Energy and Machines Physical Science

What is Energy? Is the ability to do work Two general types Kinetic Energy Potential Energy

Kinetic Energy The energy of motion Depends on mass and speed Measured in Joules (J) Symbol = KE

Potential Energy Energy that is stored (not moving) Based on position and height Measured in Joules (J) Symbol = PE

Is it Kinetic or Potential Energy? Rock sitting on the cliff’s edge A car going down a hill Sitting at the top of a slide Going down a slide

Calculating Kinetic Energy Example Problem The cart on a roller coaster has a mass of 100 kg. It is going a speed of 6 m/s along the track. How much kinetic energy does it have? Formula KE = ½ m * v2 KE= kinetic energy (J) m= mass (kg) v= velocity (m/s)

Calculating Potential Energy Example Problems A 30 kg rock is sitting on a ledge 5 m from the ground of Earth. How much potential energy does it have? Formula PE = m * g* h PE = potential energy (J) m= mass (kg) g= gravity (9.8) h= height (m)

How to also write PE formula PE = m*g*h Weight PE= Fgh

Forms of Energy Thermal Energy Mechanical Energy Motion and Position of everyday objects Examples Speeding trains Bouncing balls Thermal Energy Total PE and KE of the particles in an object Examples Liquid metals

Forms of Energy Chemical Energy Electrical Energy Energy stored in chemical bonds Examples Chemical reactions Burning coal Electrical Energy Energy associated with electric charges Examples CD players Calculators

Forms of Energy Nuclear Energy Electromagnetic Energy Form of energy that travels through space in the form of waves Examples X-rays Visible Light Nuclear Energy Energy in atomic nuclei Examples Sun Nuclear power plants

Energy Conversion Energy changing from one form to another Examples Kinetic to Potential Mechanical to Thermal

Law of Conservation of Energy States: Energy cannot be created nor destroyed, only changed Meaning Amount of energy at the start must equal amount of energy at the end

Energy Conversion: Roller Coaster PE Greatest Conversion PE decrease KE increase PE = KE KE Greatest KE decrease PE increase KE = PE

Energy Conversion: Pendulum PE Greatest Conversion PE decrease KE increase PE = KE KE Greatest KE decrease PE increase KE = PE

Energy Conversion: Throwing Ball Up KE Greatest Conversion PE increase KE decrease PE = KE PE Greatest PE decrease KE increase KE = PE

Machines

What is a Machine? A device that makes doing work easier Two divisions Simple machines Compound Machines

How do Machines make work Easier? Increase Force (ex: car jacks) Increase the distance a force is applied (ex: using a ramp) Changing the direction of an applied force (ex: ax blade)

Simple Machines Machines that does work with only one movement of the machine

Six types of Simple Machines 1). Lever A bar that is free to pivot or turn around a fixed point (ex: teeter totter) 2). Pulley Grooved wheel with a rope, chain, or cable running along the groove (ex: wishing well)

Six types of Simple Machines 3). Wheel & Axle A shaft/axle attached to the center of a larger wheel so that both rotate together (ex: door knobs) 4). Inclined Plane A sloping surface (ex: ramp)

Six types of Simple Machines 5). Screw An inclined plane wrapped in a spiral around a cylindrical post (ex: jar lids) 6). Wedge An inclined plane with one or two sloping sides (ex: knife)

What are Compound Machines? When two or more simple machines operate together Example: Can Opener Wheel & Axel Level Wedge

Input force & Output force Force applied TO the machine Symbol = Fin Output Force Force applied BY the machine Symbol = Fout

Input Work & Output Work Work done by you on a machine Symbol = Win Output Work Work done by the machine Symbol = Wout

Input & Output Work: The Relationship Input work equals Output work in an ideal machines Win = Wout Why? Law of conservation of Energy Energy is not created nor destroyed

What is Mechanical Advantage? Is the number of times that a machine increases an input force Two Versions Actual Mechanical Advantage (AMA) Ideal Mechanical Advantage (IMA)

Actual Mechanical Advantage Determined by measuring the actual forces on a machines Ratio of the output force to the input force

Actual Mechanical Advantage Formula AMA = Fr / Fe (Big/Small) AMA= actual mechanical advantage Fr= resistance force (output force) Fe=effort force (input force) AMA has no unit Fr & Fe has the unit of “N”

AMA Formula Practice 1) What is the actual mechanical advantage of a machine who’s input force is 30-N but produces an output force of 90-N? 2) You test a machine and find that it exerts a force of 10 N for each 2 N of force you exert operating the machine. What is the actual mechanical advantage of the machine?

Ideal Mechanical Advantage Is the mechanical advantage in the absence of friction Because friction is always present, the actual mechanical advantage of a machine is always less than the Ideal

Ideal Mechanical Advantage Formula IMA = de / dr (Big/Small) IMA = ideal mechanical advantage de = displacement of effort force (input distance) dr = displacement of resistant force (output distance) IMA has no unit de and dr has the unit of meters

IMA Formula Practice 1) A woman drives a car up onto wheel ramps to perform some repairs. If she drives a distance of 1.8 meters along the ramp to raise the car 0.3 meter, what is the IMA? 2) A construction worker moves a crowbar through a distance of 4 meters to lift a load 0.5 meter off the ground. What is the IMA of the crowbar?

What is efficiency? % The percentage of work input that becomes work output Because there is always some friction, the efficiency of any machine is always less than 100 percent

Efficiency Formula Wout = work output (J) Win = work input (J) Small/Big x 100

Efficiency Formula Practice 1) You have just designed a machine that used 1000 J of work from a motor for every 800 J of useful work the machine supplies. What is the efficiency of your machine?