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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?