PHYSCIS Southern Boone County HS ENERGY, WORK, and SIMPLE MACHINES Chapter 10 Bill Palmer

Physics- Energy and Work  Energy-the ability to produce a change  Thermal  Chemical  Motion

Physics- Energy and Work  Work-The process of changing the energy of a system  Work = Force x distance  W = Fd  Unit of Work is a joule

Physics- Energy and Work  A 105g hockey puck is sliding across the ice. A player exerts a constant 4.5 N force over a distance of 0.15m. How much work does the player do on the puck? What is the change in the pucks energy?

Physics- Energy and Work  Known  M = 105 g  F = 4.5 N  D= 0.15  Unknown  W = ?  Delta K = ?

Physics- Energy and Work  W = Fd = (4.5 N) (0.15 m) =.68 N m =.68 j (joules)  K = W = 0.68 j

Physics- Energy and Work  Do Practice Problems 1-5, page 227

Physics- Energy and Work  Constant Force at an Angle  Use trigonometric functions to solve

Physics- Energy and Work  A sailor pulls a boat 30.0 m along a dock using a rope that makes a angle with the horizontal. How much work does the sailor do on the boat if he exerts a force of 255 N on the rope?

Physics- Energy and Work  Known  F = 255 N  D = 30.0 m   =

Physics- Energy and Work  W = F d cos   W = (255 N) (30.0 m) (cos )  W = 6.9 X 10 3 j

Physics- Energy and Work  Do practice problems 6-8, page 229

Physics- Energy and Work  The concept of POWER includes the amount of time it takes to do the work.  For example, two people carry 100 lbs of horse feed from the car to the stable. One of them does it in 30 sec. the other does it in two min. Who had the most power?

Physics- Energy and Work  The concept of POWER includes the amount of time it takes to do the work.  For example, two people carry 100 lbs of horse feed from the car to the stable. One of them does it in 30 sec. the other does it in 2 min. (120 sec) Who had the most power?  The first guy. By how much? 4 times the power! 120/30 = 4

Physics- Energy and Work  Power is measured in watts  A watt is one joule in one second

Physics- Energy and Work  Formula P = W/t  Power equals work divided by time  W = Fd  P = Fd/t  Power equals force times distance divided by time

Physics- Energy and Work  An electric motor lifts an elevator 9.00 m in 15 s by exerting a force of 1.2 x 10 4 N. What power does the motor produce?  Known:  D = 9.0 m  t = 15 s  F= 1.2 x 10 4 N

Physics- Energy and Work  W=Fd  P=w/t  So, P= Fd/t  P = (1.2 x 10 4 N) (9.0 m) 15.0 s = 7.20 kW

Physics- Energy and Work  Do problems on page 231  Examples on board

Physics- Energy and Work  Cool Homework assignment Due Monday Oct 22 Pick any 10 problems from 19 – 47 and complete. Pages 242, 243, 244  If you pick the first 10 that is the easiest, last 10 hardest, random 10 in the middle.  Have Fun

Physics- Energy and Work  Mechanical Advantage (actual)  The ratio of the resistance force to the effort force.  MA = F r / F e

Physics- Energy and Work  Work is always the same.  So, output work equals input work  W o = W i  Work = Force X Distance  So,  F r D r = F e D e

Physics- Energy and Work  Ideal Mechanical Advantage  IMA = d e /d r  But, in real life some work is lost due to heat, friction, etc.  Efficiency ratio of output work to input work  Efficiency (%) = W o / W i X 100

Physics- Energy and Work  Efficiency (%) = MA / IMA X 100  Work together sample problem on page 237  Complete in class problems page 238  Complete as Homework problems page 244