Work and Energy

Chapter 8

Some Basic Energy Facts• Energy has a role in everything we do and is essential

to all life. • Energy is a conserved quantity – it can neither be

created nor destroyed.• Energy may be converted from one form to another.• Energy may be converted to work, or work converted

to energy.• Fossil fuels provide 90% of the energy we currently

use.• Hundreds of millions of years were needed for fossil

fuels to be created on earth.• The term energy consumption refers to the

degradation of energy resources to less useful forms

Forms of Energy• Kinetic energy – energy associated with motion (KE = ½ mv2).• Potential energy – stored energy or energy associated with

position (gravitational P.E. = mgh).• Chemical energy – associated with molecular structure and

can be released by chemical reactions such as oxidation (a form of potential energy).

• Thermal energy – associated with internal motions of atoms and molecules.

• Mass or nuclear energy – equivalence of mass and energy (E=mc2) manifested as nuclear energy.

• Light energy – associated with time-varying electric and magnetic fields (electromagnetic radiation).

• Electrical energy – associated with separation and movement of electric charges.

Energy Sources• The sun is the source of much of the energy that

is available on the earth, directly or indirectly.– Direct solar energy (thermal and photovoltaics).

– Wood and plants as fuels (photosynthesis).

– Fossil fuels (coal, oil, natural gas).

– Wind.

– Hydropower.

• Non-solar energy resources include:– Geothermal energy.

– Nuclear energy.

Energy Transformation Cycle – Power Plant

BoilerEarthchemical energy

Turbine

waste heat

steam

ElectricGenerator

mechanical energy

Loadtransmission

line

HeatLightMotorElectronics

light

Environment, Space

Sun

Energy and Work• ENERGY is the capacity for doing work.• In physics, work is defined as a force acting upon an

object to cause a displacement• WORK = FORCE × DISPLACEMENT

• In order for a force to qualify as having done work on an object, there must be a displacement and the force must cause the displacement

• Unit: Newton x meter = Joule• Work and energy have the same units.

FxW

Work??• Are the following examples of work as its

defined in physics?a) A teacher applies a force to a wall and becomes

exhausted.b) A book falls off a table and free falls to the

ground.c) A waiter carries a tray full of meals above his

head by one arm across the room.d) A rocket accelerates through space.http://www.glenbrook.k12.il.us/gbssci/phys/mmedia/energy/au.html

Work?a) No, the wall does not move, there is no

displacement.b) Yes, the force of gravity causes the book to fallc) No, there is a force, the waiter pushes up on the

tray but this force does not make it move horizontally. If the force and displace 90 degrees (perpendicular) then no work is done.

d) Yes, there is a force that accelerates the rocket. If the rocket was moving at a constant velocity, there would be no work, because no force would be needed to keep it moving at a constant velocity.

PowerPower = rate of doing work = rate of delivering energy

Units: Watt = Joule/second (SI units) Other units for power: Horsepower = 550 ft-lb/s = 746

Watts

t

WP

Work and Power Triangle

F

W

xF

P

v

P

W

tt

WP

FvP FxW

WORK = FORCE × DISPLACEMENT

Work Questions1) A 10-N forces is applied to push a block

across a friction free surface for a displacement of 5.0 m to the right.

a) Which forces are doing work on the block?b) Find the amount of work done by each force.

2) How much work is done by an applied force to lift a 15-Newton block 3.0 meters vertically at a constant speed?

3) Challenge: A tired squirrel (mass of 1 kg) does push-ups by applying a force to elevate its center-of-mass by 5 cm. Determine the number of push-ups which a tired squirrel must do in order to do a mere 5.0 Joules of work.

Work and Power Questions1) Two physics students, Will and Ben, are in the weightlifting room. Will

lifts the 100-pound barbell over his head 10 times in one minute; Ben lifts the 100-pound barbell over his head 10 times in 10 seconds. Which student does the most work? ______________ Which student delivers the most power? ______________ Explain your answers.

2) During a physics lab, Jack and Jill ran up a hill. Jack is twice as massive as Jill; yet Jill ascends the same distance in half the time. Who did the most work? ______________ Who delivered the most power? _____________ Explain your answers.

3) A tired squirrel (mass of approximately 1 kg) does push-ups by applying a force to elevate its center-of-mass by 5 cm in order to do a mere 0.50 Joule of work. If the tired squirrel does all this work in 2 seconds, then determine its power.

4) When doing a chin-up, a physics student lifts her 42.0-kg body a distance of 0.25 meters in 2 seconds. What is the power delivered by the student's biceps?

Mechanical Energy• Energy is defined as the capacity to do work.• Performing work on an object may give it the

capacity to do work, and therefore energy (e.g. winding up a spring or raising a pile driver).

• Mechanical energy is defined as the energy of an object due to its position or motion, or both.

Potential Energy• The energy of an object due to its position relative to

other objects is called potential energy (PE).• Gravitational potential energy is the energy stored

in an object as the result of its vertical position relative to the surface of the Earth or other object.

mghPE height weight energy potential nalGravitatio

Kinetic Energy

• The energy of an object due to its motion is called kinetic energy.

• Kinetic energy is the other component of mechanical energy. In this class, we will define total energy as kinetic plus potential but there are other kinds of energy. TE = PE + KE

• http://www.brainpop.com/science/energy/kineticenergy/

2

2

21

velocity mass21

energy Kinetic

mvKE

Conservation of Energy• Energy can neither be created nor destroyed.

