Conservation of Energy ► Energy  Work ► Kinetic and Potential Energy  Conservative and non-conservative forces  Other forms of energy

Introduction ► Forms of energy  Mechanical energy ► Focus for now ► Forms of energy  Energy can be transformed from one form to another ► Essential to the study of physics, chemistry, biology, geology, astronomy  Can be used in place of Newton’s law to solved certain problems more easily.

Work ► Provides a link between force and energy ► Work is the product of the component of the force along the direction of the displacement and the magnitude of the displacement  W=F(cos  )d –F(cos  ) is the component of the force in the direction of the displacement –d is the displacement

Work ► This gives no information about  The time it took for the displacement to occur  The velocity of acceleration of the object ► Note: work is zero when  There is no displacement (holding a bucket)  Force and displacement are perpendicular to each other (if we are carrying the bucket horizontally, gravity does not work)

More about Work ► Work is a scalar quantity ► Units of work are Nm or Joules (J) ► Work can be positive or negative  Positive if the force and the displacement are in the same direction  Negative if the force and the displacement are in the opposite direction ► Example lifting a cement block  Work done by the person ► Is positive when lifting the box ► Is negative when lowering the box

Examples of Work Calculations W=F(cos  )d Since there is no angle W=Fd=(100N)5m = 500J W=F(cos  )d =(100N)(cos30 )5m = 433J W=F(cos  )d Since the force required to lift up is equal and opposite to gravity then F=+mg so W=+mgd W=(15kg)(9.81m/s 2 )5m W= 735J

Example 4 ► A 10-N forces is applied to push a block across a friction free surface for a displacement of 5.0 m to the right. Since F app is the only horizontal force, it is the only force that does work W = Fd = (10N)(5.0m) = 50J

Example 5 ► A 10-N force is applied to push a block across a frictional surface at constant speed for a displacement of 5.0 m to the right Since the object moves horizontally, only horizontally forces will do work W app = F app d W = 10N 5.0 m = 50 J W frct = F f d = -10N(5.0 m) = -50J

Graphing Work ► A graph of force exerted over a displacement can be used to determine work. Since Work = Force x displacement and Area = length x width. If the axes on a graph are force and distance then the area under the line will be equivalent to work done. Find the work done over the 10 m displacement. Area = work, there are 3 distinct areas under the line the sum will equal total work done. Area = ½ bh + lw + ½ bh = ½ 3m(20N) + 5m(20N) + ½ 2m (20N) = 30 J J + 20 J = 150J Work done is 150 J

Assignment 1 ► Do questions 1 – 7 in workbook