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ENERGY

ENERGY. Energy is that property whose possession enables something to perform work. 3-types 1- Kinetic Energy- which is the energy something possesses by virtue of its motion. 2- Potential Energy- Which is the energy

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ENERGY

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  1. ENERGY • Energy is that property whose possession enables something to perform work. • 3-types 1- Kinetic Energy- which is the energy something possesses by virtue of its motion. 2- Potential Energy- Which is the energy something possesses by virtue of its position. 3- Rest Energy- which is the energy something possesses by virtue of its mass. E= mc2.

  2. The potential energy acquired by an object equals the work done against gravity or other forces to place it in position.

  3. P.E. = Work = FΔd = mgΔh • P.E. = mgΔh • Units = kg-m/s2•m = kg-m2/s2 = N-m = Joule (J) • The gravitational potential energy defined by P.E. = mgh is expressed in relation to an arbitrary reference level where h=0 eg. Sea level, street level, ground level, or floor level are useful reference levels.

  4. F=ma, a=F/m • vf2= vi2+2aΔd vi=0 vf2=2aΔd  vf2=2(F/m)Δd FΔd=1/2mvf2 Work done on the ball =K.E. of the ball Fd = 1/2mv2 K.E.=1/2mv2 units= Joules (J) • * You can set energies equal to each other. • Work = P.E. • Work = K.E. • P.E. = K.E.

  5. If a 50-kg mass of steel is raised 5-m. What is its potential energy? • P.E. = mgh • = (50kg)(9.8m/s2)(5m) • = 2.5 x 103J

  6. If a baseball has a mass of 0.14-kg and is thrown with a velocity of 7.5-m/s, what is its K.E.? • K.E. = 1/2mv2 • K.E. = 1/2(0.14-kg)(7.5-m/s)2 • = 3.9J

  7. A 600-g hammer head strikes a nail at a speed of 4-m/s and drives it 5-mm into a wooden board. What is the average force on the nail? In Newtons and in lbs. • Work=K.E. • Fd=1/2mv2 • F=mv2/2d = (.6-kg)(4m/s)2/2(.005-m) • F=960-N • 960N•.225lbs/1N=216lbs.

  8. Find the K.E. of a 1200-kg car when it is moving at 25-km/hr and when it is moving at 100-km/hr. • How much more energy does the car have when it is moving at 100-km/hr? • v1=25-km/hr=7-m/s, v2= 100-km/hr=27.8-m/s • K.E.1=1/2mv2=(.5)(1200kg)(7m/s)2 =28,935J • K.E.2=1/2mv2=(.5)(1200kg)(27.8m/s)2 =463,704J • The 100-km/hr car has 16 times as much K.E. as it does at 25-km/hr. Thus the 100km/hr car can do more work and at high speeds can cause severe auto accidents.

  9. If Meagan pushes on a lawn mower with a constant force of 90.0-N at an angle of 40° to the horizontal, how much work does she do in pushing it a horizontal distance of 7.5m? • W=(Fcos40°)d • W=(90.0N)(cos40°)(7.5M) • W=517J

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