# Energy Unit ```Energy, Work, Power, and the Law of
Conservation of Mechanical Energy
You are studying the most critical and
looked out for issue in todays world.
Energy
• The ability of a physical system to do work on
another physical system.
• The ability to move matter/energy
• Energy cannot be created or destroyed
• S.I. Units: joules, kilowatt-hours, kilocalories
• E = mc2 joules
Forms of Energy
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Matter (E=mc&sup2;)
Thermal energy (thermal energy in transit is called heat)
Chemical Energy
Electric Energy
aka light)
Nuclear energy
Magnetic energy
Elastic energy
Sound energy
Mechanical energy (This is where we study…just a small bit)
Luminous Energy
Mechanical Energy
• Sum of Potential (U) and Kinetic energy (K) of a
system
• Emechanical = U + K
• If forces acting on a system are “conservative”,
Emechanical is conserved
– E.g.: gravity, ideal spring, magnetic, and electric
forces
• If forces acting on a system are
“nonconservative”, Emechanical can be lost as heat.
– E.g.: friction and air resistence
Potential Energy (U)
• Energy of a body or system due to it is
position or arrangement of particles
• E.g.: Gravitational, elastic, chemical, nuclear,
electrostatic, and magnetic.
• Ugrav = mgh = kg * m/s2 * m = J
Kinetic Energy (K)
• The energy of an object due to its motion
• K = &frac12; mv2 = kg * m2/s2 = J
Work
• A force acting in a straight line through a
distance
• W = F * d = Fd cos Θ (where Θ is the angle
between F and the displacement vector)
= N * m = kg * m/s2 * m = joules
• Work-Energy Theorem:
W = ∆K = K2 – K1 = &frac12; m (v22 – v12)
Power
• The rate at which work is performed or energy
is converted
• Pavg = ∆W/ ∆t = J/s = Watt
• 1 Horsepower = 745.7 Watts
A kilogram of TNT contains 4.184 x
106 J of chemical energy. If a
NASCAR Sprint Cup car has a mass
of 3200lbs, how many kg worth of
TNT equivalent would it possess in
kinetic energy while travelling at
210 miles per hour?
• V = 210 mi/h = 93.9 m/s (308 ft/s, FYI)
• m = 3200 lbs = 1454.5 kg
• K = 1/2mv2 = 1/2 (1454.5 kg)(93.9 m/s)2
= 6.4 x 106 J
• 6.4 x 106 J / 4.184 x 106 J = 1.53 kg (~3.4lbs)TNT
Equivalent
If 2g of matter were converted into
energy, for how many years could you
power a 100W light
bulb?
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E=mc2 = .002kg * (3.0 x 108 m/s)2
= 1.8 x 1014 J
Since Pavg = ∆Work/ ∆t
t = Work/Pavg
= 1.8 x 1014 J / 100 Watts
= 1.8 x 1012 s
= 57,077 years
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