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PHY183-Lecture14pre

# PHY183-Lecture14pre - What is Energy Richard Feynman talks...

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1 February 7, 2012 Physics for Scientists&Engineers 1 1 Physics for Scientists & Engineers 1 Fall Semester 2012 Lecture 12 Energy, Kinetic Energy, Work What is Energy? Richard Feynman talks about the concept of energy: “It is important to realize that in physics today, we have no knowledge of what energy is. We do not have a picture that energy comes in little blobs of a definite amount. It is not that way. However, there are formulas for calculating some numerical quantity, and when we add it all together it gives ‘28’—always the same number. It is an abstract thing in that it does not tell us the mechanism or the reasons for the various formulas.” February 7, 2012 Physics for Scientists&Engineers 1 2 February 7, 2012 Physics for Scientists&Engineers 1 3 Kinetic Energy Energy contained in the motion of an object Definition Unit of kinetic energy: This energy unit has received its own name, Joule (J), named after British physicist James Joule (1818-1889) Energy unit Useful conversion: 1 J = 1 N m K = 1 2 mv 2 [ K ] = [ m ] [ v ] 2 = kg m 2 / s 2 1 J = 1 kg m 2 / s 2 February 7, 2012 Physics for Scientists&Engineers 1 4 Kinetic Energy Examples Car of mass 1,310 kg driving 55 mph (=24.6 m/s) Mass of the Earth is 6·10 24 kg, and it orbits the Sun with a speed of 30,000 m/s Baseball (mass “5 ounces” = 0.142 kg) thrown at 80 mph (= 35.8 m/s) has kinetic energy Electron ( m e = 9.1·10 -31 kg) moving with a speed of 1.3·10 6 m/s (= 0.4% of the speed of light): K car = 1 2 mv 2 = 1 2 (1310 kg)(24.6 m/s) 2 = 4.0 10 5 J K sun = 1 2 mv 2 = 1 2 (6.0 10 24 kg)(3.0 10 4 m/s) 2 = 2.7 10 33 J K baseball = 1 2 mv 2 = 1 2 (0.142 kg)(35.8 m/s) 2 = 91 J K e = 1 2 mv 2 = 1 2 (9.1 10 31 kg)(1.3 10 6 m/s) 2 = 7.7 10 19 J February 7, 2012 Physics for Scientists&Engineers 1 5 Other Energy Units Atomic and nuclear physics introduced the electron-Volt 1 eV = 1.602·10 -19 J (electron in previous example has ~5 eV kinetic energy) The energy you eat is measured in food calories 1 Cal = 4186 J (1 Cal > 40 times the kinetic energy of the baseball in previous example) For very large energy measures one need a large unit. The energy released by one million tons of TNT is large 1 Mt = 4.0·10 15 J February 7, 2012 Physics for Scientists&Engineers 1 6 Example: Falling Vase Question: A crystal vase (mass 2.40 kg) is dropped from a height of 1.30 m and falls to the floor. What is its kinetic energy just before impact? Answer: Once we know the velocity of the vase just before impact, we can put it into our equation for the kinetic energy. To obtain this velocity, we remind ourselves of the kinematics of free-falling objects. Use: Now use definition of kinetic energy and obtain in this case: Numbers: v 2 = v 0 2 2 g ( y y 0 ) = 2 g ( y 0 y ) v 0 = 0 ( ) K = 1 2 mv 2 = 1 2 m 2 g ( y 0 y ) ( ) = mg ( y 0 y ) K = (2.40 kg) (9.81 m/s 2 ) (1.30 m 0) = 30.6 J

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