Lecture 2/16/2009 Monday, February 16, 2009 11:00 AM Work and Kinetic Energy Reminders Midterm Exam I, Tuesday February 24 5:45 PM Material from Chapters 1-4 inclusive One page of notes (8.5" x 11") allowed 20 multiple choice questions, 90 minutes Scantron will be used - bring #2 HB pencils + calculators Work Force applied over a distance W = F ?x cos? Requires non-zero external force (F) and displacement (?x) in the direction (?) of the force Work is done on the object by the eternal force, if there is a displacement in the same direction Work is done on an object by an external force if there is displacement in the opposite direction Units are Newton-meter defined as Joule also 1 calorie ( energy raises a g of water 1 degree C) 1 cal = 4.186J Work and Force Direction Work is done in lifting a box Question 1 A woman holds up a bowling ball in a fixed position. The work she does on the ball is equal to zero. Although the woman is exerting force against gravity to hold the bowling ball up, she has not shifted its position. Therefore, the work done by her is zero Question 2 A man pushed a very heavy load across a horizontal floor. The work done by gravity on the load is equal to zero Preflight 1 You are towing a car up a hill with constant velocity. The work done on the car by the normal force is zero Energy Energy is the quality of a substance or object which causes something to happen. Perhaps, one could term it the capability of exerting forces. The vagueness of the definitions is due to the fact that energy can result in many effects Kinetic Energy Electrical Energy Solar Energy Chemical Energy Nuclear Energy Gravitational Energy It is convertible into other forms without loss (i.e. it is converted) Work Equations W = 1/2mv² - 1/2mv0² Hooke's Law Force exerted to compress a spring is proportional to the amount of compression Fs= -kx Pes = 1/2kx²
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