Work Energy Power Simple Machines Energy and Work Many types of energy we use the term energy freely but what is the scientific definition The ability to do work What are some types of energy Electrical Solar Heat Chemical Mechanical Work Scientific definition Work Force x Distance or W f x d Units Force N Distance m Work N m J Joulesss There must be movement of an object by the force Only the part of the force that is parallel to the motion does work on the object Example Julie pushes a large crate 5 kg across a level surface with a force of 3 0N for a distance of 2 m How much work is done on the crate Ignore any friction George lifts his physical science book 1 kg 10 N from the floor above his head for a total distance of 2 m at a constant speed What work is done by George lifting the book W f x d W 3 N x 2 m W 6 J W f x d W 10 N x 2 m W 20 J George then holds the book above his head for 5 minutes how much work is done on the book by George in this case Power Power is the rate of doing work using or producing energy The more quickly you perform work the more power you generate Power Work Time Unit for power is watts W Example If George lifted the book above his head in a time of 0 5 seconds what was his power output P W t P 20 0 5 P 40 W Energy The ability to do work Two forms of mechanical energy Potential Energy Stored Energy Energy due to position or location Examples A stretched bow has stored energy that can do work on an arrow A stretched rubber band of a slingshot has stored energy and is capable of doing work A mass held above the surface of the Earth Kinetic Energy Energy due to motion Gravitational Potential Energy Potential means the potential to do work A scalar quantity no direction Based on an object s height above some reference level Equals the work needed to lift the object to that height The path doesn t matter Potential Energy Gravitational potential energy Potential energy due to elevated position Examples Water in an elevated reservoir Raised ram of a pile driver Equal to the work done force required to move it upward x the vertical distance moved against gravity in lifting it In equation form PE mgh Units J Joules all forms of energy have the same units Kinetic Energy KE is the energy of motion A scalar quantity no direction KE 0 5m v 2 If an object has kinetic energy then it also must have momentum and vice versa If an object s speed is doubled what happens to its kinetic energy Momentum If an object s mass is doubled what happens to its kinetic energy Momentum Example What is the kinetic energy of a car 1 350 kg traveling at a speed of 29 m s 65 mi h KE 0 5m v 2 KE 0 5 1350 841 KE 567 675 J What is the car s momentum p mv p 1350 29 p 39 150 kg m s Pushing a Crate The time interval over which she pushes the block leads to a change in the block s momentum p Ft The distance over which she pushes the block leads to a change in its kinetic energy Work Energy Theorem Work energy theorem Gain or reduction of energy is the result of work In equation form work change in kinetic energy W KE Doubling speed of an object requires 4 times the work The work done by any force not just gravity on an object is equal to that object s change in kinetic energy W KE where KE KE final KE initial KE 0 5m v 2 final 0 5m v 2 initial Using the definition of W fd Fd KE Example Julie pushes a crate 5 kg across a level surface with a force of 3 0 N for a distance of 2 m How much work is done on the crate Ignore friction W fd W 3 N 2 m 6 J What is the speed of the crate after it has traveled 2 m Assume the crate starts from rest v i 0 m s KE i 1 2mv 2 0 W KE W KE f 0 6 KE f 0 KE f 6 J KE 1 2mv 2 6 1 2 5 v 2 6 2 5 v 2 2 4 v 2 v f 1 55 m s Total Mechanical Energy The total mechanical energy of a system E can be found by adding its potential PE and kinetic KE energies E KE PE In the absence of external work input or output the energy of a system remains unchanged Energy cannot be created or destroyed Energy is conserved Example Alice drops a 2 kg rock from a cliff of height 20 m Fill in the table assuming there is no air resistance and the acceleration due to gravity is 10 m s Simple Machines Machine Device for multiplying forces or changing the direction of forces Cannot create energy but can transform energy from one form to another or transfer energy from one location to another Cannot multiply work or energy same amount of work Principle of a machine Conservation of energy concept Work input Work output Input force x Input distance Output force x Output distance Force x distance input Force x distance output Lever Pulley Rotates on a point of support called the fulcrum Allows small force over a large distance and large force over a short distance Operates like a lever with equal arms changes direction of the input force