Transcript Warm up
Warm up • 1) How much energy does a 45 kg rock have when it is at the top of a 70 m cliff? • 2) How much energy does a 20 kg tricycle have when it is moving at 7 m/s? Pass Back Quizzes Questions? See me outside of class or when we have free time Work-Energy Theorem • Work and Kinetic energy – Work done = change in KE – W = KE2 - KE1 Example • A 900 kg car goes from 10 m/s to 23 m/s, how much work is done on the car by the road? Practice • If 300 J of work are done on a tricycle of mass 8 kg, that is initially at rest, what will it’s final velocity be? Work-Energy Theorem 2 • Work and Potential Energy – Work done = change in potential energy • (only depends on initial and final positions) – W = mghf – mghi Example • How much work is done lifting a 25 kg object from the ground to a height of 2 m? • This is the work done by gravity. Since this work only depends on the initial and final positions it tells us that gravity is called a ‘conservative force’ – Conservative forces • Gravity, elastic force, magnetic force, electrical force • Non Conservative Forces – Stress, Friction, drag Practice • How much work is done (by gravity) when a 30 kg rock falls 20 m? • How much work is done by gravity when a weightlifter picks up a weight, lifts it 2 m over his head, then sets it down? Total work done • W = (Kf - Ki) + (Uf – Ui) Work Lab Lab • Design a lab to measure the work done by a force • You may use anything in the lab • One lab sheet per group • Need: materials, procedure, data, calculations, what force did the work, and how much work was done – Do not use a conservative force in such a way that you get 0 work done Homework Worksheet Warm Up • An elevator lifts a person 10 floors (about 30 m). If the man in the elevator has a mass of 80 kg, and the elevator starts at rest and on the tenth floor has a velocity of 9 m/s… • A) How much work is done bye the elevator to change the man’s speed? • B) How much work is done by the elevator to overcome the force of gravity on the man? • C) How much total work is done by the elevator on the man? Total work done • W = (Kf - Ki) + (Uf – Ui) Conservation of Mechanical Energy • Usually the work done by non conservative external force = 0 so: • Kf + Uf = Ki + Ui • or • Kf - Ki = - (Uf – Ui) Example Practice Pg 191 (P 26, 32, 46, 48) Homework Pg 191 (CQ 9, 12, 13; P 25, 40)