Work, power, & Energy Chapter 12

Work Force which causes a change in the position or motion of an object in the direction of the applied force An object must have moved for work to occur Work = force X distance W = Fd SI Units: kg x m2/s2 = Joules (J)

JOULE is a unit to describe work One JOULE = a NEWTON moving one METER As you try to lift the object, it does not move… Is work being done?

Example: A weight lifter is holding a 200kg barbell above his head Example: A weight lifter is holding a 200kg barbell above his head. What is the amount of work being done?

Running up the steps and walking take the same amount of work… Running up the steps and walking take the same amount of work….but which requires more power?

Power The rate at which work is done power = work / time P = w/t SI Units Watts (W) = J/s

WATT (W) – a unit to describe power One WATT = one JOULE of work in one SECOND

Example: A girl lifts a 100-N load a height of 2. 0 m in a time of 0 Example: A girl lifts a 100-N load a height of 2.0 m in a time of 0.5 s. What power does the girl produce?

Machines Require the same amount of work, but make it easier b/c less force is applied Work input always equals work output

THE LEVER (THREE CLASSES) FIRST – fulcrum in the middle. SECOND – load in the middle. THIRD – effort in the middle.

THE PULLEY (A MODIFIED LEVER) One or more levers that changes dispersion of work

THE WHEEL AND AXLE (A LEVER OR PULLEY ON A SHAFT) Mechanical Advantage depends on size of wheel and size of axle. Some are designed for power. Some are designed for speed.

INCLINED PLANE A long gradual slope has a larger Mechanical Advantage than a short steep slope.

THE WEDGE (A DOUBLE INCLINED PLANE) The mechanical advantage depends on the length of the slope and how wide the wedge is.

THE SCREW (AN INCLINED PLANE WRAPPED AROUND A CYLINDER) Steepness of slope determines the M.A.

COMPOUND MACHINES

Energy All around us, organisms need energy to survive Energy is the ability to do work When work is done, energy is transformed or transferred to another system Can be measured by how much work is done Both energy and work are measured in Joules (J)

Potential Energy STORED energy (stretched rubber band) ‘energy of position’

Gravitational potential energy: any system where the 2 objects are separated by a distance Example: apple falling from tree when stem breaks) Depends on mass and distance b/w 2 objects PE = mgh Potential Energy = mass x acceleration due to gravity x height

Example: A 65 kg rock climber ascends a cliff Example: A 65 kg rock climber ascends a cliff. What is the climbers gravitational potential energy at a point 35m above the base?

Kinetic Energy Energy of an object in motion Is doing work b/c it is in motion KE = 1/2mv2 Kinetic Energy = ½ mass x velocity2

Kinetic Energy depends on speed more than mass; As speed increases, KE increases exponentially 2x the mass = 2x the KE 2x the velocity = 4 x the KE Example: Car crashes are more dangerous at higher speeds b/c the car has a higher KE at higher speeds, thus it can do more work = damage.

Example: What is kinetic energy of a 44 kg cheetah running at 31m/s?

Other Forms of Energy Mechanical energy – sum of potential & kinetic energy, amount of work an object can do Before the apple falls it has PE, just before it it’s the ground it has KE, b/w it has some PE & some KE

Forms cont. Chemical energy – energy stored in the bonds between atoms Energy is stored or released when bonds are broken Example is cell gets energy from C6H12O6 when the bonds are broken during respiration

Forms cont. Nuclear energy – energy released from changing the nucleus Sun’s energy comes from fusion – putting 2 H together to make He (H + H → He) E = mc2 (mass converted to energy)

Forms cont. Electrical energy – the energy of charged particles

Forms cont. Light energy – energy that can travel through empty space in electromagnetic waves (blue has more energy than red)

Conservation of Energy Energy can’t be created nor destroyed The total energy remains constant Energy only changes forms

Example: Rollercoaster PE = 354 kJ KE = 0 kJ V = 0 m/s KE = 177kJ PE = 177 kJ V = 26.2 m/s Mass = 515 kg h= 35m PE = 0 kJ KE = 354 kJ V = 37.1 m/s

Example 2: Pendulum All PE No KE All PE No KE All KE

Friction turns motion into heat Electric cords get hot Potential to kinetic Pendulum eventually will stop Energy goes into the air Friction turns motion into heat Electric cords get hot

Efficiency Not all work is useful work Always less than 100% Perpetual motion machines are impossible b/c they would have to put more out than you put in and would require complete absence of friction.