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Warm up. If you shout, your voice travels at the speed of sound (340m/s) to a surface and back , and you hear an echo. If you hear an echo 5.2s after you shout, how far away is the surface from which the sound bounces? . Energy. Chapter 4. Energy. What is energy?
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Warm up • If you shout, your voice travels at the speed of sound (340m/s) to a surface and back, and you hear an echo. If you hear an echo 5.2s after you shout, how far away is the surface from which the sound bounces?
Energy Chapter 4
Energy • What is energy? • energyis an indirectly observed quantity. It is often understood as the ability a physical system has to do work on other physical systems. Since work is defined as a force acting through a distance (a length of space), energy is always equivalent to the ability to exert pulls or pushes against the basic forces of nature, along a path of a certain length.
Energy • What is energy? • The ability of cause change • Energy is transferred and sometimes changed in form. • How do we observe energy or what does it do for us? • Motion • Heat • Electricity - light
Energy • What are some forms of energy? • Electricity • Chemical (gas for the car, food for you) • Light • Heat • Sound
Energy • Kinetic Energy • The energy an object has because of its motion. Depends on mass and speed. Kinetic energy = ½ mass x velocity squared KE = ½ mv2
Energy • A jogger whose mass is 60 kg is moving at a speed of 3 m/s. What is the joggers KE? KE = ½ mv2 KE = ½ (60 kg) (3 m/s)2 KE = ½ (60 kg) (9 m2/s2) KE = 270 kg m2/s2
Energy • The derived unit of energy is kg m2/s2 • This unit is called the Joule (J)
Energy • What is the kinetic energy of a baseball moving at a speed of 40 m/s if the baseball has a mass of 0.15 kg? KE = ½ mv2 KE = ½ (0.15 kg) (40 m/s)2 KE = ½ (0.15 kg) (1600 m2/s2) KE = 120 J
Energy • A car moving at a speed of 20 m/s has a kinetic energy of 300,000 J. What is the car’s mass? KE = ½ mv2 300,000 J = ½ (m) (20 m/s)2 KE = 1500 J
Energy • What is the velocity of a baseball with 150 J of kinetic energy if the baseball has a mass of 0.15 kg? KE = ½ mv2 2KE/m = v2 √2KE/m = v V = 44.7 m/s
Energy • Motionless objects also have energy. • Instead of energy of motion this is energy of position and is stored in an object. • This energy of position is referred to as Potential Energy.
Energy • A motionless object on a shelf has potential energy. If it falls the energy is converted to kinetic energy.
Energy • Potential Energy is the storing of energy. • Elastic PE – a stretched rubber band • Chemical PE – energy stored in chemical bonds • Electrical PE – energy stored due to position related to other charges.
Energy • Gravitational Potential Energy • Anything that can fall has stored energy called GPE. This energy is due to the height above Earth’s surface and the mass of the object.
Energy • GPE = mass (kg) x accel due to gravity (m/s2) x height (m) GPE = mgh
GPE = mass x acceleration due to gravity x height The GPE of an object can be calculated using this equation: How is GPE calculated? • Mass is measured in kilograms (kg). • Acceleration due to gravity is 9.8 m/s2 on Earth • Height is measured in meters (m). • GPE is measured in joules (j).
Calculating GPE question 1 An eagle with a mass of 2 kg flies at a height of 200 m above the ground. How much gravitational potential energy does the eagle have? GPE = mass x accel due to gravity x height = 2 kg x 9.8 m/s2x 200 m = 3920 J
changein height GPElost = mass x accel due to gravity x Calculating GPE question 2 An apple with a mass of 200 g falls 3 m from its branch to the ground. How much GPE will the apple have lost when it reaches the ground? = 0.2 kg x 9.5 m/s2x 3 m = 5.88 J
This is a web site with a 14 question quiz on ke/pe. Not too hard. Good for a couple minutes of review http://www.cstephenmurray.com/onlinequizes/physics/workandenergy/kineticvspotentialenergy.htm