1. Chapter 6 Work, Machines, Energy, and Power

2. What is Work?  Work is force exerted on an object that causes the object to move some distance  Force without moving a distance yields NO WORK!! Work = Force x Distance SI Unit for work is the Joule 1 Joule = 1Newton x 1 Meter

3. Word Problems  Word problems can be confusing; but w/ some practice they’re not that bad. Here are a few hints to make them easier –1. Be sure you remember the “Need-to-Know” formulas  v =d/t ; a = V f – V i ; F = ma ; W=Fd; P = W/t Time Time  In the word problem be sure you know the units for each of the variables in the particular formula being discussed.  Distance – Meter; Force – Newton; Volume - cm 3 or Liter –2. In the word problem, all but one of the variables is told to you in one way or another. Identify what variable is being asked to solve, then plug in the remaining variables to the formula –Solve it!! Make sure you also keep track of the units

4. How much work performed:  How much work is performed if you apply 85 newtons of force on a box causing it to move 3 meters: W = Fd= 85N x 3m = 255 J  How much work is performed if you apply 37 newtons of force and move a wagon 4.3 meters? W = Fd = 37 N x 4.3 m = 159 J  How much work is performed if you apply 118 newtons of force on a car that is stuck in the mud and doesn’t move?: W = Fd = 118 N x 0 m = 0 J You might be tired from pushing but no work was done!!

5. How much force required:  How much force was required to move an object 3 meters if 75 Joules of work were expended? –Formula: Force = work /distance –Need to solve for Force, w= 75 J & D=3M F = W/d F = W/d F = 75 NM / 3M F = 75 NM / 3M F = 25N F = 25N

6. What is a Machine?  A device that makes work easier or more effective  A machine makes work easier by changing the amount of force, the distance covered or by changing the direction of the force

7. Mechanical Advantage  A machine’s mechanical advantage is the number of times a force exerted on a machine is multiplied.  Ideal and Actual Mechanical Advantage have no unit they cancel each other out when doing the math problem.  AMA = output force / input force  IMA = effort distance / resistance distance

8. Efficiency of a Machine  The amount of work obtained from a machine is always less than the amount of work put into it. This is because work is lost to friction.  Efficiency = output work / input work x 100 Remember that work = force x distance

9. Simple Machines

10. Inclined Plane  A plane is a flat surface. When that plane is inclined, or slanted, it can help you move objects across distances. And, that's work! A common inclined plane is a ramp. Lifting a heavy box onto a loading dock is much easier if you slide the box up a ramp--a simple machine. IMA = length of incline / height of incline

11. Wedge  you can use the edge of an inclined plane to push things apart. Then, the inclined plane is a wedge. So, a wedge is actually a kind of inclined plane. An axe blade is a wedge. Think of the edge of the blade. It's the edge of a smooth slanted surface.

12. Screw  an inclined plane wrapped around a cylinder  A screw can convert a rotational force (torque) to a linear force and vice versa.torque

13. Lever  Any tool that pries something loose is a lever. A lever is a rigid bar that "pivots" (or turns) against a "fulcrum" (or a fixed point). IMA = Distance from input force to fulcrum / distance from output force to fulcrum

14. 1 st Class Levers  Notice how – The input & output forces are in opposite directions –The fulcrum is between the input & output forces  Examples include nail remover, paint can opener scissors, seesaw

15. 2 nd Class Levers  Notice how: –The input & output forces are in the same direction –Input force is farther away from the fulcrum than the output force –Examples include: wheel barrow, door, nutcracker

16. 3 rd Class Lever  Notice how: –The input & output forces are in the same direction –The input force is closer to the fulcrum than the output force –Examples include rake, shovel, baseball bat and fishing pole

17. What Class of Lever? 1 7 6 4 5 3 2 8 1._______ 2. _______ 3. _______ 4. _______ 5. _______ 6. _______ 7. _______ 8. _______ 1.3 rd Class 2. 1 st Class 3. 1 st Class 4. 2 nd Class 5. 2 nd Class 6. 3 rd Class 7. 1 st Class 8. 2 nd Class

