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

What is work? In science, work is using force to move an object a distance (both force and motion are in same direction.) http://web.pacific.edu/Images/school-engineering/physics.jpg

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

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

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

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.

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 http://www.explainthatstuff.com/howpulleyswork2.gif

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 http://www.iq.poquoson.org/dollaraire/8sci/wkmach3.gif http://www.explainthatstuff.com/howpulleyswork2.gif

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? http://www.gearseds.com/images/upload/thumbnail_trebuchert_as_lever_system.jpg

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.

Simple Machines Six simple machines are: Lever Wheel and Axle Pulley Inclined Plane Wedge Screw http://www.avon.k12.ma.us/librarymedia/Simple4.jpg

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

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

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 http://library.thinkquest.org/J002079F/images/class_1_seesaw_levers.jpg

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. http://learn.uci.edu/media/OC08/11004/OC0811004_L6Pushup.jpg

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 http://www.vectorsite.net/tpecp_03_03.jpg

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!!!!!

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.

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

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

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.

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

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)

Calculate MA of these pulleys

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)

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

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

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.

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 http://www.ehow.com/how_6544271_calculate-mechanical-advantage-wedge.html

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

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 http://homepages.cae.wisc.edu/~me231/online_notes/dimensoning/fastener_handout.pdf

Reading thread info http://homepages.cae.wisc.edu/~me231/online_notes/dimensoning/fastener_handout.pdf

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.  .625 -11NC means Diameter = .625 in. 11 threads / inch; national coarse (standard) threads 33

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) http://www.outdoorlife.com/articles/atvs/2009/11/5-quick-tips

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.

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#?

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.

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

Screw annotation example 2 Example: .88-14UNF-2B .88 = Major diameter of thread. 14 = Threads per inch. UN = Unified threads. F = Fine thread. 2 = Class 2 fit. B = Internal thread.   www.craftsmanspace.com/knowledge/bolt-and-screws-ansi.html http://www.product-category.com/p/2h_nuts_manufacturers.html

Screw annotation example 3 Example: .375-32UNEF-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. http://www.craftsmanspace.com/knowledge/bolt-and-screws-ansi.html