1. Work and Simple Machines
education.jlab.org/jsat/powerpoint/work_and_simple_machines.ppt
education.jlab.org/jsat/powerpoint/work_and_simple_machines.ppt

2. What is work?
In science, work is using force to move an object a distance (both force and motion are in same direction.)

3. Work or Not? According to scientific definition, what is work?
teacher lecturing to class
mouse pushing cheese with its nose across floor

4. Formula for work Work = Force x Distance
Force units: Newtons or pounds
Distance units: meters or feet
Work units: Nm or ft . lb
One newton-meter = one joule
Thus another unit of work is a joule

5. Simple Machines
Ancient people invented simple machines to help overcome resistive forces and allow them to do work against those forces.

6. Simple Machines
Machine: device that helps make work easier to perform by:
transferring force from one
place to another
changing direction of force
increasing magnitude of force
increasing distance or speed of force.

7. Mechanical Advantage
Machines operate using input force (EFFORT, force you apply) and output force (LOAD or RESISTANCE force -- machine uses to move something).
Mechanical advantage --machine takes small input force and increases magnitude of output force

8. Mechanical Advantage Equations
Calculated by:
(1) dividing input distance by output distance (Ideal MA).
IMA = Din / Dout or
IMA = DE / DR
(2) using ratio of output force divided by input force (Actual MA)
AMA = Fout / Fin, or
AMA = FR/ FE

9. Mechanical Advantage Equations (cont.)
If output force > input force, MA >1
Solve: Machine increases 10 pound input force to 100 pound output force. (MA) = ________.
Suppose the IMA = 8, and you move a force of 12#. What is ideal effort force?

10. Mechanical Advantage
NOTE: No machine can increase both the magnitude and the distance of a force at the same time. What you win on the force, you lose on the distance.

11. Simple Machines Six simple machines are: Lever Wheel and Axle Pulley
Inclined Plane
Wedge
Screw

12. Lever
Lever -- rigid bar that rotates around fixed point called fulcrum
In use, lever has both effort (applied) force and load (resistance force).

13. 3 Classes of Levers
Class of lever determined by location of effort force and load relative to fulcrum.
1, 2, 3 F, R, E

14. First Class Lever
Fulcrum (F) between effort (E) and resistance (R); i.e., fulcrum is in middle of 1st class lever.
First-class lever always changes direction of force (i.e., downward effort force on lever results in upward movement of resistance force).
Common examples: crowbars, scissors, pliers, tin snips, seesaws.
MA <1, =1, >1 possible

15. Second Class Lever
Load (resistance R) in middle between fulcrum (F) and effort (E).
Common examples: nut crackers, wheel barrows, doors, bottle openers.
Does not change direction of force.
Fulcrum always closer to load than to effort force; mechanical advantage >1.

16. Third Class Lever
Effort between fulcrum and resistance (effort in middle).
Examples: tweezers, hammers, shovels, arm.
Does not change direction of force
Allow for precision, gain in speed and distance with corresponding decrease in force.
MA < 1

17. ALWAYS MEASURED FROM THE FULCRUM!!!!!
M.A. of Lever
To find MA of lever, divide output force by input force (Fout/Fin), or divide length of effort arm by length of resistance arm (Din/Dout).
LEVER ARM LENGTH IS
ALWAYS MEASURED FROM THE FULCRUM!!!!!

18. Wheel and Axle
Wheel and axle -- large wheel rigidly secured to smaller wheel or shaft, called axle
When either wheel or axle turns, other part also turns. One full revolution of either part causes one full revolution of other part.

19. MA in wheel and axle systems
IMA = radius of effort
radius of load

20. MA in wheel and axle systems
Applying force to axle increases speed and distance moved by wheel
Applying force to wheel increases accuracy and reduces input force

21. Pulley Pulley -- grooved wheel that turns freely on axle
Can simply change direction of force, or can increase mechanical advantage, depending on how pulley system is arranged.

