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Work and Energy
Section 3: What is Energy?
Preview
Section 3
Work and Energy Section 3
Key Ideas
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What is the relationship between energy and work?
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Why is potential energy called energy of position?
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What factors does kinetic energy depend on?
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What is nonmechanical energy?
Work and Energy Section 3
Bellringer
You should already have learned that energy is always conserved. Instead of being created or destroyed, energy just changes from one form to another. For example, sunlight is the ultimate source of energy on Earth. Look at the illustration below, and identify the types of energy involved.
Work and Energy
Bellringer, continued
Section 3
1.
How does sunlight provide the energy the girl needs to swing the bat? ( Hint: What do you need to have energy?)
2.
When the girl hits the ball, she exerts a force on it. Does she do work on the ball in the scientific sense of the term? Explain your answer.
3.
After the girl hits the ball, the ball moves very fast and has energy. When the ball hits the fielder’s glove, it stops moving.
Given that energy can never be destroyed but merely changes form, what happens to the energy the ball once had?
( Hint: If you are the fielder, what do you hear and feel as you catch the ball?)
Work and Energy Section 3
Energy and Work
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What is the relationship between energy and work?
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Whenever work is done, energy is transformed or is transferred from one system to another system.
• energy : the capacity to do work
• Energy is measured in joules (J).
Work and Energy Section 3
Potential Energy
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Why is potential energy called energy of position?
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Potential energy ( PE ) is sometimes called energy of position because it results from the relative positions of objects in a system.
• potential energy: the energy that an object has because of the position, shape, or condition of the object
Work and Energy Section 3
Potential Energy, continued
• Any object that is stretched or compressed to increase or decrease the distance between its parts has elastic potential energy.
– Examples: stretched bungee cords, compressed springs
• Any system of two or more objects separated by a vertical distance has gravitational potential energy.
– Example: a roller coaster at the top of a hill
Work and Energy Section 3
Visual Concept: Potential Energy
Work and Energy Section 3
Potential Energy, continued
• Gravitational potential energy depends on both mass and height.
• grav. PE = mass
free-fall acceleration
height, or
PE = mgh
• The height can be relative.
Work and Energy
Math Skills
Section 3
Gravitational Potential Energy
A 65 kg rock climber ascends a cliff. What is the climber’s gravitational potential energy at a point 35 m above the base of the cliff?
1. List the given and unknown values.
Given: mass, m = 65 kg height, h = 35 m free-fall acceleration, g = 9.8 m/s 2
Unknown: gravitational potential energy, PE = ? J
Work and Energy Section 3
Math Skills, continued
2. Write the equation for gravitational potential energy.
PE = mgh
3. Insert the known values into the equation, and solve.
PE = (65 kg)(9.8 m/s 2 )(35 m)
PE = 2.2
10 4 kg•m 2 /s 2
PE = 2.2
10 4 J
Work and Energy Section 3
Kinetic Energy
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What factors does kinetic energy depend on?
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Kinetic energy depends on both the mass and the speed of an object.
• kinetic energy: the energy of an object due to the object’s motion
• KE = ½ mass
speed squared, or KE= ½mv 2
Work and Energy Section 3
Visual Concept: Kinetic Energy
Work and Energy
Kinetic Energy, continued
Section 3
• Kinetic energy depends on speed more than mass.
• Atoms and molecules have kinetic energy.
Work and Energy
Math Skills
Section 3
Kinetic Energy
What is the kinetic energy of a 44 kg cheetah running at
31 m/s?
1. List the given and unknown values.
Given: mass, m = 44 kg speed, v = 31 m/s
Unknown: kinetic energy, KE = ? J
Work and Energy Section 3
Math Skills, continued
2. Write the equation for kinetic energy.
kinetic energy
1
mass
speed squared
2
KE
1
2 mv
2
3. Insert the known values into the equation, and solve.
KE = ½ (44 kg)(31 m/s) 2 = 2.1 × 10 4 kg•m 2 /s 2
KE = 2.1 × 10 4 J
Work and Energy Section 3
Other Forms of Energy
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What is nonmechanical energy?
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Energy that lies at the level of the atom is sometimes called nonmechanical energy .
• mechanical energy: the amount of work an object can do because of the object’s kinetic and potential energies
• In most cases, nonmechanical forms of energy are just special forms of either kinetic or potential energy.
Work and Energy Section 3
Other Forms of Energy, continued
• Chemical reactions involve potential energy.
– The amount of chemical energy associated with a substance depends in part on the relative positions of the atoms it contains.
• Living things get energy from the sun.
– Plants use photosynthesis to turn the energy in sunlight into chemical energy.
• The sun gets energy from nuclear reactions.
– The sun is fueled by nuclear fusion reactions in its core.
Work and Energy Section 3
Other Forms of Energy, continued
• Energy can be stored in fields.
– Electrical energy results from the location of charged particles in an electric field.
– When electrons move from an area of higher electric potential to an area of lower electric potential, they gain energy.
Work and Energy Section 3
Other Forms of Energy, continued
• Light can carry energy across empty space.
– Light energy travels from the sun to Earth across empty space in the form of electromagnetic waves .
– Electromagnetic waves are made of electric and magnetic fields, so light energy is another example of energy stored in a field.
