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# l7 - Today HW#4 pushed back to 8:00 am Thursday Exam#1 is...

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ISP209s10 Lecture 7 -1- Today HW#4 pushed back to 8:00 am Thursday Exam #1 is on Thursday Feb. 11 Bring a calculator and a #2 pencil Allowed 1 page notes (front and back) • E=mc 2 , General Relativity, and exam review

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ISP209s10 Lecture 7 -2- Comments on Exam 1 40 questions, multiple choice Bring #2 pencil and calculator Level of difficulty HW sets Extra credit opportunity if you bomb In LONCAPA, take a retake of a similar 40 point exam For each point higher than original in-class score, I will add .30 points to your original score say you got a 30/40 on the exam, and a 40/40 on the retake. Then you would get (40-30)*0.3 = 3 extra points for an adjusted score of 33/40.
ISP209s10 Lecture 7 -3- Special Relativity: Time, Length, Mass Time Dilation: “Moving clocks slow down” Time slows down Length Contraction: “Moving rulers shrink” – Lengths in the direction of motion are shorter than when at rest. Mass increases the faster you go (but you can never reach “c” since mass would be infinite)

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ISP209s10 Lecture 7 -4- A few words about Energy Energy is the ability to do work. It comes in two main types – Kinetic energy: the energy of motion – Potential energy: the energy of position Work = force x distance (it’s a scalar measured in Joules, J (same as Nm)) Power is the rate work is done (it’s a scalar measured in J/s (same as a Watt W)
ISP209s10 Lecture 7 -5- Any increase in a system’s energy results in an increase to its mass! • The change in mass is equal to the change in energy, divided by the square of the speed of light. • The speed of light is very large, so the change in mass is undetectable in ordinary situations. • This idea (that mass can “change”) was very odd at the time. E = mc 2 : What it means

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ISP209s10 Lecture 7 -6- E = mc 2 in action Schematic picture of a chemical or nuclear reaction: Start with some initial mass (kg) Something happens End up with some final mass (kg) Some fraction, f, is converted to energy ENERGY (Joules, J) The amount of energy is E = m converted c 2 m converted = (M initial - M final ) = M initial x fraction converted
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