The ability to make something happen, to create a change
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Transcript of The ability to make something happen, to create a change
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EnergyThe ability to make
something happen, to create a change
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I. Types of Mechanical Energy
• A. Potential Energy• 1. potential energy is stored energy• 2. Does not involve motion or change• 3. The object has potential to cause
change
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• B. Gravitational Potential Energy (PE)• 1. determined by its position above the
earth.• 2. PE = mgh• m - mass (kilograms)• g – gravity (9.8 m/s2)• h – height (meters)
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• Both mass and height are proportional to the amount of potential energy!
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• C. Kinetic Energy• 1. the energy of motion• 2. Anything moving has kinetic energy• 3. KE = 1/2mv2
• m – mass (kilograms)• v – velocity (m/s)
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• Both mass & velocity are proportional to the amount of energy
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II. Units of Measurement• A. Energy is measured in Joules (J)• B. 1J = kg • m/s2
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III. Law of Conservation of Energy
• Energy cannot be created or destroyed, but can change forms. The total of all the energies is constant.
• Look at the sum of the energies in the examples.
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IV. Mechanical Energy
• 1. The total amount of kinetic and potential energy in a system.
• 2. ME = KE + PE• 3. Also measured in Joules• 4. What happens to the mechanical
energy as KE & PE are converted into each other?
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Example Problems
1. What is the potential energy of a 50N brick on top of a 4m ladder?
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Example Problems
2. If a student drops a book having a gravitational potential energy of 5 joules from a height of 1.1 meters what is the mass of the book?
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Example Problems
3. An SUV with a mass of 2,450 kilograms is traveling at 21 meters/second southeast. What is the kinetic energy of the SUV?
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Example Problems
4. A jet skier with a total mass of 250 kilograms is traveling south with a kinetic energy of 40,500 J. What is the velocity of the jet skier?
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Example Problems
5. A cat with a kinetic energy of 160 J is traveling up a tree at a velocity of 8 meters per second. What is the cat’s mass?
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Practice Problems1. Calculate the kinetic energy of a 3kg toy
cart that moves at 4m/s.
a. Calculate the kinetic energy of the same cart at twice the speed.
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Practice Problems2. Calculate the potential energy of a 90N
block of ice sitting on a table 3m above the ground.
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Practice Problems3. Calculate the potential energy of a
weightlifter holding a 100kg barbell above his head at a height of 2.2 m.
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Practice Problems4. A 0.5kg sphere at the top of an incline
has a potential energy of 6.0 joules. How tall is the incline?
a. The sphere rolls halfway down the incline. What will the object’s new potential energy be?
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Practice Problems5. A 0.173kg baseball is thrown with a
velocity of 2m/s. What I the kinetic energy of the baseball?
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Practice Problems6. A 3.1 kg cat sitting on a tree branch has
a potential energy of 153.7 joules. How far above ground is the tree branch?
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Practice Problems7. A 1000kg roller coaster car sits at the top
of a 30m drop. What is the gravitational potential energy of the roller coaster car?
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Practice Problems8. When a 5kg mass is lifted from the
ground to a height of 10 meters, the gravitational potential energy of the mass is increased by approximately how much?
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Practice Problems9. A 50kg shell is shot from a cannon at
Earth’s surface to a height of 450m. What is the gravitational potential energy of the shell at this height?
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Practice Problemsa. What is the change in the potential
energy when the shell falls to a height of 225m?
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Practice Problemsb. What is the gain in kinetic energy when
the shell falls to the height of 225m?
c. What is the velocity of the shell at this new height?