work, energy and power introduction energy is used to do work mechanical – motion of objects...
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Work, Energy and PowerWork, Energy and Power
Introduction
Energy is used to do work• Mechanical – motion of objects and
gravity
Types of energy: solar, chemical, nuclear, petroleum, geothermal, hydroelectric, wind, …
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Work, Energy and PowerWork, Energy and Power
Work
The product of displacement and the force acting in the direction of the displacement.
Unit: joule ( J ) or Nm• James Prescott Joule (1818-1889) known for his mechanical equivalent of heat experiments
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Work, Energy and PowerWork, Energy and Power
Energy
It is hard to measure, can be transferred between objects and can change form.
• Atoms, kicking a ball, wind to electric to light, etc.
A scalar quantity, may be positive or negative and has units of joules like work.
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Work, Energy and PowerWork, Energy and Power
Potential Energy
Gravitational potential energy (GPE) depends on the position of the object relative to an arbitrary point where the object’s GPE = 0.
• It can be positive or negative• GPE = m g y
Other forms: springs, rubber bands, food, …
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Work, Energy and PowerWork, Energy and Power
Work and Gravitational Potential Energy
An object’s GPE will change if work is done against gravity or by gravity.
• Lifting a ball: work done against gravity or +GPE
• Drop a ball: work done by gravity or –GPE
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Work, Energy and PowerWork, Energy and Power
Kinetic Energy
The energy of motion
It is always positive: KE = ½ m v2
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Work, Energy and PowerWork, Energy and Power
Power
The rate at which work is done
Unit: watts (W) or J/s• Horsepower (hp) is still used for
engines• 1 hp = 746 W• James Watt (1736-1819) improved the
steam engine
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Work, Energy and PowerWork, Energy and Power
Conservative and Non-conservative Forces
Conservative: gravity• Path independent
Non-conservative: kinetic friction, air resistance
• Path dependent
Example: designing roller coasters04/20/2304/20/23 APHY101APHY101 88
Work, Energy and PowerWork, Energy and Power
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Work, Energy and PowerWork, Energy and Power
Machines
They multiply forces, not energy !!!
Work input = Work output• Assumes thermal losses are small
Examples: ramps, levers, pulleys
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