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Unformatted text preview: Spring 2011 PHYS 172: Modern Mechanics Lecture 19 Collisions Read 10.9 10.11 Example: headon car crash Two identical cars of mass m=1000 kg move toward each other with speeds v 1 =v 2 =30 m/s (~65 mph), collide headon and stick together. CLICKER: What is the speed of the center of mass of the system consisting of these two cars after the collision? A. 30 m/s B. 15 m/s C. 60 m/s D. 0 m/s Example: headon car crash Two identical cars of mass m=1000 kg move toward each other with speeds v 1 =v 2 =30 m/s (~65 mph), collide headon and stick together. CLICKER: What was the speed of the center of mass of the system consisting of these two cars before the collision? A. 30 m/s B. 15 m/s C. 60 m/s D. 0 m/s Example: headon car crash Two identical cars of mass m=1000 kg move toward each other with speeds v 1 =v 2 =30 m/s (~65 mph), collide headon and stick together. A. is conserved B. increases C. decreases CLICKER: During the collision, the translational kinetic energy of the system consisting of these two cars: Example: headon car crash Two identical cars of mass m=1000 kg move toward each other with speeds v 1 =v 2 =30 m/s (~65 mph), collide headon and stick together. A. is conserved B. increases C. decreases CLICKER: During the collision, the internal kinetic energy of the system consisting of these two cars: Example: headon car crash CLICKER POLL: How much energy is released as heat? ( ) ( )( ) 2 2 1 1 2 2 1000 kg 30 m/s 900 kJ w w m C T mv = = = m w = 900 kJ/(4.18 J/g/K 100 K) = 2.2 kg (~1/2 gal) Enough to heat up from freezing to boiling: A) 250 gallons of water B) 50 gallons of water C) 10 gallons of water D) 2 gallons of water E) gallon of water Two identical cars of mass m=1000...
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