Download Potential Energy is Always Potential Energy of a System Isolated and Non-isolated Systems | PHY 2048 and more Exams Physics in PDF only on Docsity! Chapter 8, part 3 Potential Energy is always Potential Energy of a System Isolated and Non-isolated Systems • Energy Conservation in Isolated System • Energy Change due to External Forces Potential Energy of a System U is always potential energy of a system, not of an individual object U depends on relative position of objects in a system • apple and the Earth • pendulum and the Earth • two blocks attached with a spring • a block on a spring hanging from a support standing on the Earth Isolated System This is true only if there are no non-conservative forces… Friction is nearly always there and results in dissipation of mechanical energy—friction transfers kinetic energy of macroscopic objects into heat… Heat is the “random” kinetic energy of molecules making up macroscopic objects EARTH 1 2 ...mec systemE U K K const= + + + = 1 2( ...)system system thermalE U K K E const= + + + + = Example 1 A block of mass m starts out with the same speed v in the three situations shown below and moves until it is topped by friction. The surfaces have identical friction coefficients μ. Inclination angle is α. Find increases in thermal energy for these three cases. v v v What the textbook does not say Etotal = U + K + Ethermal = Const Ethermal = “random” kinetic energy of molecules K = kinetic energy of a macroscopic object “as a whole” U ↔ K OK K → Ethermal OK Ethermal is the final destination! Ethermal → K: impossible Ethermal → U: impossible yet another fundamental law of physics Example 3 Resistance force experienced by a car of mass m = friction + drag force: F=μN+bv2. Car’s engine can deliver power P. Write the equation for the maximum speed for this car on a flat road? on a road with 15% slope going up? m=1200 kg, μ=0.03, b=1.8, P=140 hp 0 20 40 60 80 100 120 140 160 180 200 0 10 20 30 40 50 60 70 80 90 100 speed (mph) ne ed ed h or se po w er 15% slope flat road HITT A ball is suspended on a massless spring and oscillates back and forth between levels 1 and 3. The level 2 is right in the middle between 1 and 3. Which of the three levels corresponds to the maximum potential energy of the system Earth-ball-spring? 3 2 1 (A) 1 (B) 2 (C) 3 (D) 1 & 3 (E) all equal
