Energy and MachinesPhysical 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?

1. Rock sitting on the cliff’s edge

2. A car going down a hill

3. Sitting at the top of a slide

4. Going down a slide

Calculating Kinetic Energy• Formula

–KE = ½ m * v2

• KE= kinetic energy (J)• m= mass (kg)• v= velocity (m/s)

Example ProblemThe 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?

Calculating Potential Energy• Formula

–PE = m * g* h

• PE = potential energy (J)• m= mass (kg)• g= gravity (9.8)• h= height (m)

Example ProblemsA 30 kg rock is sitting on a ledge 5 m from the ground of Earth. How much potential energy does it have?

How to also write PE formula

•PE = m*g*h

•Weight•PE= Fgh

Forms of 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– Energy stored in

chemical bonds– Examples• Chemical reactions• Burning coal

• Electrical Energy– Energy associated

with electric charges– Examples• CD players• Calculators

Forms of 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 Coaster1. PE Greatest2. Conversion

a) PE decreaseb) KE increasec) PE = KE

3. KE Greatest4. Conversion

a) KE decreaseb) PE increasec) KE = PE

5. PE Greatest

Energy Conversion: Pendulum1. PE Greatest2. Conversion

a) PE decreaseb) KE increasec) PE = KE

3. KE Greatest4. Conversion

a) KE decreaseb) PE increasec) KE = PE

5. PE Greatest

Energy Conversion: Throwing Ball Up1. KE Greatest2. Conversion

a) PE increaseb) KE decreasec) PE = KE

3. PE Greatest4. Conversion

a) PE decreaseb) KE increasec) KE = PE

5. KE Greatest

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). LeverA bar that is free to

pivot or turn around a fixed point (ex: teeter

totter)

2). PulleyGrooved wheel with a rope, chain, or cable

running along the groove (ex: wishing well)

Six types of Simple Machines3). Wheel & Axle

A shaft/axle attached to the center of a

larger wheel so that both rotate together

(ex: door knobs)

4). Inclined PlaneA sloping surface

(ex: ramp)

Six types of Simple Machines

5). ScrewAn inclined plane

wrapped in a spiral around a cylindrical

post (ex: jar lids)

6). WedgeAn 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• Input Force–Force applied

TO the machine–Symbol = Fin

• Output Force–Force applied BY

the machine–Symbol = Fout

Input Work & Output Work• Input 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 Practice1) 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 Practice1) 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?