Transcript 2(190 j)
Energy The ability to do work Objectives • Explain the relationship between energy and work. • Define potential energy and kinetic energy. • Calculate Kinetic energy and potential energy. • Distinguish between mechanical and nonmechanical energy • Identify nonmechanical forms of energy Energy Types • Potential Energy • Kinetic Energy • Mechanical Energy • All are measured in joules, J • Work is also measured in J Potential Energy • Sometimes called gravitational potential energy. • PE = mass x gravity x height • PE = mgh g = 9.8 m/s2 PE m g h Add this to your circle sheet. Potential Energy problem samples A. Calculate the PE of a car with mass of 1200 kg at the top of a 42 m high hill. Ans.: PE=mgh=1200kg(9.8m/s2)(42m) =490 000 = 4.9 x 105 J, for the car B. What is the PE of a 65 kg climber on top of Mount Everest (8800m high)? Ans.: PE=mgh=65kg(9.8 m/s2)(8800m) =5 600 000 = 5.6 x 106 J, climber Kinetic Energy The energy an object has as a result of its motion. In the picture at right, which block has the greatest kinetic energy? Which one has the greatest potential energy? Formula for Kinetic Energy KE = ½ mass x velocity2 KE = ½ mv2 Put these other forms of the formula at the bottom of your circle sheet. • Calculate the kinetic energy of 1500 kg car moving at 29 m/s and at 42 km/h KE = ½ mv2 = ½ (1500kg)(29m/s)2 = 6.3 x 105J 42km/h: V = 42km 1000m 1h = 12m/s h 1km 3600s KE = ½ mv2 KE = ½ (1500kg)(12m/s)2 = 1.1 x 105J • A 39 kg child has 190 joule of kinetic energy after sledding down a hill. What is the child’s speed at the bottom of the hill in m/s? • V = 2KE • m • V = 2(190 j) • 39kg • V = 3.3 m/s Your assignment is to finish 1, 2, and 3 • But first, add these formulas to the bottom of your circle sheet for other forms of the kinetic energy formula: • V = 2KE m = 2KE • m v2 Answers to1bc,2,3 1b. KE=1/2mv2 = ½(1500kg)(18m/s)2 = 240000 =2.4 x 105 J 1c. V = 42km 1000m 1h = 12m/s h 1km 3600s KE = ½ (1500kg)(12m/s)2 = 1.1 x 105J • #2 • V = 2KE = 2(190J) • m 35kg = 3.3m/s • #3. m = 2KE = 2(16J) = 8.0 kg • v2 (2.0 m/s)2 Mechanical Energy • The sum of potential energy and the kinetic energy in a system. • Like a pendulum. Potential energy changes to kinetic energy and vice versa as the pendulum swings back and forth. • The total mechanical energy for “the system” remains constant. Other Forms of Energy Nonmechanical • Energy at the atomic level that does not generally affect the motion of an object. Thermal Energy The temperature of an object affects how fast the atoms of the object move. (Remember the atomic theory?) Faster motion greater Kinetic Energy!! Chemical Reactions • When chemical reactions occur, energy can be released • Examples: heat from a burning log in the fireplace or fireworks! Solar Energy • Light from the sun sustains life here on earth. • The sun gets its energy from nuclear reactions. • Light from the sun travels through space in the form of electromagnetic waves. Electrical Energy • Moving electrons through conducting materials allow machines to do a tremendous amount of work for us. • Can you think of any? Assignment • 1-9ab on the next slide • For EXTRA CREDIT: • Solve 9d on next slide. Hand this in separately with formula, work, and answer…(you know the routine…) for full credit. Worth 3 test points Answers to 7-9ab • • • • • • • • 7.PE=mgh=93.0kg(9.8m/s2)(550m)= =500 000 J 8.KE=1/2mv2 = ½(0.02kg)(300m/s)2 =900 J 9.a. PE=mgh=2.5kg)(9.8m/s2)(2.0m) = 49 J 9.b. KE=1/2mv2=1/2(0.015kg)(3.5m/s)2 = 0.092 = 9.2 x 10-2 J Book assignments • Read chapter 15 • Write and answer Key Concepts for each section • Define all terms • Add new formulas to list (see notes) • 469-70/1-22,26 • 471/FCAT practice