Work and Simple Machines

1. Work and Simple Machines

2. What is work? • In science, the word work has a different meaning than you may be familiar with. • The scientific definition of work is: using a force to move an object a distance (when both the force and the motion of the object are in the same direction.)

3. Work or Not? • According to the scientific definition, what is work and what is not? • a teacher lecturing to his/her class • a mouse pushing a piece of cheese with its nose across the floor

4. WORK

5. Formula for work Work = Force x Distance • The unit of force is Newtons • The unit of distance is meters • The unit of work is Newton-meters • One Newton-meter is equal to one Joule • So, the unit of work is a Joule

6. W=FD Work = Force x Distance Calculate: If a man pushes a concrete block 10 meters with a force of 20 N, how much work has he done?

7. W=F * D Work = Force x Distance Calculate: If a man pushes a concrete block 10 meters with a force of 20 N, how much work has he done? Work = F X D = 20N*10m 200Nm=200 joules

8. History of Work Before engines and motors were invented, people had to do things like lifting or pushing heavy loads by hand. Using an animal could help, but what they really needed were some clever ways to either make work easier or faster.

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

10. Simple Machines • The six simple machines are: • Lever • Wheel and Axle • Pulley • Inclined Plane • Wedge • Screw

11. Simple Machines • A machine is a device that helps make work easier by accomplishing one or more of the following functions: • Increasing the magnitude of a force • Increasing the distance of a force • Increasing the speed of a force • Changing the direction of a force • Transferring a force from one place to another

12. Mechanical Advantage • Input force (the force you apply) • Outputforce (force which the machine applies to the task). • When a machine takes a small input force and increases the magnitude of the output force, a Mechanical Advantage has been produced.

13. Ideal Mechanical Advantage • Friction is ignored when calculating IMA. • IMA > 1 means it increases force • Each machine calculates IMA differently • As we cover each machine, put the IMA formula on the grid sheet for that machine

14. Actual Mechanical Advantage • AMA is the ratio of output force/input force (R/E). • If an input force of 20 newtons and the output force of 100 newtons, the machine has an Actual Mechanical Advantage (AMA) of 5. • AMA = R / E • Formula is the SAMEfor all machines • Friction decreases the AMA.

15. There is no such thing as a free lunch No machine can increase both the magnitude and the distance of a force at the same time.

16. The Lever • A lever is a rigid bar that rotates around a fixed point called the Fulcrum. • Effort Force supplied by you • Resistance Force: force supplied by the machine to move something • There are 3 Classes of levers

17. The 3 Classes of Levers • The class of a lever is determined by the location of the effort, resistance and fulcrum.

18. Types of Levers • Note the location of the F, E and R for each type of lever • For all levers the following formulas are correct • IMA = LE/LR = divide the length of the Effort arm by the length of the Resistance arm (both measured from force to the fulcrum) • AMA = Resistance / Effort .

19. First Class Lever • The fulcrum is located at some point between the effort and resistance forces. • Common examples of first-class levers include see-saw, crowbar, scissors, pliers, tin snips • A first-class lever ALWAYS changes the direction of force .

20. FIRST CLASS LEVERSThe Fulcrum is between E and R . If F is closer to R the Effort moves farther than Resistance, multiplies E and changes its direction

21. SECOND CLASS LEVERS • the Resistance is located between the fulcrum and the Effort. • Common examples of second-class levers include nut crackers, wheel barrows and bottle openers. • Advantage: Always increases the force • Never changes the direction of force.

22. R is between fulcrum and E. Effort moves farther than Resistance. Multiplies force, but NEVER changes its direction

23. Effort is between Fulcrum and Resistance Cannot increase the force Resistance moves farther than Effort. Multiplies the distance or speed that the effort force travels

24. Third Class Lever • Effort is applied between the Fulcrum and the Resistance. • Examples of third-class lever: tweezers, a rake and your arm • Never changes the direction of the force • Always produces a gain in speed and distance • Always DECREASES the force

25. Wheel and Axle • The wheel and axle is a large wheel rigidly secured to a smaller wheel or shaft, called an axle. • When the wheel or axle is turned, the other part also turns. One full revolution of either part causes one full revolution of the other .