OR

• The total amount of energy in an isolated system never changes.

Energy simulation: http://phet.colorado.edu/web-pages/simulations-base.html

In physics, energy conservation means energy may be transformed from one form to another, or to work, with no net loss or gain.

Transformation of energy• PE is transformed into KE, then some goes back to PE,

and back again and so on. If there is no friction, the total amount of energy (KE + PE) does not change.

Total is always 40,000 J

More transformation of energy examples• http://www.physicsclassroom.com/mmedia/

energy/pe.html

• http://www.physicsclassroom.com/mmedia/energy/ce.html

Energy Questions1) In the following specify if the energy is changing from PE to KE or KE to PE.

Explain each answer.a) A ball falls from a height of 2 meters in the absence of air resistance.b) A baseball is traveling upward towards a man in the bleachers.c) A bungee chord begins to exert an upward force upon a falling bungee jumper.

2) In the following specify if there is a change in PE, KE or both. Explain each answer.

a) Rusty Nales pounds a nail into a block of wood. The hammer head is moving horizontally when it applies force to the nail.

b) A weightlifter applies a force to lift a barbell above his head at constant speed.

3) An object which weighs 10 N is dropped from rest from a height of 4 meters above the ground. When it has free-fallen 1 meter its total mechanical energy with respect to the ground is

4) During a certain time interval, a 20-N object free-falls 10 meters. The object gains _____ Joules of kinetic energy during this interval.

5) Determine the kinetic energy of a 1000-kg roller coaster car that is moving with a speed of 20.0 m/s.

Work-Energy Theorem• The kinetic energy gained or lost by an object is

equal to the work done by a net force acting on the object.

• The work- energy tells that when a car’s speed doubles, it will skid 4 times as far when stopping

• Example of work-energy theorem and its applications:– http://www.glenbrook.k12.il.us/gbssci/phys/mmedia/energy/se.html– http://www.physicsclassroom.com/mmedia/energy/hw.html– http://www.physicsclassroom.com/mmedia/energy/cs.html

KE Fx

KE Net work

Simple Machines• A machine is defined as a

device that can increase, decrease or change the direction of the applied force.

• A simple machine has few or no moving parts.

• Simple machines do not increase work done, energy output or power, they can only change increase output force.

Machines –lever and pulley• Machines like levers and pulleys multiply forces

or change the direction in which forces act.

PulleyPulleyLeverLever

• The lever and the pulley enable a smaller force to be applied through a larger distance to yield an equivalent amount of work.

Types of levers• In a first class lever the

fulcrum is in the middle and the load and effort is on either side. Example: see-saw

• In a second class lever the fulcrum is at the end, with the load in the middle. Example: wheelbarrow

• In a third class lever the fulcrum is again at the end, but the effort is in the middle. Example: tweezers

Machines – inclined plane• Another example of a simple machine is an inclined

plane, or ramp.• When doing work, this type of slanted surface

reduces the force you need to exert on something.• For an ideal inclined plane, where there is no loss

due to friction:distanceoutput forceoutput distance applied force applied

3 N6 m2 m

W = (6 m)(1 N)

W = (2 m)(3 N)

1 N

(1 N)(6 m) = (3 N)(2 m)Work to lift straight up

Work to push up ramp

Why Use Simple Machines?• We use simple machines for the

mechanical advantage. Even simple machines can offer a significant increase in output force.

• Mechanical advantage can be thought of as the number of times a machine multiplies the applied force. It is the ratio of applied force to output force. For machines that increase output force, MA is greater than one.

distanceoutput

distance applied

force applied

forceoutput MA

Efficiency of Energy Conversion• In ideal machine, the work output equals the

work input (efficiency is 100%). There are no ideal machines.

• All real machines convert some work to heat through friction (efficiency is less than 100%).

• An automobile has an overall efficiency of 15-20% (80-85% is waste heat due to friction, to the cooling system or carried off by exhaust gases).

%100inputenergy total

outputenergy useful Efficiency

Finding efficiency and MA for a ramp• If the ramp is not 100% efficient, extra force will be required

to push box up the ramp• Useful energy output is the energy required to lift the box

straight up• Total energy input is the energy required to push the box up

the ramp.• e = 6 J/12J x 100% = 50% (efficiency always less than 100%)• MA = 3 N/2 N (MA is usually greater than 1)

3 NW = (3 N)(2 m)

W = (2 N)(6 m)2 N

6 m2 m

Machine Questions1) What do machines do?2) Try to figure out the MA and efficiency of this inclined planes:

3) Try to figure out the MA and efficiency of this pulley system:

1500 N

60 N

A 12 N force is required to pull the 20 N bucket straight up. When the rope is pulled down 4 m, the bucket rises 2 m.

20 N

12 N

Machine Questions cont.4) What effort force is required to lift a person on this lever?

5) An ideal lever 5 meters long is used to lift up a load of 1000 N. The lever gives a mechanical advantage of 4. Sketch the lever and show the location of the fulcrum and the forces at each end of the lever.

Sources

• Conceptual Physics by Paul Hewitt• www.physicsclassroom.com• http://rigel.physics.unr.edu/faculty/phaneuf/

classinfo/index100.html• www.generalpatton.org/education/

lesson_plans/Simple_machines.ppt