Work and Simple Machines

1. Work and Simple Machines

2. What is work? • What do you think of when you hear the word WORK? • The scientific definition of work is: exerting a force to move an object a distance • both the force and the motion of the object are in the same direction

3. Work is done on an object when the object moves in the same direction in which the force is exerted.

4. Work or Not? • According to the scientific definition, Which of the following is work? • a teacher lecturing to her class • a mouse pushing a piece of cheese with its nose across the floor

5. Work or Not? • a mouse pushing a piece of cheese with its nose across the floor

7. What’s work? • A scientist delivers a speech to an audience of his peers. • A body builder lifts 350 pounds above his head. • A mother carries her baby from room to room. • A father pushes a baby in a carriage. • A woman carries a 20 kg grocery bag to her car?

8. What’s work? • A scientist delivers a speech to an audience of his peers. No • A body builder lifts 350 pounds above his head. Yes • A mother carries her baby from room to room. No • A father pushes a baby in a carriage. Yes • A woman carries a 20 km grocery bag to her car? No

9. 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

10. Machines

11. Simple Machines • A machine is a device that helps make work easier to perform by accomplishing one or more of the following functions: • changing the direction of a force • increasing the amount of a force • increasing the distance the force is applied

12. Mechanical Advantage • Machines have both an input forceand an outputforce • Input force: the force you apply to the machine • Output force: force the machine applies to an object • When a machine takes a small input force and increases the amount of the output force, a mechanical advantagehas been produced.

13. Input and Output Work • The amount of input work done by the gardener equals the amount of output work done by the shovel.

14. Mechanical Advantage • Mechanical advantage is the ratio of output force divided by input force. • If the output force is bigger than the input force, a machine has a mechanical advantage greater than one. • A machine that increases distance instead of force has a smaller input force & a greater output force • Machines can not increase both the size and the distance of a force at the same time.

15. A machine makes work easier by changing at least one of three factors. • change the amount of force you exert • Changing the distance over which you exert your force • or changing the direction in which you exert your force.

16. The 6 Simple Machines Screw Wedge Inclined Plane Pulley Wheel and Axle Lever

17. Inclined Plane • An inclined plane is a flat, sloped surface.

18. Inclined Planes

19. Inclined Plane -Mechanical Advantage • The mechanical advantage of an inclined plane is equal to the length of the slope divided by the height of the inclined plane. • While the inclined plane produces a mechanical advantage, it does so by increasing the distance through which the force must move.

20. Wedge • A wedge is a device that is thick at one end and tapers to a thin edge at the other end.

21. Wedges

22. Wedge – Mechanical Advantage • The mechanical advantage of a wedge can be found by dividing the length of either slope (S) by the thickness (T) of the big end. S • As an example, assume that the length of the slope is 10 inches and the thickness is 4 inches. The mechanical advantage is equal to 10/4 or 2 1/2. As with the inclined plane, the mechanical advantage gained by using a wedge requires a corresponding increase in distance. T

23. Screws • A screw can be thought of as an inclined plane wrapped around a cylinder.

24. Screw The mechanical advantage of an screw can be calculated by dividing the circumference by the pitch of the screw. Pitch equals 1/ number of turns per inch.

25. Levers • A lever is a ridged bar that is free to pivot, or rotate, on a fixed point.

26. Levers • Levers are classified according to the location of the fulcrum relative to the input and output forces.

27. First Class Lever • In a first-class lever the fulcrum is located at some point between the effort and resistance forces. • Common examples of first-class levers include crowbars, scissors, pliers, tin snips and seesaws. • A first-class lever always changes the direction of force (I.e. a downward effort force on the lever results in an upward movement of the resistance force).

28. First Class Lever

29. Second Class Lever • With a second-class lever, the load is located between the fulcrum and the effort force. • Common examples of second-class levers include nut crackers, wheel barrows, doors, and bottle openers. • A second-class lever does not change the direction of force. When the fulcrum is located closer to the load than to the effort force, an increase in force (mechanical advantage) results.

30. Second Class Lever

31. Third Class Lever • With a third-class lever, the effort force is applied between the fulcrum and the resistance force. • Examples of third-class levers include tweezers, hammers, and shovels. • A third-class lever does not change the direction of force; third-class levers always produce a gain in speed and distance and a corresponding decrease in force.

32. Third Class Lever

33. Wheel and Axle • A wheel and axle is a simple machine made of two circular or cylindrical objects fastened together that rotate about a common axis.

34. Wheel and Axle • You can find the ideal mechanical advantage of a wheel and axle by dividing the radius of the wheel by the radius of the axle.

35. Wheel and Axel • The mechanical advantage of a wheel and axle is the ratio of the radius of the wheel to the radius of the axle. • In the wheel and axle illustrated above, the radius of the wheel is five times larger than the radius of the axle. Therefore, the mechanical advantage is 5:1 or 5. • The wheel and axle can also increase speed by applying the input force to the axle rather than a wheel. This increase is computed like mechanical advantage. This combination would increase the speed 5 times. 5 1

36. Pulley • A pulley is a simple machine made of a grooved wheel with a rope or cable wrapped around it.

37. Pulleys • Fixed Pulley: changes the direction of a force; however, it does not create a mechanical advantage. • Movable Pulley: rises and falls with the load that is being moved. A single moveable pulley creates a mechanical advantage; however, it does not change the direction of a force. • Block & Tackle Pulley: a system of two or more pulleys with a rope or cable threaded between them, usually used to lift or pull heavy loads.

39. Simple Machines in the Body • Most of the machines in your body are levers that consist of bones and muscles.

40. Compound Machines • A compound machine is a machine that utilizes two or more simple machines.

41. Rube Goldberg Machines • Rube Goldberg machines are examples of complex machines. • All complex machines are made up of combinations of simple machines. • Rube Goldberg machines are usually a complicated combination of simple machines. • By studying the components of Rube Goldberg machines, we learn more about simple machines

42. Safety Device for Walking on Icy Pavements When you slip on ice, your foot kicks paddle (A), lowering finger (B), snapping turtle (C) extends neck to bite finger, opening ice tongs (D) and dropping pillow (E), thus allowing you to fall on something soft.