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EnergyChapter 5 Section 2
What is Energy?
• Energy – A scalar quantity that is often understood as the ability for a physical system to produce changes on a different physical system.
• Units for energy is a “Joule”– The variable for a Joule is a capital “J”
Kinetic Energy
• Kinetic Energy – The energy of an object due to it motion.
• Kinetic energy depends upon the object’s velocity and mass.
• Any mass that is moving has kinetic energy.
Equation for Kinetic Energy
Influencing Kinetic Energy
• Mass and speed both influence the kinetic energy of an object.
• Which one influences it more?
Example Problem
• A car traveling at 50m/s has a mass of 1000kg. Calculate the kinetic energy.
Example Answer
• KE = ½mv²
= ½ (1000kg) (50m/s) ²
=1,250,000
KE = 1,250,000J or 1.25MJ
Potential Energy
• Potential Energy – The energy associated with an object due to its position.
• In other words, potential energy is stored energy that has the potential to do work when the energy is released.
Different Forms of Potential Energy
• There are two different forms of potential energy:– Gravitational Potential Energy– Elastic Potential Energy
Gravitational Potential Energy
• Gravitational Potential Energy – Potential energy associated with an object due to its position relative to the Earth or some other gravitational source.
Gravitational Potential Energy Equation
Converting Gravitational Potential Energy into Kinetic Energy
• Imagine a rock falling off a cliff. As it falls, it gains kinetic energy since it picks up velocity as it falls to the ground below.
• But where does the kinetic energy come from?
• It comes from the gravitational potential energy that is associated with the rock’s initial position on the cliff relative to the ground below.
Origin of Gravitational Potential Energy
• Gravitational potential energy is a result of an object’s position, so it must be measured relative to some ZERO level.
• Doesn’t matter where you place the zero level (origin), but it must remain consistent through out the problem.
Example Problem
• How much potential energy does a rock have that has a mass of 10kg and is 100m off the ground on top of a cliff?
Example Problem Answer
• PEg = mgh
=(10kg)(9.8m/s²)(100m)
=9800J
PEg = 9800J or 9.8KJ
Elastic Potential Energy
• Elastic Potential Energy – The potential energy in a stretched or compressed elastic object.– Examples:
• Springs• Rubber bands• Shocks
Relaxed Length of a Spring
• The length of a spring when no external forces are acting on it is called the relaxed length of a spring.
• When an external force compresses or stretches a spring, elastic potential energy is stored in the spring.
Elastic Potential Energy Equation
Spring Constant
• Spring Constant – A parameter that expresses how resistant a spring is to being compressed or stretched.
• For a flexible spring the constant is small and for a stiff spring the constant is large.
• The units for the spring constant is “N/m”
Example Problem
• A child has a rubber band with a length of 0.10m. The child pulls back on the rubber band to a distance of 0.20m. The spring constant of the rubber band is 5 N/m. What is the potential energy with in the rubber band?
Example Problem Answer
PEe= ½kx²
= ½ (k) (xf – xi)²
= ½ (5 N/m) (0.20m – 0.10m)²
= ½ (5 N/m) (0.10m)²
=0.025J
PEe = 0.025J or 25.0 mJ
Mechanical Energy
• Mechanical Energy – The sum of kinetic energy and all forms of potential energy.– Example:
• Any kind of mechanical device– Clocks– Air compressor– Basically anything that has moving pieces…
– Doesn’t include electrical, chemical, or heat energy.
Mechanical Energy Equation
ME = KE + ΣPE
ME = Mechanical Energy
KE = Kinetic Energy
ΣPE = The sum of all the potential energy
Classification of Energy
Energy
Mechanical
Kinetic Potential
Gravitational
Elastic
Non-mechanical