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ENERGY
Part I
Learning Goals for Section 1
The Nature of Energy
 Distinguish between kinetic and potential energy.
 Calculate kinetic energy.
 Describe different forms of potential energy.
 Calculate gravitational potential energy.
What is Energy?
• the ability to do work
• the ability to cause a change
Work, Power, Efficiency
 Work = (Force) (distance)
 Work measured in Nm
1 Nm = 1 Joule
 Power = Work / time
 Power measured in J/s
1 J/s = 1 Watt
1 HP = 764 Watts
 Efficiency = (Wout / Win) x 100
 Efficiency is a percentage (%)
Work Power Efficiency Problems
 Amy uses 20N of force to push a lawn mower 10
meters. How much work does she do?
 Frank does 4.8 E4 J of work in climbing a set of
stairs. If he climbs the stairs for 2 minutes, what is
his power output?
 A block and tackle does 1.25 E5 J of useful work, but
friction limits the block and tackle to an efficiency of
45%. What is the amount of input on work block and
tackle?
Kinetic Energy
 energy of motion
 depends on mass and speed
 depends on speed more than it depends on mass –
LOOK AT THE EQUATION!!!
Kinetic Energy Equation
kinetic energy (joules) = ½ mass (kg) × [speed (m/s)]2
KE = ½ mv2
Kinetic Energy Equations
 KE = ½ mv2
 m = 2 KE
(Joules)
(kilograms)
v2
v = √2KE
m
(m/s)
KE Problems
 What is the KE of an 8 kg mass at 5 m/s?
 What is the mass of an object that has 100 J of KE when
moving at 5 m/s?
 Determine the velocity of a 6 kg mass has 75 J of KE.
 A rocket with a mass of 1.5 E4 kg accelerates at 220 m/s2
for 29s from an initial speed of 5200 m/s. Determine the
kinetic energy of the rocket before and after the
acceleration.
Potential Energy
 stored energy
 stored energy in an object
means the object has
potential to cause change
Gravitational Potential Energy
 stored energy due to position
 depends on both mass and height
Gravitational Potential Energy Equation
GPE (J) = mass (kg) × acceleration due to gravity (m/s2) × height (m)
GPE = mgh
remember… mass x acceleration due to gravity = weight
acceleration due to gravity is 9.8 m/s2
GPE Equations
 GPE = mgh
(Joules)
 m = GPE
(kg)
gh
 h = = GPE
(m)
mg
 g = GPE
mh
(m/s2)
GPE Problems
 What is the GPE of a 3 kg ball that is 2 m above the
floor?
 How high do you have to lift a 5 kg box to give it 98 J
of GPE?
 What is the weight of an stone that has 1200 J of
GPE when setting on a 275 m cliff
Elastic Potential Energy
 energy stored by something that can stretch or
compress – rubber band, spring
 PEE = ½ kd2

K is the spring constant for the material


Measured in N/m
d is the compression/expansion distance of the material

Measured in meters
PEE Equations
 PEE = ½ kd2
(Joules)
 k = 2PEE
(N/m)
d2
 d = √2PEE
k
(m)
PEEProblems
 The elastic force constant of a spring in a toy is
550 N/m. If the spring is compressed 1.2 cm,
compute the potential energy stored in the spring.
 The spring constant (k) of a car’s front coil is 1800
N/cm. When the front tire rolls over a rock, the
spring is compressed 15 cm from its equilibrium
position. How much PEE is stored in the spring?
PEEProblems continued
 A 100.0 g arrow is pulled back 30.0 cm against a
bow, which has an elastic force constant of
125 N/m. If the entire potential energy stored in the
bow is converted to the kinetic energy of the arrow,
compute the speed of the arrow.