18. Wheel and Axle  two circular objects attached together about a common axis  Wheel is the large cylinder  Axle is the small cylinder IMA = Radius of the wheel / Radius of the axle

19. The Nature of Energy  Energy – the ability to do work or cause a change. –work is the transfer of energy –SI unit for energy is the same as the SI unit for work – Joule  Two main types of energy: Kinetic and Potential –Kinetic Energy: the energy of motion –Potential Energy: Energy stored for use at a later time

20. Calculating Kinetic Energy  Kinetic Energy: the energy of motion  The amount of kinetic energy depends on the objects mass and velocity  Energy is transferred during work –The more work one does on an object… –The more energy one imparts on the object  Kinetic energy = Mass x Velocity 2 2 When mass is doubled; Kinetic Energy is doubled When velocity is doubled; Kinetic Energy is quadrupled!!

21. What’s the Kinetic Energy?  What is the Kinetic Energy (in Joules) of an object with a mass of 10 kg and a velocity of 10 m/s?  When mass is doubled; Kinetic Energy is doubled  When velocity is doubled; Kinetic Energy is quadrupled!!

22. Potential Energy:  Energy stored for use at a later time  2 Types: –Elastic Potential Energy:  Energy stored in springs, bow and arrow, stretched elastic or rubber bands.  Associated w/ objects that can be stretched or compressed. –Gravitational Potential Energy:  Height and weight dependant (notice its weight, NOT mass!)  GPE = work done to lift and object to a height  GPE = Weight x Height (remember that weight = mass x 9.8 m/s2)  GPE = mass x 9.8 m/s2 x Height GPE = 100 N x 300 m = 30,000 Nm = 30,000 Joules

23. Different Forms of Energy  6 different types : –Mechanical –Thermal Energy –Chemical Energy –Electrical Energy –Electromagnetic Energy –Nuclear Energy

24. Mechanical Energy  associated w/ the motion (kinetic) or position of an object (potential)  Kinetic Energy exists whenever an object which has mass is in motion with some velocity. Everything you see moving about has kinetic energy.  Potential Energy exists whenever an object which has mass has a position within a force field. The most everyday example of this is the position of objects in the earth's gravitational field. GPE = Weight x Height

25. Thermal Energy  associated w/ the total energy of the particles (atoms and molecules) in an object. As thermal energy increases, the particles increase in speed and the thermal energy (temperature) of the object increases.

26. Chemical Energy  the energy stored in chemical bonds. The potential energy stored in compounds.

27. Electrical Energy  Moving electrical charges. Electricity!!

28. Electromagnetic Energy  Travels in waves, associated w/ light, infrared, ultraviolet, microwaves, x-rays, etc  Longer wavelength yields low frequency & low energy  Shorter wavelength yield high frequency & high energy

29. Nuclear Energy  Associated w/ the fusion or fission of nuclear atoms. The fusion of hydrogen into helium fuels the power of the sun

30. Energy Conversion and Conservation  Most forms of energy can be converted from one type to another.  Law of the Conservation of Energy - states that energy cannot be created or destroyed. It simply changes from one form into another  Einstein’s theory of Relativity - E = mc 2  a small amount of mass can be changed directly into a tremendous amount of energy  E = the energy produced  m = the mass being converted  c = the speed of light (186,000 miles/second)

31. Energy Conversion

32. Power  Power: the rate at which work is done  Power = work / time and since: –Work = force x distance….  Power = Force x Distance Time Time  SI Unit for Power is the Watt  1 Watt = 1Joule / 1 Second  Horsepower : An American unit of power –The amount of work a horse does when it lifts 33,000 pounds of coal to a height of 1 foot in 1 minute. 33,000 pounds of coal to a height of 1 foot in 1 minute. –1 horsepower = 746 watts James Watt Same amount of work was done; however there was more power in lifter B since his took less time AB

33. Power Problems W = F x D P = F x D / T P = 35 N x 10 m / 5 sec P = 350 J / 5 sec = 70 J/sec P = 70 Watts W = P x T convert 1 hour into seconds: 1 hour 60 min x 60 sec 1 hr 1 min Work = 60 watts x 3600 sec = 216,000 Joules = 216 Kilojoules