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Unformatted text preview: PHYS171 Final Exam Practice Fall 2011 Name: _____________________ Student ID #: ________ 1 Short Answer Questions (respond in a few sentences using physical principals) 5 points each 1) Describe in a sentence or two why heat will not flow from a cold material to a hot material. Use whatever physical concepts you find useful, such as the kinetic theory of gasses, laws of thermodynamics, etc. (5 points) Heat energy consists of random energy of molecules. When molecules collide, they tend to exchange energy, so the slower molecule tends to gain energy and the faster molecule tends to close energy, on average. 2) Consider two containers of gas. They contain the same number of atoms, but the atoms in one container have twice the velocity as the other. What is the ratio of pressures, and why? (5 points) If the velocity is doubled, the pressure increases by 4. Faster molecules have greater momentum change when the bounce off the wall, and they bit the wall more often. The velocity enters twice, so the pressure is proportional to v 2 . 3) Explain why an object that is less dense than water floats. Any object immersed in water experiences a buoyancy force equal to the weight of the water displaced. If the object is less dense than water, the weight of the object is less than the buoyancy force, and the net force is upward. 4) A mass slides down an inclined plane without friction. Another mass, in the shape of a sphere, rolls down the same plane. Which has greater velocity upon reaching the bottom? When the mass rolls, some of the potential energy is converted to rotational energy, which reduces increase in center-of-mass energy. For the sliding mass, all of the potential energy becomes kinetic energy of the center-of-mass. Therefore the rolling object goes slower. PHYS171 Final Exam Practice Fall 2011 Name: _____________________ Student ID #: ________ 2 5) You are in a car that collides head-on with an identical vehicle. Which would result in lower chance of injury. Elastic collision, or inelastic collision, why? In elastic in-elastic collision the cars come to a stop. In an elastic collosion they bounce off, resulting in a larger change of velocity. Assuming the collisions occur during a similar time interval, elastic collision is worse. (This is why cars are designed to crumple during collisions, rather than remain rigidly intact.) 6) Sketch position and velocity vs time for car which slows to a stop at constant acceleration. Make sure slope and curvature are appropriate....
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