CME - CME Conservation of Mechanical Energy revised...

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1 Cons. of Mechanical Energy Learning Objectives: During this lab, you will 1. learn how to communicate scientific results in writing. 2. estimate the uncertainty in a quantity that is calculated from quantities that are uncertain. 3. be introduced to the concept of relative (or fractional) change. 4. be introduced to a technique for accounting for a systematic error. 5. test a physical law experimentally. A. Introduction Conservation laws play an important role in physics. In classical physics, quanti- ties such as energy, linear and angular momentum and the amount of electric charge are conserved. The list of conserved quanti- ties expanded significantly during the devel- opment of modern physics. For example, the total number of heavy elementary particles, or baryons ( including protons and neutrons ), appears to be constant in the universe. The same rule applies to the number of light par- ticles, or leptons ( such as the electrons, muons and neutrinos ). Quantities named par- ity and strangeness are also conserved while relativity requires expanding the law of con- servation of energy to include the equiva- lence of mass and energy in the form of Einstein’s famous law, E = mc 2 . Physicists place such confidence in these laws that they have postulated the existence of new elementary particles to explain apparent failures of conservation laws in carefully performed experiments. No violation of the law of conservation of energy has ever been substantiated. Because energy takes many forms, it is easy to believe that it has been lost or gained in an isolated experiment. For example, a rolling cart coming to a halt suggests that kinetic energy has disappeared from the sys- tem. However, the kinetic energy of the roll- ing cart may have been transformed into gravitational potential energy or into thermal energy. Similarly, a high-energy photon may disappear, reappearing as a low-energy particle-antiparticle pair. Part of the kinetic energy of the high-energy photon has been converted into the rest-mass energy of the particle-antiparticle pair through Einstein’s relation. In this experiment, you make two types of measurements: a. conversion of gravitational potential energy of a falling body into the kinetic energy of the body and a cart to which it is tied, b. conversion of gravitational potential energy into the potential energy stored in a stretched spring. You must hand in a report for this lab worth 45 points. Detailed guidance for writing this report is at Section F of this write-up. B. Apparatus You will use a PASCO track with encoded pulley, a low-friction cart, weight hanger and weights. You will also use a block, a spring and a spirit level. Data will be recorded using a computer running the Logger Pro program. C. Theory Several equations are useful in this experiment: Kinetic energy: K = ½ mv 2 (1) Gravitational potential energy: U g = mgh (2) CME Conservation of Mechanical Energy revised July 11, 2005
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Cons. of Mechanical Energy 2 Potential energy of a stretched spring: U k = ½ kx 2 (3) Hooke’s Law: F = - kx (4)
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CME - CME Conservation of Mechanical Energy revised...

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