22. Types of Pulleys
Single fixed pulley: changes direction of force, but does not increase MA
(MA = 1)
Movable pulleys increase MA. Direction of pull and direction load moves are same
MA > 1

23. Types of Pulleys
MA of pulleys can be estimated by counting number of strands of rope holding the weight (load)
Strand holding load counts only if direction load moves is in same direction as force applied (up and up, for instance)

24. Calculate MA of these pulleys

25. Inclined Plane
Wagon trail or road on steep hill traverses back and forth. Reduces slope angle, increases distance
Sloping flat surface. Move weight from lower to higher elevation. (Rolling something down does not give MA)

26. Mechanical Advantage of Inclined Plane
IMA = length of slope / height of ramp
or hypotenuse / vertical side of triangle
AMA = force used to move object up ramp
weight of object
Small force applied
over long distance gives
large MA

27. Takes less force for car A to get to top of ramp, but all the cars do same amount of work. (What you win on force, you lose on distance, remember?)
A B C

28. Wedge
Wedge -- modified inclined plane. Used to separate or hold objects.
Consists of 1 or 2 inclined planes. Double wedge (two inclined planes) join together with sloping surfaces outward.

29. Then calculate MA for the double wedge
IMA = length of either slope (L) / thickness (T) of big end.
IMA = L / T
Calculate MA for single wedge if hypotenuse = 10 in. and vertical = 3 in.
Then calculate MA for the double wedge

30. Screw Screw – modified inclined plane.
Threads of screw type of spiraling ramp (or inclined plane).

31. MA Calculation for screws
MA of a screw: divide circumference of screw by 1/ number of threads per inch (pitch).
Gentler pitch (i.e. finer threads) means the screw is easier to move, but a lot of turns are needed

32. Reading thread info

33. Screw size annotations
Unified Annotations - Screw thread specified by giving major diameter, number of threads per inch, class of fit, and sometimes external (A) or internal (B). If thread is to be left-hand, letters LH after class symbol. Size of thread may be given on drawing by using either fractional-inch sizes or decimal-inch sizes.
  NC means Diameter = .625 in.
11 threads / inch; national coarse (standard) threads 33

34. Bolt and wrench MA question
Calculate MA of a bolt and wrench system if a wrench with 8 in. handle turns a
.75-20UNC-2A bolt using 12 lb ft. torque.
P d (wrench) / pitch (screw)
(Hint: this type questions has
showed up on a core test)

35. Efficiency In ideal conditions
Input Force x Distance = Output Force x Distance
However, some output force always lost due to friction.
Comparison of work output to work input (Wout / Win)
is efficiency.
No machine has 100 percent efficiency due to friction.
Some useful work input is always wasted and shows up as heat.

36. Practice Questions
1. Explain who is doing more work and why: a bricklayer carrying bricks and placing them on the wall of a building being constructed, or a project supervisor observing and recording the progress of the workers from an observation booth.
2. How much work is done in pushing an object 7.0 m across a floor with a force of 50 N and then pushing it back to its original position? How much power is used if this work is done in 20 sec? (Work / time = power)
3. Using a single fixed pulley, how heavy a load could you lift if you exerted a force of 75#?

37. Practice Questions
4. Give an example of a machine in which friction is both an advantage and a disadvantage. Indicate why/where friction is needed and why/where it is problematic.
5. Power is described as work / time. If a man pulls on an 8-rope block and tackle to lift a 900-lb. engine 4 ft. in 48 seconds, how much power does he use (in watts)? If the engine were lifted in 32 seconds, how much power is used? What affect does power have on amount of work done?
6. What is effort force? What is work input? Explain the relationship between effort force, effort distance, and work input.

38. Screw annotation example 1
Example: UNC-2A
.75” = Major diameter of the thread.
10 = Threads per inch.
UN = Unified threads.
C = Coarse thread.
2 = Class 2 fit.
A = External thread.

39. Screw annotation example 2
Example: UNF-2B
.88 = Major diameter of thread.
14 = Threads per inch.
UN = Unified threads.
F = Fine thread.
2 = Class 2 fit.
B = Internal thread.

40. Screw annotation example 3
Example: UNEF-4A-LH
.375 = Major diameter of the thread.
32 = Threads per inch.
UN = Unified threads.
EF = Extra Fine thread.
4 = Class 4 fit.
A = External thread.
LH = Left-hand thread.