Work and Energy ENERGY The ability to do work WORK A change in energy.
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Transcript of Work and Energy ENERGY The ability to do work WORK A change in energy.
Work and
Energy
ENERGYThe ability to
do work
WORKA change in energy
Work
Energy
Circular Definition!
Radiant ElectricalChemical
Thermal Nuclear
Magnetic
Sound
Mechanical
At home – check out http://www.youtube.com/watch?
v=3ff_AXVlo9U for the Singing Tesla Coil!
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 (PEg)
–Elastic PE (PEelastic)
Kinetic Energy
Energy due to motion
KE = ½ mv2
Potential EnergyEnergy due to
position
Bowling Ball – Conservation of Energy
Conserving Energy
Gravitational PE
PEg = 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!!
WORKW = Fs
(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
Wnet = KE
POWERThe rate at which
work is done
P= Wt
Or…
P=t
Or…
P=Pt
Or…
P = Fv= Force * velocity
The SI unit of poweris the Watt,
named in honor ofJames WattJames Watt.
One Watt, W, of powerOne Watt, W, of poweris the power achievedis the power achieved
when when 1.0 J of work1.0 J of work is done or is done or1.0 J of energy is transferred1.0 J of energy is transferred
in a in a time of 1.0 stime of 1.0 s..
Watt = 1 J/sHorsepower (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 8.1-8.6Do RQ 1-13; P&C 1-3,5-7;
T&E 3-8;T&S 1-4