1. Energy and Work Part 2

2. What is Work Work is the transfer of energy through motion –When force acts over a distance in the direction of an object’s motion W = F x D W = Work F = Force D = Distance W = Work F = Force D = Distance Measured in Joules (J) J = N x m Measured in Joules (J) J = N x m

3. Example A dancer lifts a 400 N ballerina overhead a distance of 1.4 m and holds her there for several seconds. How much work is done during the lift? W = F x D F = 400 N D = 1.4 m W = 400N x 1.4m W= 560 J or 560 N x m How much work was being done while the ballerina was overhead?

4. Why was no work being done while the ballerina was overhead? Work – transfer of energy through motion If the ballerina is not moving what type of energy does she represent –Potential What if she was dropped. What type of energy would she represent? –Kinetic

5. Conservation of Energy Law of Conservation of Energy –Energy may change form but it cannot be created nor destroyed under ordinary conditions

6. Calculating Kinetic Energy Kinetic Energy - KE =1/2 x M x V 2 –Example: Determine the kinetic evergy of a 25 kg roller coaster car that is moving with a speed of 18.3 m/s KE = ½ x 25 kg x 18.3 2 KE = 4186 Joules

7. Calculating Potential Energy Potential Energy – PE grav =mgh –m = mass –g = gravitational acceleration –h =height –A 4 kg book is placed on a shelf 3 meters above the ground. What is the potential energy of this book? PE grav = 4kg x 9.8 m/s 2 x 3 m PE grav = 117.6 Joules

8. Mechanical Energy Mechanical Energy – is the total amount of kinetic and potential energy in a system

9. Mechanical Energy A B C D E At what points do you have the greatest potential energy? Answer: A and E At what point do you have increasing/decreasing potential energy? Answer: B (decreasing) and D (increasing) At what point do you have the greatest kinetic energy? Answer: C At what point do you have increasing/decreasing kinetic energy? Answer: B (increasing) and D (decreasing)

10. Power The rate at which work is done. Measure of the amount of work done in a certain amount of time. –Calculating Power: power = Work/Time p = W/t

11. Calculating Power The units for power are Watts –Watts = 1 joule per second Work = Joule Time = second –Hence Watts = Joule/Second –Example: A figure skater lifts his partner, who weighs 450 N, 1.0 m in 3.0 s. How much power is required?

12. Solution What is known? F = 450 N F = 450 N D = 1.0 m D = 1.0 m T = 3.0 s T = 3.0 s What is not known? W = ? W = ? W = f x d W = 450 N x 1.0 m W = 450 J Now you can calculate power: p = 450 J/3.0 s p = 150 W