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 her class • a mouse pushing a piece of cheese with its nose across the floor

5. 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?

6. 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

7. 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

8. 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?

9. 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? 200 joules (W = 20N x 10m)

10. Power • Power is the rate at which work is done. • Power = Work*/Time *(force x distance) • The unit of power is the watt.

11. Check for Understanding 1.Two physics students, Ben and Bonnie, are in the weightlifting room. Bonnie lifts the 50 kg barbell over her head (approximately .60 m) 10 times in one minute; Ben lifts the 50 kg barbell the same distance over his head 10 times in 10 seconds. Which student does the most work? Which student delivers the most power? Explain your answers.

12. Ben and Bonnie do the same amount of work; they apply the same force to lift the same barbell the same distance above their heads. Yet, Ben is the most powerful since he does the same work in less time. Power and time are inversely proportional.

13. 2. How much power will it take to move a 10 kg mass at an acceleration of 2 m/s/s a distance of 10 meters in 5 seconds? This problem requires you to use the formulas for force, work, and power all in the correct order. Force=Mass x Acceleration Work=Force x Distance Power = Work/Time

14. 2. How much power will it take to move a 10 kg mass at an acceleration of 2 m/s/s a distance of 10 meters in 5 seconds? This problem requires you to use the formulas for force, work, and power all in the correct order. Force=Mass x Acceleration Force=10 x 2 Force=20 N Work=Force x Distance Work = 20 x 10 Work = 200 Joules Power = Work/Time Power = 200/5 Power = 40 watts

15. 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.

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

17. 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.

18. Definitions: Energy: Ability to do work Work= Force x Distance Force: A Push or a Pull

19. Inclined Plane

20. Inclined Plane • The Egyptians used simple machines to build the pyramids. One method was to build a very long incline out of dirt that rose upward to the top of the pyramid very gently. The blocks of stone were placed on large logs (another type of simple machine - the wheel and axle) and pushed slowly up the long, gentle inclined plane to the top of the pyramid.

21. Inclined Planes • An inclined plane is a flat surface that is higher on one end • Inclined planes make the work of moving things easier

22. Work input and output • Work input is the amount of work done on a machine. • Input force x input distance • Work output is the amount of work done by a machine. • Output force x output distance Wout = Win Fout x Dout = Fin x Din 10N x 3m = 2N x 15m 15 m Din Dout 3 m Fin 10 N

23. 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.

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

25. Wedges • Two inclined planes joined back to back. • Wedges are used to split things.

26. 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

27. First Class Lever Fulcrum is between EF (effort) and RF (load)Effort moves farther than Resistance.Multiplies EF and changes its directionThe mechanical advantage of a lever is the ratio of the length of the lever on the applied force side of the fulcrum to the length of the lever on the resistance force side of the fulcrum.

28. First Class Lever . • Common examples of first-class levers include crowbars, scissors, pliers, tin snips and seesaws.

29. Second ClassLever RF (load) is between fulcrum and EF Effort moves farther than Resistance.Multiplies EF, but does not change its directionThe mechanical advantage of a lever is the ratio of the distance from the applied force to the fulcrum to the distance from the resistance force to the fulcrum.

30. Second Class Lever • Examples of second-class levers include nut crackers, wheel barrows, doors, and bottle openers.

31. Third Class Lever EF is between fulcrum and RF (load) Does not multiply force Resistance moves farther than Effort.Multiplies the distance the effort force travelsThe mechanical advantage of a lever is the ratio of the distance from the applied force to the fulcrum to the distance of the resistance force to the fulcrum

32. Third Class Lever • Examples of third-class levers include tweezers, arm hammers, and shovels.

33. Pulleys • Pulley are wheels and axles with a groove around the outside • A pulley needs a rope, chain or belt around the groove to make it do work

34. Diagrams of Pulleys Fixed pulley: A fixed pulley changes the direction of a force; however, it does not create a mechanical advantage. Movable Pulley: The mechanical advantage of a moveable pulley is equal to the number of ropes that support the moveable pulley.

35. COMBINED PULLEY • The effort needed to lift the load is less than half the weight of the load. • The main disadvantage is it travels a very long distance.

36. WHEEL AND AXEL • The axle is stuck rigidly to a large wheel. Fan blades are attached to the wheel. When the axel turns, the fan blades spin.

37. 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

38. GEARS-Wheel and Axel • Each gear in a series reverses the direction of rotation of the previous gear. The smaller gear will always turn faster than the larger gear.

39. 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

40. 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.