Work and Energy
ENERGY The ability to do work
WORK A change in energy
WorkEnergy
Circular Definition!
Radiant Electrical Chemical Thermal Nuclear Magnetic Sound Mechanical
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Potential energy + Kinetic energy = Mechanical energy
Potential energy + Kinetic energy = Mechanical energy Example of energy changes in a swing or pendulum.
2 Kinds of Mechanical Energy Kinetic energy (KE) Potential Energy (PE) –Gravitational PE (PE g ) –Elastic PE (PE elastic )
Kinetic Energy Energy due to motion KE = ½ mv 2
Potential Energy Energy due to position
Bowling Ball – Conservation of Energy
Conserving Energy
Gravitational PE PE g = mgh
A 25 kg boy runs with a speed of 6.5 m/s. How much kinetic energy does the boy have? 530 Joules
How much potential energy does a 0.50 kg apple have if it hangs from a tree 13m above the ground? 64 Joules
A 220 gram fly has a kinetic energy of 55 Joules. How fast is the fly flying? 22 m/s
ENERGY IS CONSERVED!!
WORK W = F s (Work = Force x displacement) Units: Joule (J) = N · m
Work is done only when components of a force are parallel to a displacement.
Force helps motion Positive Work Force opposes motion Negative Work Force 90 ° to motion No work Object not in motion No work
A student drags his book bag 15 meters across the classroom by pulling on it with a force of 5.0 Newtons. How much work does the student do on the bag? 75 Joules
Larry carries his 35N physics book across the room 25m away. How much work does he do on the book? 0 Joules
Work-Kinetic Energy Theorem W net = KE
POWER The rate at which work is done
P= W tt Or…
P= tt Or…
P= PP tt Or…
P = Fv = Force * velocity
The SI unit of power is the Watt, named in honor of James Watt James Watt James Watt James Watt. One Watt, W, of power is the power achieved when 1.0 J of work is done or 1.0 J of energy is transferred in a time of 1.0 s.
Watt = 1 J/s Horsepower (hp) = 746 W Unit of Power
How much power is generated by a motor that can lift a 5300 kg crate of bricks with a velocity of 15 m/s? 780,000 W
How much power is generated by a car engine that does 5500 Joules of work in 35 seconds? 160 W
How long will it take a 36 Watt motor to pull a box across the floor 25 m with a force of 16 N? 11 s
Homework: Chapter 8, Hewitt: Read Do RQ 1-13; P&C 1-3,5-7; T&E 3-8; T&S 1-4