Chapter 13 WORK & ENERGY

TN Standards CLE – Demonstrate the relationship among work, power, and machines CLE – Investigate the Law of Conservation of Energy

Bellwork What is Work?

Work, Power, and Machines Work is conducted only when an applied ( net ) force results in the change in position of an object Work is measured in Joules ( J ) –1 Joule = 1 N x 1m Work is calculated by multiplying force and distance ( over which the force is applied ) –W = F x d ( if given mass of an object – find weight by multiplying by gravity which is 9.81 m/s 2 )

Power is the amount of work conducted over a certain time interval –The rate at which work is conducted P = W / Δt Power is measured in Watts Work, Power, and Machines

Machines help to do work by changing the size of an input force, the direction of a force, or both Different forces can do the same amount of work –Figure 3 ( a box lifted vs a box pushed up a ramp )

Simple Machines The six types of simple machines are: –Simple lever –Wheel and axle –Pulley –Simple inclined plane –Wedge –Screw

Classes of Machines What are the two families of simple machines? –Inclined plane and lever Levers are divided into 3 classes based on the location of the fulcrum and of the input and output forces. 1 st class ( most common ) – hammer 2 nd class – wheelbarrow, nutcrackers, hinged doors 3 rd class – human forearm

Compound Machines A combination of any of those six simple machines Example – SCISSORS Combination of lever and wedge

Bellwork – 11/17/15 How efficient is a machine that uses 150 J to lift a 60 N load 2 m? Input = 150 J ; Output = 60 N x 2 m = 120 J Efficiency = out/in = 120 J / 150 J = 0.8 For percentage, multiply by 100 ( 80 % ) What is the work output by an 85% efficient wheel and axle when 65 J of work is input? 0.85 = out / 65 J  output = 55.3 J

Mechanical Advantage / Efficiency

Bellwork – 11/11/15 What is Energy?

Energy is a property of an object due to its motion or its position When work is done, energy is transferred or transformed from one system to another –Carry a tennis ball up in the stadium –You add potential energy to it by carrying it up

Bellwork – 11/11/15 An 1500 kg car is driving at 20 m/s. What kind of energy does it have? How much of this energy is there? Kinetic KE = (½)mv 2 = (1/2)(1500 kg)(20 m/s)^2 KE = 300,000 J

What is Energy? Energy is measured also in Joules ( J ) Potential energy – energy of position – PE = hmg –h is height ; g is gravity; m is mass –Height is relative Kinetic Energy – energy of motion – KE = ( ½ ) mv 2 – m is mass ; v is velocity

Other Forms of Energy Mechanical energy ( sum of Potential and Kinetic ) Non-mechanical energy –( atomic level ) Chemical reactions involves chemical energy ( a form of potential energy ) –Sun gets energy from nuclear reactions Electrical energy ( stored in electric fields )

Conservation of Energy Energy easily changes from one form to another ( battery ) PE can become KE ( tennis ball dropped from the top of stadium ) –Vice-versa ( projectile shot / reaches max height ) Law of Conservation of Energy? –( neither created nor destroyed )

Roller Coaster Example

Thermodynamics For any system, the net change in energy equals the energy transferred as work and heat Efficiency ( how much energy/work you get out of a machine or process compared to how much is put in ) –Internal Combustion Engine