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
•
•
•
•
•
•
•
•
•
•
•
Matter (E=mc²)
Thermal energy (thermal energy in transit is called heat)
Chemical Energy
Electric Energy
Radiant Energy (the energy of electromagnetic radiation,
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 = ½ 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 = ½ 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?
•
•
•
•
•
•
•
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