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 = KE 2 - KE 1

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 = mgh f – mgh i

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 = (K f - K i ) + (U f – U i )

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 = (K f - K i ) + (U f – U i )

Conservation of Mechanical Energy Usually the work done by non - conservative external force = 0 so: K f + U f = K i + U i or K f - K i = - (U f – U i )

Example

Practice Pg 191 (P 26, 32, 46, 48)

Homework Pg 191 (CQ 9, 12, 13; P 25, 40)