Chapter 9

Chapter 9

1. Work

   • product of the net force on an object
   • distance object moves
   • force acts on object, object moves in direction of force
   • work=net force*distance
     • W=Fd
   • work done against another force
   • change the speed of an object
   • unit
     • combines N with m
       • Joules/J

2. Mechanical Energy

   • energy
     • Prroperty of an object/system that allows it to do work
       • J
   • Energy due to position/movement of something
   • Kinetic & Potential

3. Potential Energy (PE)

   • energy that is stored & held in readiness
     • elastic
       • stretched rubber band
         • potential b/c of position
     • chemical
       • Any substance that can do work through chemical reactions has chemical PE
     • gravitational
       • potential energy due to elevated positions
         • PE=mgh

4. Kinetic Energy (KE)

   • All moving objects
     • Equal to the work required to bring it to speed from rest or vice versa (for moving objects)
       • force*distance=KE
   • depends on mass and speed
     • KE=1/2mv^2

5. Work-energy theorem

   • describes the relationship between work and energy
   • whenever work is done, energy changes.
     • work=ΔKE
   • emphasizes the role of change
     • no change in KE, no work

6. law of conservation of energy

   • energy cannot be created or destroyed.
     • Can be transformed from one form into another
     • total amt. never changes

7. Machines

   • device for multiplying or changing direction of forces
   • transfers energy from one place to another or transforms it from one form to another
   • levers
     • bar that turns at a fixed point
       • We do work on one end of the lever, the other end does work on the load.
         • work input=work output
           • (force*distance)input =(force*distance)output
   • ratio of output force to input force
     • mechanical advantage
   • Pulley
     • a kind of lever that can be used to change the direction of a force

8. Efficiency

   • In all machines, some energy is transformed into KE
     • This makes the machine warmer
   • ratio of useful energy output to total energy input
     • percentage of the work input that is converted to work output
     • useful work output/total work input