26. Wheel and Axle • IMA = • Radius(wheel) _____________ Radius(axle) • AMA = R/E

27. Pulleys • Can change the direction of a force • or • Can gain a Mechanical Advantage depending on how the pulley(s) is(are) arranged.

28. Fixed Pulley • Fixed pulley :if it does not rise or fall with the load being moved. A fixed pulley changes the direction of a force; however, it does not increase the force.

29. Moveable Pulley • A Moveable pulleyrises and falls with the load that is being moved. A single moveable pulley creates an IMA of 2. It does not change the direction of a force. • The IMA of a moveable pulley is equal to the number of ropes that support the moveable pulley. Pulling down strand does not count.

30. Compound Pulley systems • Composed of at least 1 fixed and 1 moveable pulley linked • IMA = # of supporting strands. • PULLING DOWN DOES NOT COUNT AS A SUPPORTING STRAND.

31. Inclined Plane • An inclined plane is an even sloping surface. A Ramp. • The inclined plane makes it easier to move a weight from a lower to higher elevation. • IMA = Run/Rise • IMA = Effort Distance/ Resistance Distance

32. Inclined Plane • The IMA of an inclined plane is equal to the length of the slope divided by the height of the inclined plane. • IMA(Slope) = run/rise • Mechanical Advantage, is derived by increasing the effort distance through which the force must move.

33. Wedge

34. Screw • The screw is also a modified version of the inclined plane. • While this may be somewhat difficult to visualize, it may help to think of the threads of the screw as a type of circular ramp (or inclined plane).

35. IMA of a SCREW

36. INPUT WORK = Effort X Distance the Effort moved • IW = E x DE • DE is not the same as LE. LE is measured from fulcrum to the E. DE is measured along the direction of movement. • OUTPUT WORK = Resistance X Distance the Resistance moved • OW = R x DR • DR is not the same as LR. LR is measured from fulcrum to the R. DR is measured along the direction of movement.

37. Efficiency Some output force is always lost due to friction. • Efficiency=(Output Work/Input Work)*100 • Formula for Efficiency is the same for ALL machines • No machine has 100% efficiency due to friction.

38. Theoretical Effort (TE)

39. Calculations related to Simple Machines KEY

40. 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? 3. Using a single fixed pulley, how heavy a load could you lift?

41. Practice Questions 4. Give an example of a machine in which friction is both an advantage and a disadvantage. 5. Why is it not possible to have a machine with 100% efficiency? 6. What is effort force? What is work input? Explain the relationship between effort force, effort distance, and work input.

42. Practice Question answers 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. Work is defined as a force applied to an object, moving that object a distance in the direction of the applied force. The bricklayer is doing more work. 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 = 7 m X 50 N X 2 = 700 N-m or J 3. Using a single fixed pulley, how heavy a load could you lift? Since a fixed pulley has a mechanical advantage of one, it will only change the direction of the force applied to it. You would be able to lift a load equal to your own weight, minus the negative effects of friction.

43. Practice Question answers 4. Give an example of a machine in which friction is both an advantage and a disadvantage. One answer might be the use of a car jack. Advantage of friction: It allows a car to be raised to a desired height without slipping. Disadvantage of friction: It reduces efficiency. 5. Why is it not possible to have a machine with 100% efficiency? Friction lowers the efficiency of a machine. Work output is always less than work input, so an actual machine cannot be 100% efficient. 6. What is effort force? What is input work? Explain the relationship between effort force, effort distance, and input work. The effort force (E) is the force applied to a machine. Input work (IW) is the work done on a machine. The input work (IW) of a machine is equal to the effort force (E) times the distance (DE) over which the effort force is exerted.