Ch 11 ENERGY and its CONSERVATION. work causes a change in the energy of a system KE (an increase or decrease in KE) ket.

Transcription

1 Ch 11 ENERGY and its CONSERVATION 11.1 The Many Forms of Energy work causes a change in the energy of a system W = KE (an increase or decrease in KE) work energy theorem object + work object work increase in KE decrease in KE ket. 30 9:43 AM energy can be stored when energy is spent, it affects the motion of a system example: throwing vs catching a ball throwing: you exert force to throw the ball (do work) you put work into the system this increases the KE of the ball 0 KE + work = KE catching: ball is moving (already has KE) you exert a force in the opposite direction of the ball's motion ( Work) this decreases the KE of the ball initial KE work = 0 KE ket. 30 9:47 AM 1

2 Kinetic Energy 1 KE = mv Work = KE If a 50 kg car is traveling at 15 m/s and comes to a stop, what work is done on the car? ket :35 AM Kinetic Energy 1 KE = mv Work = KE Bill weighs 530 N. What is his kinetic energy as he swims at a constant speed, covering a distance of 7m in 1.0 min? ket :38 AM

3 Potential Energy : "stored" energy, or energy of position Examples: height above ground stretched rubber band compressed spring in chemical bonds (food, gasoline) stored energy is made useful when it changes forms and causes motion ket :40 AM PE: gravitational potential energy ex: tossing a ball into the air PE = mgh B h At A: KE upward is max PE is 0 At B: KE = 0 (v = 0) PE is max At C: KE downward is max magnitude is the same direction is different height PE is 0 A max v h= 0 max v C ket :43 AM 3

4 gravity is ALWAYS working on an object as it moves if the object moves away from earth, energy is stored in the object due to the gravitational force between the object and earth ket :03 AM work can also be a change in potential energy W = PE PE = mgh How much work do you do if you lift a 4 kg mass from the ground to above your head, a height of.5 meters? How much work do you do if you lower a 4 kg mass from.5 m to the ground? ket :06 AM 4

5 What is the change in gravitational potential energy when a 7.0 x 10 kg elevator moves from the second floor (6.0m above the street) to the top floor (5.0m above the street) of a building? ket :1 AM 1 KE = mv Work = KE PE PE = mgh A 14,700 N car is traveling at 5 m/s. The brakes are applied suddenly and the car slides to a stop. The average braking force between the tires and the road is 7100N. How far will the car slide once the brakes are applied? ket :31 AM 5

6 Bob can consistently throw a 0.00 kg ball at a speed of 1.0 m/s. On one such throw, Bob throws the ball straight upward and it passes the top of a flagpole when it is 6.00m above the ball's initial position. a) what is the ball's gravitational potential energy when it passes the top of the flagpole? top of flagpole b) what is the ball's kinetic energy when it is released from Bob's hand? Bob's hand (ball's initial position) ket :30 AM 11. Conservation of energy no objects enter or leave the system no external forces act on system Law of Conservation of Energy: in a closed, isolated system, energy is neither created nor destroyed, energy is conserved. energy can change forms, but the total energy of the system remains constant mechanical energy: sum of the KE and PE of a system Apr 3 9:4 AM 6

7 ball has a weight of 10N m 1 m Known mg = 10N h 1 = m h = 1m h 3 = 0m we do NOT know any velocities, so we can't solve for KE directly 0 m Apr 3 9:3 AM the path an object takes does NOT affect the final KE (in theory) if h = 0, PE = 0 PE= mgh final KE = initial PE for both paths yay! we don't have to worry about angles!! of course, there is usually friction that affects stuff, but we can ignore it unless it is mentioned in the problem. Apr 3 9:44 AM 7

8 Roller coaster has weight of mg PE = mgh PE 1 = mgh 1 h h 1 h = 0 so PE = 0, all the PE has converted to KE Apr 3 9:49 AM PE = mgh Roller coaster has weight of mg this energy difference is still KE when you get to the top of the next hill, so there is still a velocity for the roller coaster PE 1 = mgh 1 h h 1 h = 0 so PE = 0, all the PE has converted to KE Apr 3 9:49 AM 8