amount of kinetic energy an object has is equal to the amount of work you do to get the object moving

Potential Energy Is energy stored in an object due to its position Potential Energy is also considered energy at rest GPE = mass x acceleration due to gravity x height Potential Energy Potential Energy (joules) m - m - mass of object in kg g - g - acceleration of gravity in m/s 2 h - h - height of object, in m GPE – GPE – gravitational potential energy in J GPE = mgh

Potential Energy Gravitational potential energy depends on an objects mass and height Objects with potential energy don’t use their energy until they start moving. The further the object is from the ground the more potential energy it has Chemical potential energy is energy stored in the form of chemical bonds.

Law of Conservation of Energy Energy can never be created or destroyed just turned into another form.

Law of Conservation of Energy When a ball flies through the air it does not lose energy even though it slows down and speeds up The energy in the ball changes from kinetic to potential and then back to kinetic energy